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    "primer": "# TypeScript Primer: Four Common Syntax Patterns\n\nThe tutorial repeatedly uses the following four kinds of syntax. When reading a type, first identify the data structure described to the right of the colon. When reading a function, identify the operation it performs at runtime.\n\n## Type Annotations Enable Early Checks\n\n```ts\nconst step: number = 1;\n```\n\n`step` is a variable, `1` is its runtime value, and `: number` gives the editor and compiler an instruction for type checking. The built JavaScript omits this type annotation.\n\n## interface Describes an Object's Fields\n\n```ts\ninterface ToolCall {\n  name: string;\n  arguments: JsonValue;\n}\n```\n\nAn `interface` lists the fields that an object must contain. This Tool Call contains a string field named `name` and an `arguments` field represented using JavaScript Object Notation (JSON). JSON is a structured data format.\n\n## async / await Waits for a Result\n\n```ts\nconst response = await llm.complete(request);\n```\n\nAn `async` function can use `await`. Here, the code sends a model request and continues the Agent Loop after receiving the result. Parallel execution and retries after failure require separate logic.\n\n## The type Field Identifies the Event Kind\n\n```ts\nif (event.type === \"tool/result\") {\n  console.log(event.content);\n}\n```\n\nThe Session Log contains multiple event types. After the code checks `type`, TypeScript can determine that the current branch has fields such as `content` and `name`. The execution trace on the right also classifies events by this field.\n"
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  "chapters": [
    {
      "id": "chapter-1",
      "number": "01",
      "shortTitle": "Agent Loop",
      "title": "How the model cycles between actions and feedback",
      "question": "What does DSH's Agent Loop look like?",
      "codeGuide": {
        "title": "Put model actions behind a verifiable execution boundary",
        "description": "Start with one small baseline: a Step fixes its current input, the model proposes a tool call, and the runtime validates and executes it before passing the result to the next Step.",
        "observations": [
          {
            "title": "Each request captures the current state when it is built",
            "text": "The user message first enters the history. Each Step then copies the current messages, Tool schemas, and dynamic instructions. The model receives a fixed input, so later concurrent changes cannot alter a request that has already been built.",
            "lines": [
              53,
              76
            ]
          },
          {
            "title": "Tool calls determine whether execution continues",
            "text": "The model response first enters the history. If it contains no tool calls, the entire Turn ends. If it contains calls, the Harness runs them one by one, appends all results, and starts the next Step.",
            "lines": [
              73,
              95
            ]
          },
          {
            "title": "Failures also become feedback",
            "text": "Unknown tools, argument validation failures, and execution errors all become structured results. In the next Step, the model can read the reason for a failure and adjust its approach.",
            "lines": [
              98,
              113
            ]
          }
        ],
        "fills": [
          {
            "label": "Interface declarations and Agent structure",
            "kind": "skeleton",
            "ranges": [
              [
                1,
                40
              ],
              [
                115,
                116
              ]
            ]
          },
          {
            "label": "Constructor: inject the model adapter and tools",
            "kind": "body",
            "ranges": [
              [
                41,
                52
              ]
            ]
          },
          {
            "label": "runTurn: request assembly, tool round-trips, and stopping",
            "kind": "body",
            "ranges": [
              [
                53,
                97
              ]
            ]
          },
          {
            "label": "Tool execution: validation and error wrapping",
            "kind": "body",
            "ranges": [
              [
                98,
                114
              ]
            ]
          }
        ]
      },
      "changeStory": {
        "title": "Establish a verifiable execution baseline",
        "summary": "Nano DSH puts consecutive Steps inside one Turn: the model proposes a tool call, the runtime validates and executes it, and the result joins the next request. The loop closes a tool round-trip while leaving the tool catalog, history, and record in memory for later chapters to improve.",
        "harnessRole": "Execution layer: convert model output into validated operations",
        "connection": "This chapter proves that the model, tools, and feedback can form a loop. Chapter 2 controls the history each request sees, Chapter 3 makes tools and rules composable capabilities, and Chapter 4 establishes a shared record for every fact.",
        "outcomes": [
          "Trace the execution order of requests, responses, tool results, and the next Step through Agent.runTurn()",
          "Explain that the model proposes a Tool Call, while the Harness finds the tool, validates its arguments, and runs it",
          "Distinguish a Step, a Turn, and maxSteps, and identify the runtime problems this baseline still leaves open"
        ]
      },
      "lesson": "# One Tool Round-Trip: Where the Agent Loop Starts\n\nConsider one concrete task: the user asks for the current time in Beijing. The model understands the question, but it has no live data. It needs to call `get_time`, read the returned value, and then answer. That sequence puts the model, a tool, and the runtime on one execution path, which every later DSH mechanism relies on.\n\nMany Agent Harnesses can complete one tool call. DSH becomes distinct in the way it organizes the surrounding runtime: tools, prompts, sessions, and the Agent Loop can all be assembled as plugins, while execution facts enter one shared log. This chapter first establishes the smallest loop so that the later mechanisms have a clear place to attach.\n\n`agent.ts` declares the model adapter, tool definitions, the result of one Turn, and the state held by `Agent`. Keep one boundary in view before reading the implementation: **the model proposes the next step; the runtime prepares input, executes actions, and saves feedback.**\n\n<!-- fill {\"index\":0} -->\n\n## 1.1 Why One Answer Can Span Multiple Steps\n\nOn its first request, the model receives system rules, the parameter definition for `get_time`, the user question, and an instruction for the current Step. None of that reveals the current time, so it returns a Tool Call. The runtime executes the query and adds the result to message history. A second request includes that new fact. When the model then returns ordinary text, the Turn ends.\n\n**A Step is one model request together with the tool execution it triggers; a Turn can contain several Steps.** To keep every Step within the same boundary, the `Agent` constructor fixes the model adapter, tool table, argument validator, system rules, and maximum Step count. The loop can then start without rediscovering those dependencies on every request.\n\n<!-- fill {\"index\":1} -->\n\nOnly after that setup does `runTurn()` process user input. It writes the input into history, builds an independent request for each Step, calls the model, and writes the reply back into history. A Tool Call keeps the loop moving; a reply without one becomes the final answer. Request construction, the model reply, and the tool round-trip sit in one control flow, so one complete Turn can be read in order.\n\n<!-- fill {\"index\":2} -->\n\n<!-- evidence {\"id\":\"m01-request-feedback\",\"tab\":\"request\",\"requestStep\":2,\"label\":\"How a tool result enters the next request\",\"description\":\"Compare the two requests: the first has the task and tool definitions; the second appends the tool call and query result to that content.\"} -->\n\n## 1.2 Tool Execution Needs One Feedback Path\n\nThe model can supply a tool name and arguments, but it cannot decide whether the action is valid or succeeds. The runtime has to find the registered tool, check that the arguments follow its Schema, and execute the function. A successful query, an unknown tool, invalid arguments, and a thrown exception all need to return through the same kind of tool-result message.\n\n`runTurn()` therefore keeps validation details out of the loop body and calls `#executeTool()`. **Successful values and failures both return as the same structured feedback.** The helper converts the three failure cases into results with `ok: false` and preserves a successful value. The next request can read one consistent message shape, allowing the model to adjust its arguments or choose another action.\n\n<!-- fill {\"index\":3} -->\n\n## 1.3 Where This Loop Sits in DSH\n\nThis minimal implementation turns a tool round-trip into an inspectable program flow, while still keeping the tool catalog, system rules, and message history directly inside `Agent`. Longer tasks make request input grow; runtime changes crowd the constructor; and history held only in memory cannot support replay after the process ends.\n\nFull DSH preserves this execution boundary while assigning input projection, capability assembly, and session recording to dedicated runtime mechanisms. Understanding the Step and Turn in this chapter makes it possible to see which part of the loop each later mechanism supports.\n",
      "source": {
        "path": "src/agent.ts",
        "content": "import { Ajv, type ValidateFunction } from \"ajv\";\nimport type {\n  AssistantMessage,\n  JsonValue,\n  Llm,\n  ModelMessage,\n  ToolDefinition,\n  UnifiedRequest,\n} from \"./protocol.js\";\n\nexport interface AgentTraceEntry {\n  kind: \"turn\" | \"step\" | \"request\" | \"assistant\" | \"tool\";\n  label: string;\n  detail?: string;\n}\n\nexport interface RunTurnResult {\n  finalMessage: AssistantMessage;\n  steps: number;\n  trace: AgentTraceEntry[];\n  requests: UnifiedRequest[];\n}\n\nexport interface AgentOptions {\n  llm: Llm;\n  tools: ToolDefinition[];\n  systemPrompt: string;\n  dynamicContext?: (step: number) => string;\n  maxSteps?: number;\n}\n\nexport class Agent {\n  readonly #llm: Llm;\n  readonly #tools: Map<string, ToolDefinition>;\n  readonly #validators: Map<string, ValidateFunction>;\n  readonly #systemPrompt: string;\n  readonly #dynamicContext: (step: number) => string;\n  readonly #maxSteps: number;\n  readonly #messages: ModelMessage[] = [];\n\n  constructor(options: AgentOptions) {\n    this.#llm = options.llm;\n    this.#tools = new Map(options.tools.map((tool) => [tool.name, tool]));\n    const ajv = new Ajv({ allErrors: true, strict: false });\n    this.#validators = new Map(\n      options.tools.map((tool) => [tool.name, ajv.compile(tool.inputSchema)]),\n    );\n    this.#systemPrompt = options.systemPrompt;\n    this.#dynamicContext = options.dynamicContext ?? ((step) => `Current step: ${step}`);\n    this.#maxSteps = options.maxSteps ?? 8;\n  }\n\n  async runTurn(userInput: string): Promise<RunTurnResult> {\n    this.#messages.push({ role: \"user\", content: userInput });\n    const trace: AgentTraceEntry[] = [{ kind: \"turn\", label: \"turn/start\", detail: userInput }];\n    const requests: UnifiedRequest[] = [];\n\n    for (let step = 1; step <= this.#maxSteps; step += 1) {\n      trace.push({ kind: \"step\", label: `step/${step}/start` });\n      const request: UnifiedRequest = {\n        system: this.#systemPrompt,\n        tools: [...this.#tools.values()].map(({ name, description, inputSchema }) => ({\n          name,\n          description,\n          inputSchema,\n        })),\n        messages: structuredClone(this.#messages),\n        dynamicContext: this.#dynamicContext(step),\n      };\n      requests.push(structuredClone(request));\n      trace.push({ kind: \"request\", label: \"llm/request\", detail: `${request.tools.length} tools` });\n\n      const response = await this.#llm.complete(request);\n      const assistant = response.message;\n      this.#messages.push(assistant);\n      trace.push({ kind: \"assistant\", label: \"assistant/message\", detail: assistant.content });\n\n      if (assistant.toolCalls.length === 0) {\n        trace.push({ kind: \"turn\", label: \"turn/end\", detail: \"completed\" });\n        return { finalMessage: assistant, steps: step, trace, requests };\n      }\n\n      for (const call of assistant.toolCalls) {\n        const result = await this.#executeTool(call.name, call.arguments);\n        this.#messages.push({\n          role: \"tool\",\n          toolCallId: call.id,\n          name: call.name,\n          content: JSON.stringify(result),\n        });\n        trace.push({ kind: \"tool\", label: call.name, detail: JSON.stringify(result) });\n      }\n    }\n\n    throw new Error(`Agent exceeded maxSteps (${this.#maxSteps}) before finishing the turn.`);\n  }\n\n  async #executeTool(name: string, input: JsonValue): Promise<JsonValue> {\n    const tool = this.#tools.get(name);\n    if (!tool) return { ok: false, error: `Unknown tool: ${name}` };\n    const validate = this.#validators.get(name);\n    if (!validate?.(input)) {\n      return {\n        ok: false,\n        error: \"Invalid tool arguments\",\n        issues: (validate?.errors ?? []).map((error) => `${error.instancePath || \"/\"} ${error.message}`),\n      };\n    }\n    try {\n      return await tool.execute(input);\n    } catch (error) {\n      return { ok: false, error: error instanceof Error ? error.message : String(error) };\n    }\n  }\n}\n",
        "excerpt": "  async runTurn(userInput: string): Promise<RunTurnResult> {\n    this.#messages.push({ role: \"user\", content: userInput });\n    const trace: AgentTraceEntry[] = [{ kind: \"turn\", label: \"turn/start\", detail: userInput }];\n    const requests: UnifiedRequest[] = [];\n\n    for (let step = 1; step <= this.#maxSteps; step += 1) {\n      trace.push({ kind: \"step\", label: `step/${step}/start` });\n      const request: UnifiedRequest = {\n        system: this.#systemPrompt,\n        tools: [...this.#tools.values()].map(({ name, description, inputSchema }) => ({\n          name,\n          description,\n          inputSchema,\n        })),\n        messages: structuredClone(this.#messages),\n        dynamicContext: this.#dynamicContext(step),\n      };\n      requests.push(structuredClone(request));\n      trace.push({ kind: \"request\", label: \"llm/request\", detail: `${request.tools.length} tools` });\n\n      const response = await this.#llm.complete(request);\n      const assistant = response.message;\n      this.#messages.push(assistant);\n      trace.push({ kind: \"assistant\", label: \"assistant/message\", detail: assistant.content });\n\n      if (assistant.toolCalls.length === 0) {\n        trace.push({ kind: \"turn\", label: \"turn/end\", detail: \"completed\" });\n        return { finalMessage: assistant, steps: step, trace, requests };\n      }\n\n      for (const call of assistant.toolCalls) {\n        const result = await this.#executeTool(call.name, call.arguments);\n        this.#messages.push({\n          role: \"tool\",\n          toolCallId: call.id,\n          name: call.name,\n          content: JSON.stringify(result),\n        });\n        trace.push({ kind: \"tool\", label: call.name, detail: JSON.stringify(result) });\n      }\n    }\n\n    throw new Error(`Agent exceeded maxSteps (${this.#maxSteps}) before finishing the turn.`);\n  }\n\n  async #executeTool(name: string, input: JsonValue): Promise<JsonValue> {\n    const tool = this.#tools.get(name);\n    if (!tool) return { ok: false, error: `Unknown tool: ${name}` };\n    const validate = this.#validators.get(name);\n    if (!validate?.(input)) {\n      return {\n        ok: false,\n        error: \"Invalid tool arguments\",\n        issues: (validate?.errors ?? []).map((error) => `${error.instancePath || \"/\"} ${error.message}`),\n      };\n    }\n    try {\n      return await tool.execute(input);\n    } catch (error) {\n      return { ok: false, error: error instanceof Error ? error.message : String(error) };\n    }",
        "startLine": 53,
        "endLine": 113
      },
      "extraFiles": [],
      "diff": "diff --git a/src/agent.ts b/src/agent.ts\nnew file mode 100644\nindex 0000000..16b87c4\n--- /dev/null\n+++ b/src/agent.ts\n@@ -0,0 +1,115 @@\n+import { Ajv, type ValidateFunction } from \"ajv\";\n+import type {\n+  AssistantMessage,\n+  JsonValue,\n+  Llm,\n+  ModelMessage,\n+  ToolDefinition,\n+  UnifiedRequest,\n+} from \"./protocol.js\";\n+\n+export interface AgentTraceEntry {\n+  kind: \"turn\" | \"step\" | \"request\" | \"assistant\" | \"tool\";\n+  label: string;\n+  detail?: string;\n+}\n+\n+export interface RunTurnResult {\n+  finalMessage: AssistantMessage;\n+  steps: number;\n+  trace: AgentTraceEntry[];\n+  requests: UnifiedRequest[];\n+}\n+\n+export interface AgentOptions {\n+  llm: Llm;\n+  tools: ToolDefinition[];\n+  systemPrompt: string;\n+  dynamicContext?: (step: number) => string;\n+  maxSteps?: number;\n+}\n+\n+export class Agent {\n+  readonly #llm: Llm;\n+  readonly #tools: Map<string, ToolDefinition>;\n+  readonly #validators: Map<string, ValidateFunction>;\n+  readonly #systemPrompt: string;\n+  readonly #dynamicContext: (step: number) => string;\n+  readonly #maxSteps: number;\n+  readonly #messages: ModelMessage[] = [];\n+\n+  constructor(options: AgentOptions) {\n+    this.#llm = options.llm;\n+    this.#tools = new Map(options.tools.map((tool) => [tool.name, tool]));\n+    const ajv = new Ajv({ allErrors: true, strict: false });\n+    this.#validators = new Map(\n+      options.tools.map((tool) => [tool.name, ajv.compile(tool.inputSchema)]),\n+    );\n+    this.#systemPrompt = options.systemPrompt;\n+    this.#dynamicContext = options.dynamicContext ?? ((step) => `Current step: ${step}`);\n+    this.#maxSteps = options.maxSteps ?? 8;\n+  }\n+\n+  async runTurn(userInput: string): Promise<RunTurnResult> {\n+    this.#messages.push({ role: \"user\", content: userInput });\n+    const trace: AgentTraceEntry[] = [{ kind: \"turn\", label: \"turn/start\", detail: userInput }];\n+    const requests: UnifiedRequest[] = [];\n+\n+    for (let step = 1; step <= this.#maxSteps; step += 1) {\n+      trace.push({ kind: \"step\", label: `step/${step}/start` });\n+      const request: UnifiedRequest = {\n+        system: this.#systemPrompt,\n+        tools: [...this.#tools.values()].map(({ name, description, inputSchema }) => ({\n+          name,\n+          description,\n+          inputSchema,\n+        })),\n+        messages: structuredClone(this.#messages),\n+        dynamicContext: this.#dynamicContext(step),\n+      };\n+      requests.push(structuredClone(request));\n+      trace.push({ kind: \"request\", label: \"llm/request\", detail: `${request.tools.length} tools` });\n+\n+      const response = await this.#llm.complete(request);\n+      const assistant = response.message;\n+      this.#messages.push(assistant);\n+      trace.push({ kind: \"assistant\", label: \"assistant/message\", detail: assistant.content });\n+\n+      if (assistant.toolCalls.length === 0) {\n+        trace.push({ kind: \"turn\", label: \"turn/end\", detail: \"completed\" });\n+        return { finalMessage: assistant, steps: step, trace, requests };\n+      }\n+\n+      for (const call of assistant.toolCalls) {\n+        const result = await this.#executeTool(call.name, call.arguments);\n+        this.#messages.push({\n+          role: \"tool\",\n+          toolCallId: call.id,\n+          name: call.name,\n+          content: JSON.stringify(result),\n+        });\n+        trace.push({ kind: \"tool\", label: call.name, detail: JSON.stringify(result) });\n+      }\n+    }\n+\n+    throw new Error(`Agent exceeded maxSteps (${this.#maxSteps}) before finishing the turn.`);\n+  }\n+\n+  async #executeTool(name: string, input: JsonValue): Promise<JsonValue> {\n+    const tool = this.#tools.get(name);\n+    if (!tool) return { ok: false, error: `Unknown tool: ${name}` };\n+    const validate = this.#validators.get(name);\n+    if (!validate?.(input)) {\n+      return {\n+        ok: false,\n+        error: \"Invalid tool arguments\",\n+        issues: (validate?.errors ?? []).map((error) => `${error.instancePath || \"/\"} ${error.message}`),\n+      };\n+    }\n+    try {\n+      return await tool.execute(input);\n+    } catch (error) {\n+      return { ok: false, error: error instanceof Error ? error.message : String(error) };\n+    }\n+  }\n+}\n",
      "diffStats": {
        "filesChanged": 1,
        "additions": 115,
        "deletions": 0,
        "files": [
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            "path": "src/agent.ts",
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          "request": {
            "system": "Use a tool when the answer requires external facts. Answer directly after receiving the result.",
            "tools": [
              {
                "name": "get_time",
                "description": "Get the current time in a city.",
                "inputSchema": {
                  "type": "object",
                  "properties": {
                    "city": {
                      "type": "string"
                    }
                  },
                  "required": [
                    "city"
                  ],
                  "additionalProperties": false
                }
              }
            ],
            "messages": [
              {
                "role": "user",
                "content": "Find the current time in Beijing."
              }
            ],
            "dynamicContext": "Step 1: retrieve the current facts."
          },
          "parts": [
            {
              "id": "system",
              "kind": "system",
              "stability": "stable",
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              "approximateTokens": 5
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                {
                  "name": "get_time",
                  "description": "Get the current time in a city.",
                  "inputSchema": {
                    "type": "object",
                    "properties": {
                      "city": {
                        "type": "string"
                      }
                    },
                    "required": [
                      "city"
                    ],
                    "additionalProperties": false
                  }
                }
              ],
              "approximateTokens": 43
            },
            {
              "id": "message-0",
              "kind": "message",
              "stability": "append-only",
              "label": "user",
              "value": {
                "role": "user",
                "content": "Find the current time in Beijing."
              },
              "approximateTokens": 10
            },
            {
              "id": "dynamic",
              "kind": "dynamic",
              "stability": "step-variable",
              "label": "Step context",
              "value": "Step 1: retrieve the current facts.",
              "approximateTokens": 4
            }
          ],
          "totalApproximateTokens": 62,
          "prefix": {
            "sharedParts": 0,
            "sharedApproximateTokens": 0,
            "previousParts": 0,
            "currentParts": 4,
            "firstInvalidation": "First request has no predecessor.",
            "note": "Teaching estimate only; no provider cache was queried."
          }
        },
        {
          "step": 2,
          "request": {
            "system": "Use a tool when the answer requires external facts. Answer directly after receiving the result.",
            "tools": [
              {
                "name": "get_time",
                "description": "Get the current time in a city.",
                "inputSchema": {
                  "type": "object",
                  "properties": {
                    "city": {
                      "type": "string"
                    }
                  },
                  "required": [
                    "city"
                  ],
                  "additionalProperties": false
                }
              }
            ],
            "messages": [
              {
                "role": "user",
                "content": "Find the current time in Beijing."
              },
              {
                "role": "assistant",
                "content": "I will check the time first.",
                "toolCalls": [
                  {
                    "id": "call-time",
                    "name": "get_time",
                    "arguments": {
                      "city": "Beijing"
                    }
                  }
                ]
              },
              {
                "role": "tool",
                "toolCallId": "call-time",
                "name": "get_time",
                "content": "{\"city\":\"Beijing\",\"time\":\"14:30\"}"
              }
            ],
            "dynamicContext": "Step 2: answer from the Tool Result."
          },
          "parts": [
            {
              "id": "system",
              "kind": "system",
              "stability": "stable",
              "label": "System prompt",
              "value": "Use a tool when the answer requires external facts. Answer directly after receiving the result.",
              "approximateTokens": 5
            },
            {
              "id": "tools",
              "kind": "tools",
              "stability": "stable",
              "label": "Tool schemas",
              "value": [
                {
                  "name": "get_time",
                  "description": "Get the current time in a city.",
                  "inputSchema": {
                    "type": "object",
                    "properties": {
                      "city": {
                        "type": "string"
                      }
                    },
                    "required": [
                      "city"
                    ],
                    "additionalProperties": false
                  }
                }
              ],
              "approximateTokens": 43
            },
            {
              "id": "message-0",
              "kind": "message",
              "stability": "append-only",
              "label": "user",
              "value": {
                "role": "user",
                "content": "Find the current time in Beijing."
              },
              "approximateTokens": 10
            },
            {
              "id": "message-1",
              "kind": "message",
              "stability": "append-only",
              "label": "assistant",
              "value": {
                "role": "assistant",
                "content": "I will check the time first.",
                "toolCalls": [
                  {
                    "id": "call-time",
                    "name": "get_time",
                    "arguments": {
                      "city": "Beijing"
                    }
                  }
                ]
              },
              "approximateTokens": 30
            },
            {
              "id": "message-2",
              "kind": "message",
              "stability": "append-only",
              "label": "tool · get_time",
              "value": {
                "role": "tool",
                "toolCallId": "call-time",
                "name": "get_time",
                "content": "{\"city\":\"Beijing\",\"time\":\"14:30\"}"
              },
              "approximateTokens": 27
            },
            {
              "id": "dynamic",
              "kind": "dynamic",
              "stability": "step-variable",
              "label": "Step context",
              "value": "Step 2: answer from the Tool Result.",
              "approximateTokens": 4
            }
          ],
          "totalApproximateTokens": 119,
          "prefix": {
            "sharedParts": 3,
            "sharedApproximateTokens": 58,
            "previousParts": 4,
            "currentParts": 6,
            "firstInvalidation": "assistant",
            "note": "Longest identical canonical prefix; approximate tokens, not provider cache usage."
          }
        }
      ],
      "events": [],
      "trace": [],
      "graphs": []
    },
    {
      "id": "chapter-2",
      "number": "02",
      "shortTitle": "Context and Cache Reuse",
      "title": "Keep the stable prefix as long as possible",
      "question": "How is context organized, and how does DSH optimize cache reuse?",
      "codeGuide": {
        "title": "Build a controllable model view from complete facts",
        "description": "The context module retains original messages, shortens long tool results only for the current request, and orders stable content, accumulated history, and Step-specific instructions by how often they change.",
        "observations": [
          {
            "title": "Projection preserves the original messages",
            "text": "The code checks only Tool Messages. It copies short results in full. For long results, it retains the beginning and end and records the number of omitted characters. The caller still holds the unprojected messages in memory; Chapter 4 moves them into the Session Log.",
            "lines": [
              38,
              67
            ]
          },
          {
            "title": "Request content is ordered by how often it changes",
            "text": "System rules and tools come first, followed by historical messages in order. Dynamic instructions for the current Step come last. This order directly determines where the first change can appear.",
            "lines": [
              70,
              99
            ]
          },
          {
            "title": "Prefix comparison stops at the first difference",
            "text": "The code first serializes both requests deterministically, then compares them item by item from the beginning. The page labels the shared length and first change position as teaching estimates. Data returned by the model provider indicates the actual cache hit.",
            "lines": [
              101,
              137
            ]
          }
        ],
        "fills": [
          {
            "label": "Projection rules, request components, and comparison results",
            "kind": "skeleton",
            "ranges": [
              [
                1,
                37
              ]
            ]
          },
          {
            "label": "Create a model view: retain facts and clip long results",
            "kind": "body",
            "ranges": [
              [
                38,
                69
              ]
            ]
          },
          {
            "label": "Order request components by their rate of change",
            "kind": "body",
            "ranges": [
              [
                70,
                100
              ]
            ]
          },
          {
            "label": "Compare stable prefixes and perform supporting calculations",
            "kind": "body",
            "ranges": [
              [
                101,
                174
              ]
            ]
          }
        ]
      },
      "changeStory": {
        "title": "Complete facts and model input serve different purposes",
        "summary": "Nano DSH retains original messages and lets projectMessages() produce the model view for the current request. Long tool results are clipped, system rules and tools come first, history is appended, and Step-specific instructions come last. Full DSH derives its model view from the Session Log and continues to manage tool results and earlier history under context pressure.",
        "harnessRole": "Input layer: determine exactly what the model receives in each Step",
        "connection": "Chapter 1 needs a request; this chapter defines how to project it from complete facts. Chapter 3 turns tools and rules into composable capabilities, and Chapter 4 makes the complete facts enter one event source.",
        "outcomes": [
          "Distinguish among the complete record, the model projection, and the provider's final request, and explain how each relates to messages",
          "Explain why clipToolResult() changes only the model view and preserves the original Tool Result",
          "Use the order of system, tools, history, and dynamicContext to identify where the cache becomes invalid"
        ]
      },
      "lesson": "# Keep the Shared Prefix as Long as Possible\n\nEach Step in the first chapter carries more messages into the next request. That is easy to follow in a short task. A long tool result can quickly fill the model's context window. At that point, the runtime needs to retain two forms of data: **complete facts for review and recovery, and a model view for the current inference.**\n\nDSH's context design first makes input controllable, then uses a stable request order to encourage prefix-cache reuse. **Control the input first; then keep the stable prefix as long as possible.** Caching is not an isolated optimization. It depends on which request content stays fixed and which content is appended at the end. Follow the division between complete facts and the model view before looking at the code that preserves it.\n\n`context.ts` opens with projection rules, request components, and comparison results. These give later functions a common vocabulary: which tool results need clipping, which parts compose a request, and where two requests first differ.\n\n<!-- fill {\"index\":0} -->\n\n## 2.1 The Model View Can Shrink; Complete Facts Must Remain\n\nWhen an Agent answers a question about a memo, `read_note` may return a long text. Carrying the full value through every Step steadily increases input length. Overwriting that value would make later replay lose its evidence. Nano DSH therefore creates a projection before sending a request: it changes only the messages the model reads for this request and leaves the original messages intact.\n\nThe projection function copies messages one by one and calls its clipping helper only for Tool Results. Short results remain unchanged. A long result keeps its beginning, end, and an omission count. The beginning helps the model identify the source; the end often contains a conclusion, status, or error; the marker makes clear that the middle is absent from this view.\n\nThe code intentionally keeps copying and clipping together: **clipping happens while the request view is generated, so the complete facts are never overwritten.** Full DSH stores those original facts in the Session Log for later reconstruction, replay, and recovery.\n\n<!-- fill {\"index\":1} -->\n\n## 2.2 Request Order Determines Where Change Appears\n\nClipping bounds the size of one result. The order of request components determines where new content changes an otherwise shared request. Nano DSH places system rules and the Tool Schema, the names and argument rules of available tools, first. It then appends message history in time order and places instructions that change on every Step at the end. Comparing adjacent requests from the start leaves a longer shared prefix for content that changes slowly.\n\n`describeRequest()` makes that order an explicit array. **Slow-changing content comes first; new content is appended near the end.** The function does not call a model service or manage a cache. It separates a request into observable components, so the page and the comparison function use the same order to understand one model call. When a tool round-trip adds messages, the new part naturally lands at the tail of history.\n\n<!-- fill {\"index\":2} -->\n\n<!-- evidence {\"id\":\"m02-prefix\",\"tab\":\"request\",\"requestStep\":2,\"label\":\"Change starts at the end of history\",\"description\":\"Compare the stable opening and first changed position of two requests to see how a new tool round-trip affects their prefix.\"} -->\n\n## 2.3 Check Structural Stability with a Repeatable Comparison\n\nThe page can describe how much of two requests starts the same way, but it cannot know a model service's actual cache hit. `compareRequestPrefix()` therefore performs a deterministic structural check. It normalizes components, compares them from the beginning, and records the count of shared components, an estimated text size, and the first changed position. Actual cache hits and billing remain the model service's own result.\n\nThe comparison also estimates tokens, the units a model uses to process text, and sorts object fields consistently. These helpers do not alter request content. They make a local comparison of the same structure produce the same conclusion, so readers do not mistake a display estimate for the model service's cache implementation.\n\n<!-- fill {\"index\":3} -->\n\nWhen tools, prompts, or the model adapter change during a run, the stable part of a request changes with them. DSH organizes those capabilities in the runtime; the context module continues to arrange the currently effective capabilities and history into model input.\n",
      "source": {
        "path": "src/context.ts",
        "content": "import type { ModelMessage, ToolMessage, ToolSchema, UnifiedRequest } from \"./protocol.js\";\n\nexport interface ProjectionSettings {\n  maxToolResultChars: number;\n  toolResultHeadChars: number;\n  toolResultTailChars: number;\n}\n\nexport const DEFAULT_PROJECTION: ProjectionSettings = {\n  maxToolResultChars: 720,\n  toolResultHeadChars: 430,\n  toolResultTailChars: 170,\n};\n\nexport interface RequestPart {\n  id: string;\n  kind: \"system\" | \"tools\" | \"message\" | \"dynamic\";\n  stability: \"stable\" | \"append-only\" | \"step-variable\";\n  label: string;\n  value: unknown;\n  approximateTokens: number;\n}\n\nexport interface PrefixComparison {\n  sharedParts: number;\n  sharedApproximateTokens: number;\n  previousParts: number;\n  currentParts: number;\n  firstInvalidation: string | null;\n  note: string;\n}\n\nexport interface ClippedToolResult {\n  content: string;\n  omittedCharacters: number;\n}\n\nexport function projectMessages(\n  messages: ModelMessage[],\n  settings: ProjectionSettings = DEFAULT_PROJECTION,\n): ModelMessage[] {\n  return messages.map((message) => {\n    if (message.role !== \"tool\") return structuredClone(message);\n    const clipped = clipToolResult(message.content, settings);\n    return { ...message, content: clipped.content } satisfies ToolMessage;\n  });\n}\n\nexport function clipToolResult(\n  content: string,\n  settings: ProjectionSettings = DEFAULT_PROJECTION,\n): ClippedToolResult {\n  if (content.length <= settings.maxToolResultChars) {\n    return { content, omittedCharacters: 0 };\n  }\n  const omittedCharacters = Math.max(\n    0,\n    content.length - settings.toolResultHeadChars - settings.toolResultTailChars,\n  );\n  const marker = `\\n[… ${omittedCharacters} characters omitted from the model projection …]\\n`;\n  return {\n    content:\n      content.slice(0, settings.toolResultHeadChars) +\n      marker +\n      content.slice(content.length - settings.toolResultTailChars),\n    omittedCharacters,\n  };\n}\n\nexport function describeRequest(request: UnifiedRequest): RequestPart[] {\n  const parts: RequestPart[] = [\n    part(\"system\", \"system\", \"stable\", \"System prompt\", request.system),\n    part(\"tools\", \"tools\", \"stable\", \"Tool schemas\", request.tools),\n  ];\n  for (const [index, message] of request.messages.entries()) {\n    const suffix = message.role === \"tool\" ? ` · ${message.name}` : \"\";\n    parts.push(\n      part(\n        `message-${index}`,\n        \"message\",\n        \"append-only\",\n        `${message.role}${suffix}`,\n        message,\n      ),\n    );\n  }\n  if (request.dynamicContext) {\n    parts.push(\n      part(\n        \"dynamic\",\n        \"dynamic\",\n        \"step-variable\",\n        \"Step context\",\n        request.dynamicContext,\n      ),\n    );\n  }\n  return parts;\n}\n\nexport function compareRequestPrefix(\n  previous: UnifiedRequest | undefined,\n  current: UnifiedRequest,\n): PrefixComparison {\n  const currentParts = describeRequest(current);\n  if (!previous) {\n    return {\n      sharedParts: 0,\n      sharedApproximateTokens: 0,\n      previousParts: 0,\n      currentParts: currentParts.length,\n      firstInvalidation: \"First request has no predecessor.\",\n      note: \"Teaching estimate only; no provider cache was queried.\",\n    };\n  }\n  const previousParts = describeRequest(previous);\n  let sharedParts = 0;\n  while (\n    sharedParts < previousParts.length &&\n    sharedParts < currentParts.length &&\n    canonical(previousParts[sharedParts]?.value) === canonical(currentParts[sharedParts]?.value)\n  ) {\n    sharedParts += 1;\n  }\n  return {\n    sharedParts,\n    sharedApproximateTokens: currentParts\n      .slice(0, sharedParts)\n      .reduce((total, item) => total + item.approximateTokens, 0),\n    previousParts: previousParts.length,\n    currentParts: currentParts.length,\n    firstInvalidation:\n      sharedParts === Math.min(previousParts.length, currentParts.length)\n        ? null\n        : currentParts[sharedParts]?.label ?? \"Request end\",\n    note: \"Longest identical canonical prefix; approximate tokens, not provider cache usage.\",\n  };\n}\n\nexport function approximateTokens(value: unknown): number {\n  const text = typeof value === \"string\" ? value : canonical(value);\n  return Math.max(1, Math.ceil([...text].length / 4));\n}\n\nexport function toolSchemas(tools: Array<{ name: string; description: string; inputSchema: Record<string, unknown> }>): ToolSchema[] {\n  return tools.map(({ name, description, inputSchema }) => ({ name, description, inputSchema }));\n}\n\nfunction part(\n  id: string,\n  kind: RequestPart[\"kind\"],\n  stability: RequestPart[\"stability\"],\n  label: string,\n  value: unknown,\n): RequestPart {\n  return { id, kind, stability, label, value, approximateTokens: approximateTokens(value) };\n}\n\nfunction canonical(value: unknown): string {\n  return JSON.stringify(sortObject(value));\n}\n\nfunction sortObject(value: unknown): unknown {\n  if (Array.isArray(value)) return value.map(sortObject);\n  if (value !== null && typeof value === \"object\") {\n    return Object.fromEntries(\n      Object.entries(value as Record<string, unknown>)\n        .sort(([left], [right]) => left.localeCompare(right))\n        .map(([key, item]) => [key, sortObject(item)]),\n    );\n  }\n  return value;\n}\n",
        "excerpt": "export function projectMessages(\n  messages: ModelMessage[],\n  settings: ProjectionSettings = DEFAULT_PROJECTION,\n): ModelMessage[] {\n  return messages.map((message) => {\n    if (message.role !== \"tool\") return structuredClone(message);\n    const clipped = clipToolResult(message.content, settings);\n    return { ...message, content: clipped.content } satisfies ToolMessage;\n  });\n}\n\nexport function clipToolResult(\n  content: string,\n  settings: ProjectionSettings = DEFAULT_PROJECTION,\n): ClippedToolResult {\n  if (content.length <= settings.maxToolResultChars) {\n    return { content, omittedCharacters: 0 };\n  }\n  const omittedCharacters = Math.max(\n    0,\n    content.length - settings.toolResultHeadChars - settings.toolResultTailChars,\n  );\n  const marker = `\\n[… ${omittedCharacters} characters omitted from the model projection …]\\n`;\n  return {\n    content:\n      content.slice(0, settings.toolResultHeadChars) +\n      marker +\n      content.slice(content.length - settings.toolResultTailChars),\n    omittedCharacters,\n  };\n}\n\nexport function describeRequest(request: UnifiedRequest): RequestPart[] {\n  const parts: RequestPart[] = [\n    part(\"system\", \"system\", \"stable\", \"System prompt\", request.system),\n    part(\"tools\", \"tools\", \"stable\", \"Tool schemas\", request.tools),\n  ];\n  for (const [index, message] of request.messages.entries()) {\n    const suffix = message.role === \"tool\" ? ` · ${message.name}` : \"\";\n    parts.push(\n      part(\n        `message-${index}`,\n        \"message\",\n        \"append-only\",\n        `${message.role}${suffix}`,\n        message,\n      ),\n    );\n  }\n  if (request.dynamicContext) {\n    parts.push(\n      part(\n        \"dynamic\",\n        \"dynamic\",\n        \"step-variable\",\n        \"Step context\",\n        request.dynamicContext,\n      ),\n    );\n  }\n  return parts;\n}\n\nexport function compareRequestPrefix(\n  previous: UnifiedRequest | undefined,\n  current: UnifiedRequest,\n): PrefixComparison {\n  const currentParts = describeRequest(current);\n  if (!previous) {\n    return {\n      sharedParts: 0,\n      sharedApproximateTokens: 0,\n      previousParts: 0,\n      currentParts: currentParts.length,\n      firstInvalidation: \"First request has no predecessor.\",\n      note: \"Teaching estimate only; no provider cache was queried.\",\n    };\n  }\n  const previousParts = describeRequest(previous);\n  let sharedParts = 0;\n  while (\n    sharedParts < previousParts.length &&\n    sharedParts < currentParts.length &&\n    canonical(previousParts[sharedParts]?.value) === canonical(currentParts[sharedParts]?.value)\n  ) {\n    sharedParts += 1;\n  }\n  return {\n    sharedParts,\n    sharedApproximateTokens: currentParts\n      .slice(0, sharedParts)\n      .reduce((total, item) => total + item.approximateTokens, 0),\n    previousParts: previousParts.length,\n    currentParts: currentParts.length,\n    firstInvalidation:\n      sharedParts === Math.min(previousParts.length, currentParts.length)\n        ? null\n        : currentParts[sharedParts]?.label ?? \"Request end\",\n    note: \"Longest identical canonical prefix; approximate tokens, not provider cache usage.\",\n  };",
        "startLine": 38,
        "endLine": 137
      },
      "extraFiles": [
        {
          "path": "src/agent.ts",
          "content": "import { Ajv, type ValidateFunction } from \"ajv\";\nimport {\n  DEFAULT_PROJECTION,\n  projectMessages,\n  toolSchemas,\n  type ProjectionSettings,\n} from \"./context.js\";\nimport type {\n  AssistantMessage,\n  JsonValue,\n  Llm,\n  ModelMessage,\n  ToolDefinition,\n  UnifiedRequest,\n} from \"./protocol.js\";\n\nexport interface AgentTraceEntry {\n  kind: \"turn\" | \"step\" | \"request\" | \"assistant\" | \"tool\";\n  label: string;\n  detail?: string;\n}\n\nexport interface RunTurnResult {\n  finalMessage: AssistantMessage;\n  steps: number;\n  trace: AgentTraceEntry[];\n  requests: UnifiedRequest[];\n}\n\nexport interface AgentOptions {\n  llm: Llm;\n  tools: ToolDefinition[];\n  systemPrompt: string;\n  dynamicContext?: (step: number) => string;\n  projection?: ProjectionSettings;\n  maxSteps?: number;\n}\n\nexport class Agent {\n  readonly #llm: Llm;\n  readonly #tools: Map<string, ToolDefinition>;\n  readonly #validators: Map<string, ValidateFunction>;\n  readonly #systemPrompt: string;\n  readonly #dynamicContext: (step: number) => string;\n  readonly #maxSteps: number;\n  readonly #projection: ProjectionSettings;\n  readonly #messages: ModelMessage[] = [];\n\n  constructor(options: AgentOptions) {\n    this.#llm = options.llm;\n    this.#tools = new Map(options.tools.map((tool) => [tool.name, tool]));\n    const ajv = new Ajv({ allErrors: true, strict: false });\n    this.#validators = new Map(\n      options.tools.map((tool) => [tool.name, ajv.compile(tool.inputSchema)]),\n    );\n    this.#systemPrompt = options.systemPrompt;\n    this.#dynamicContext = options.dynamicContext ?? ((step) => `Current step: ${step}`);\n    this.#maxSteps = options.maxSteps ?? 8;\n    this.#projection = options.projection ?? DEFAULT_PROJECTION;\n  }\n\n  async runTurn(userInput: string): Promise<RunTurnResult> {\n    this.#messages.push({ role: \"user\", content: userInput });\n    const trace: AgentTraceEntry[] = [{ kind: \"turn\", label: \"turn/start\", detail: userInput }];\n    const requests: UnifiedRequest[] = [];\n\n    for (let step = 1; step <= this.#maxSteps; step += 1) {\n      trace.push({ kind: \"step\", label: `step/${step}/start` });\n      const request: UnifiedRequest = {\n        system: this.#systemPrompt,\n        tools: toolSchemas([...this.#tools.values()]),\n        messages: projectMessages(this.#messages, this.#projection),\n        dynamicContext: this.#dynamicContext(step),\n      };\n      requests.push(structuredClone(request));\n      trace.push({ kind: \"request\", label: \"llm/request\", detail: `${request.tools.length} tools` });\n\n      const response = await this.#llm.complete(request);\n      const assistant = response.message;\n      this.#messages.push(assistant);\n      trace.push({ kind: \"assistant\", label: \"assistant/message\", detail: assistant.content });\n\n      if (assistant.toolCalls.length === 0) {\n        trace.push({ kind: \"turn\", label: \"turn/end\", detail: \"completed\" });\n        return { finalMessage: assistant, steps: step, trace, requests };\n      }\n\n      for (const call of assistant.toolCalls) {\n        const result = await this.#executeTool(call.name, call.arguments);\n        this.#messages.push({\n          role: \"tool\",\n          toolCallId: call.id,\n          name: call.name,\n          content: JSON.stringify(result),\n        });\n        trace.push({ kind: \"tool\", label: call.name, detail: JSON.stringify(result) });\n      }\n    }\n\n    throw new Error(`Agent exceeded maxSteps (${this.#maxSteps}) before finishing the turn.`);\n  }\n\n  async #executeTool(name: string, input: JsonValue): Promise<JsonValue> {\n    const tool = this.#tools.get(name);\n    if (!tool) return { ok: false, error: `Unknown tool: ${name}` };\n    const validate = this.#validators.get(name);\n    if (!validate?.(input)) {\n      return {\n        ok: false,\n        error: \"Invalid tool arguments\",\n        issues: (validate?.errors ?? []).map((error) => `${error.instancePath || \"/\"} ${error.message}`),\n      };\n    }\n    try {\n      return await tool.execute(input);\n    } catch (error) {\n      return { ok: false, error: error instanceof Error ? error.message : String(error) };\n    }\n  }\n}\n"
        }
      ],
      "diff": "diff --git a/src/context.ts b/src/context.ts\nnew file mode 100644\nindex 0000000..7ecce66\n--- /dev/null\n+++ b/src/context.ts\n@@ -0,0 +1,173 @@\n+import type { ModelMessage, ToolMessage, ToolSchema, UnifiedRequest } from \"./protocol.js\";\n+\n+export interface ProjectionSettings {\n+  maxToolResultChars: number;\n+  toolResultHeadChars: number;\n+  toolResultTailChars: number;\n+}\n+\n+export const DEFAULT_PROJECTION: ProjectionSettings = {\n+  maxToolResultChars: 720,\n+  toolResultHeadChars: 430,\n+  toolResultTailChars: 170,\n+};\n+\n+export interface RequestPart {\n+  id: string;\n+  kind: \"system\" | \"tools\" | \"message\" | \"dynamic\";\n+  stability: \"stable\" | \"append-only\" | \"step-variable\";\n+  label: string;\n+  value: unknown;\n+  approximateTokens: number;\n+}\n+\n+export interface PrefixComparison {\n+  sharedParts: number;\n+  sharedApproximateTokens: number;\n+  previousParts: number;\n+  currentParts: number;\n+  firstInvalidation: string | null;\n+  note: string;\n+}\n+\n+export interface ClippedToolResult {\n+  content: string;\n+  omittedCharacters: number;\n+}\n+\n+export function projectMessages(\n+  messages: ModelMessage[],\n+  settings: ProjectionSettings = DEFAULT_PROJECTION,\n+): ModelMessage[] {\n+  return messages.map((message) => {\n+    if (message.role !== \"tool\") return structuredClone(message);\n+    const clipped = clipToolResult(message.content, settings);\n+    return { ...message, content: clipped.content } satisfies ToolMessage;\n+  });\n+}\n+\n+export function clipToolResult(\n+  content: string,\n+  settings: ProjectionSettings = DEFAULT_PROJECTION,\n+): ClippedToolResult {\n+  if (content.length <= settings.maxToolResultChars) {\n+    return { content, omittedCharacters: 0 };\n+  }\n+  const omittedCharacters = Math.max(\n+    0,\n+    content.length - settings.toolResultHeadChars - settings.toolResultTailChars,\n+  );\n+  const marker = `\\n[… ${omittedCharacters} characters omitted from the model projection …]\\n`;\n+  return {\n+    content:\n+      content.slice(0, settings.toolResultHeadChars) +\n+      marker +\n+      content.slice(content.length - settings.toolResultTailChars),\n+    omittedCharacters,\n+  };\n+}\n+\n+export function describeRequest(request: UnifiedRequest): RequestPart[] {\n+  const parts: RequestPart[] = [\n+    part(\"system\", \"system\", \"stable\", \"System prompt\", request.system),\n+    part(\"tools\", \"tools\", \"stable\", \"Tool schemas\", request.tools),\n+  ];\n+  for (const [index, message] of request.messages.entries()) {\n+    const suffix = message.role === \"tool\" ? ` · ${message.name}` : \"\";\n+    parts.push(\n+      part(\n+        `message-${index}`,\n+        \"message\",\n+        \"append-only\",\n+        `${message.role}${suffix}`,\n+        message,\n+      ),\n+    );\n+  }\n+  if (request.dynamicContext) {\n+    parts.push(\n+      part(\n+        \"dynamic\",\n+        \"dynamic\",\n+        \"step-variable\",\n+        \"Step context\",\n+        request.dynamicContext,\n+      ),\n+    );\n+  }\n+  return parts;\n+}\n+\n+export function compareRequestPrefix(\n+  previous: UnifiedRequest | undefined,\n+  current: UnifiedRequest,\n+): PrefixComparison {\n+  const currentParts = describeRequest(current);\n+  if (!previous) {\n+    return {\n+      sharedParts: 0,\n+      sharedApproximateTokens: 0,\n+      previousParts: 0,\n+      currentParts: currentParts.length,\n+      firstInvalidation: \"First request has no predecessor.\",\n+      note: \"Teaching estimate only; no provider cache was queried.\",\n+    };\n+  }\n+  const previousParts = describeRequest(previous);\n+  let sharedParts = 0;\n+  while (\n+    sharedParts < previousParts.length &&\n+    sharedParts < currentParts.length &&\n+    canonical(previousParts[sharedParts]?.value) === canonical(currentParts[sharedParts]?.value)\n+  ) {\n+    sharedParts += 1;\n+  }\n+  return {\n+    sharedParts,\n+    sharedApproximateTokens: currentParts\n+      .slice(0, sharedParts)\n+      .reduce((total, item) => total + item.approximateTokens, 0),\n+    previousParts: previousParts.length,\n+    currentParts: currentParts.length,\n+    firstInvalidation:\n+      sharedParts === Math.min(previousParts.length, currentParts.length)\n+        ? null\n+        : currentParts[sharedParts]?.label ?? \"Request end\",\n+    note: \"Longest identical canonical prefix; approximate tokens, not provider cache usage.\",\n+  };\n+}\n+\n+export function approximateTokens(value: unknown): number {\n+  const text = typeof value === \"string\" ? value : canonical(value);\n+  return Math.max(1, Math.ceil([...text].length / 4));\n+}\n+\n+export function toolSchemas(tools: Array<{ name: string; description: string; inputSchema: Record<string, unknown> }>): ToolSchema[] {\n+  return tools.map(({ name, description, inputSchema }) => ({ name, description, inputSchema }));\n+}\n+\n+function part(\n+  id: string,\n+  kind: RequestPart[\"kind\"],\n+  stability: RequestPart[\"stability\"],\n+  label: string,\n+  value: unknown,\n+): RequestPart {\n+  return { id, kind, stability, label, value, approximateTokens: approximateTokens(value) };\n+}\n+\n+function canonical(value: unknown): string {\n+  return JSON.stringify(sortObject(value));\n+}\n+\n+function sortObject(value: unknown): unknown {\n+  if (Array.isArray(value)) return value.map(sortObject);\n+  if (value !== null && typeof value === \"object\") {\n+    return Object.fromEntries(\n+      Object.entries(value as Record<string, unknown>)\n+        .sort(([left], [right]) => left.localeCompare(right))\n+        .map(([key, item]) => [key, sortObject(item)]),\n+    );\n+  }\n+  return value;\n+}\n",
      "diffStats": {
        "filesChanged": 1,
        "additions": 173,
        "deletions": 0,
        "files": [
          {
            "path": "src/context.ts",
            "additions": 173,
            "deletions": 0
          }
        ]
      },
      "requests": [
        {
          "step": 1,
          "request": {
            "system": "Retrieve the facts first, then answer briefly.",
            "tools": [
              {
                "name": "read_note",
                "description": "Read a note by name.",
                "inputSchema": {
                  "type": "object",
                  "properties": {
                    "name": {
                      "type": "string"
                    }
                  },
                  "required": [
                    "name"
                  ],
                  "additionalProperties": false
                }
              }
            ],
            "messages": [
              {
                "role": "user",
                "content": "What does the note say?"
              }
            ],
            "dynamicContext": "Step 1: read the note."
          },
          "parts": [
            {
              "id": "system",
              "kind": "system",
              "stability": "stable",
              "label": "System prompt",
              "value": "Retrieve the facts first, then answer briefly.",
              "approximateTokens": 4
            },
            {
              "id": "tools",
              "kind": "tools",
              "stability": "stable",
              "label": "Tool schemas",
              "value": [
                {
                  "name": "read_note",
                  "description": "Read a note by name.",
                  "inputSchema": {
                    "type": "object",
                    "properties": {
                      "name": {
                        "type": "string"
                      }
                    },
                    "required": [
                      "name"
                    ],
                    "additionalProperties": false
                  }
                }
              ],
              "approximateTokens": 43
            },
            {
              "id": "message-0",
              "kind": "message",
              "stability": "append-only",
              "label": "user",
              "value": {
                "role": "user",
                "content": "What does the note say?"
              },
              "approximateTokens": 10
            },
            {
              "id": "dynamic",
              "kind": "dynamic",
              "stability": "step-variable",
              "label": "Step context",
              "value": "Step 1: read the note.",
              "approximateTokens": 4
            }
          ],
          "totalApproximateTokens": 61,
          "prefix": {
            "sharedParts": 0,
            "sharedApproximateTokens": 0,
            "previousParts": 0,
            "currentParts": 4,
            "firstInvalidation": "First request has no predecessor.",
            "note": "Teaching estimate only; no provider cache was queried."
          }
        },
        {
          "step": 2,
          "request": {
            "system": "Retrieve the facts first, then answer briefly.",
            "tools": [
              {
                "name": "read_note",
                "description": "Read a note by name.",
                "inputSchema": {
                  "type": "object",
                  "properties": {
                    "name": {
                      "type": "string"
                    }
                  },
                  "required": [
                    "name"
                  ],
                  "additionalProperties": false
                }
              }
            ],
            "messages": [
              {
                "role": "user",
                "content": "What does the note say?"
              },
              {
                "role": "assistant",
                "content": "I will read the note first.",
                "toolCalls": [
                  {
                    "id": "call-note",
                    "name": "read_note",
                    "arguments": {
                      "name": "today"
                    }
                  }
                ]
              },
              {
                "role": "tool",
                "toolCallId": "call-note",
                "name": "read_note",
                "content": "Note: today · meeting preparation\nBackground: Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. \nConclusion: the meeting starts at 3:00 p.m."
              }
            ],
            "dynamicContext": "Step 2: answer only from the note."
          },
          "parts": [
            {
              "id": "system",
              "kind": "system",
              "stability": "stable",
              "label": "System prompt",
              "value": "Retrieve the facts first, then answer briefly.",
              "approximateTokens": 4
            },
            {
              "id": "tools",
              "kind": "tools",
              "stability": "stable",
              "label": "Tool schemas",
              "value": [
                {
                  "name": "read_note",
                  "description": "Read a note by name.",
                  "inputSchema": {
                    "type": "object",
                    "properties": {
                      "name": {
                        "type": "string"
                      }
                    },
                    "required": [
                      "name"
                    ],
                    "additionalProperties": false
                  }
                }
              ],
              "approximateTokens": 43
            },
            {
              "id": "message-0",
              "kind": "message",
              "stability": "append-only",
              "label": "user",
              "value": {
                "role": "user",
                "content": "What does the note say?"
              },
              "approximateTokens": 10
            },
            {
              "id": "message-1",
              "kind": "message",
              "stability": "append-only",
              "label": "assistant",
              "value": {
                "role": "assistant",
                "content": "I will read the note first.",
                "toolCalls": [
                  {
                    "id": "call-note",
                    "name": "read_note",
                    "arguments": {
                      "name": "today"
                    }
                  }
                ]
              },
              "approximateTokens": 31
            },
            {
              "id": "message-2",
              "kind": "message",
              "stability": "append-only",
              "label": "tool · read_note",
              "value": {
                "role": "tool",
                "toolCallId": "call-note",
                "name": "read_note",
                "content": "Note: today · meeting preparation\nBackground: Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. Project status, open questions, and discussion notes. \nConclusion: the meeting starts at 3:00 p.m."
              },
              "approximateTokens": 172
            },
            {
              "id": "dynamic",
              "kind": "dynamic",
              "stability": "step-variable",
              "label": "Step context",
              "value": "Step 2: answer only from the note.",
              "approximateTokens": 4
            }
          ],
          "totalApproximateTokens": 264,
          "prefix": {
            "sharedParts": 3,
            "sharedApproximateTokens": 57,
            "previousParts": 4,
            "currentParts": 6,
            "firstInvalidation": "assistant",
            "note": "Longest identical canonical prefix; approximate tokens, not provider cache usage."
          }
        }
      ],
      "events": [],
      "trace": [],
      "graphs": []
    },
    {
      "id": "chapter-3",
      "number": "03",
      "shortTitle": "Everything Is a Plugin",
      "title": "Manage runtime capabilities through plugins",
      "question": "How does DSH make everything a plugin?",
      "codeGuide": {
        "title": "Give each capability a source, dependency, and exit path",
        "description": "Nano DSH mounts plugins through Context. When a plugin provides services, tools, prompts, or listeners, Context records the owner and adds the related cleanup action to one effect stack.",
        "observations": [
          {
            "title": "Registered contributions are removed after setup fails",
            "text": "Context creates an installation record before setup and records the current plugin as the source of each contribution. If setup throws an error, Context runs the registered cleanup functions in reverse order.",
            "lines": [
              45,
              60
            ]
          },
          {
            "title": "Removal uses the same effect stack",
            "text": "A successful mount returns a removal function that can run only once. Removal first removes the contributions, then clears the plugin and its dependency relationships. Repeated calls do not run cleanup operations again.",
            "lines": [
              62,
              83
            ]
          },
          {
            "title": "Registered content automatically records its source",
            "text": "During setup, a plugin registers services, tools, prompts, and event listeners. For each registration, Context records the source plugin and registers a cleanup function that removes the corresponding contribution.",
            "lines": [
              86,
              123
            ]
          }
        ],
        "fills": [
          {
            "label": "Plugin contract and runtime state held by Context",
            "kind": "skeleton",
            "ranges": [
              [
                1,
                44
              ],
              [
                194,
                215
              ]
            ]
          },
          {
            "label": "mount: install, roll back, and unload",
            "kind": "body",
            "ranges": [
              [
                45,
                78
              ]
            ]
          },
          {
            "label": "effect, provide, and use",
            "kind": "body",
            "ranges": [
              [
                79,
                106
              ]
            ]
          },
          {
            "label": "Register contributions, read state, and clean up consistently",
            "kind": "body",
            "ranges": [
              [
                107,
                193
              ]
            ]
          }
        ]
      },
      "changeStory": {
        "title": "Runtime capabilities can be composed and fully removed",
        "summary": "Full DSH uses a Cordis plugin tree to assemble model adapters, tools, sessions, and the Agent Loop. Nano DSH retains unified registration and reversible effects: a plugin contributes services, tools, prompts, and listeners, and Context removes those contributions through the same cleanup path when installation fails or the plugin unloads.",
        "harnessRole": "Assembly layer: determine which capabilities the Agent currently has",
        "connection": "Chapter 1 consumes the tool catalog, and Chapter 2 places tools and rules into a request. This chapter explains how plugins supply those capabilities. Chapter 4 will add plugin changes to the shared log.",
        "outcomes": [
          "Distinguish a Tool, which is an action available to the model, from a Plugin, which is a unit of installed capability",
          "Trace the success, failure, and removal paths through Context.mount(), effect(), and the disposer cleanup function",
          "Use the source records and service relationships returned by inspect() to explain which plugins provide the current capabilities"
        ]
      },
      "lesson": "# Every Runtime Capability Comes from a Plugin\n\nChapter 1 gives `Agent` its tools and system rules directly, and Chapter 2 arranges them into a request. That starting point makes the loop easy to understand. Once capabilities multiply, changes begin to gather in a few constructors and global states: adding a tool, replacing a model adapter, or changing workspace settings can touch the same core code.\n\nDSH uses Cordis to organize this change. Cordis is the TypeScript plugin runtime used by DSH: **the model adapter, tool registry, Session Log, and Agent Loop all join one runtime tree as plugins.** Profiles, named assemblies; Bundles, packages of plugins; and Patches, configuration overrides, decide which plugins the current tree contains. Nano DSH omits configuration loading and retains the central runtime constraint: **each capability has a source and can be fully withdrawn after installation.**\n\n`runtime.ts` first expresses that constraint through the plugin contract, service tokens, and the internal state of `Context`. `Context` keeps the current plugins, services, tools, prompts, listeners, and dependency relationships. Together, these containers answer two questions: what can the Agent use now, and where did each capability come from?\n\n<!-- fill {\"index\":0} -->\n\n## 3.1 Give a Capability a Lifecycle Before Installing It\n\nA Plugin is one unit of installation and removal. With a `Context`, it can provide a Service for other plugins, register a Tool the model can call, add a Prompt fragment to system rules, or listen for runtime events. In the capability graph, `clock-core` provides the `clock` service; `clock-tools` uses that service and registers `get_time`. Ownership and dependency can therefore be traced through the graph.\n\nThe difficult state appears when installation fails halfway through. A plugin can register a service and then throw while registering a tool. Leaving its earlier contributions in `Context` makes later runtime state unreliable. `mount()` creates an installation record, runs `setup()`, and gathers every cleanup action in one effect stack. **A failed installation and an intentional unload follow the same reverse cleanup path.**\n\n<!-- fill {\"index\":1} -->\n\n<!-- evidence {\"id\":\"m03-owners\",\"tab\":\"graph\",\"requestStep\":1,\"label\":\"Capability ownership and dependencies\",\"description\":\"Inspect the plugins, services, tools, and prompts to confirm how a tool obtains its dependency through a service relationship.\"} -->\n\n## 3.2 Registering a Capability Also Records How It Leaves\n\n`effect()` is the common lifecycle entry point. It can run only during plugin installation and adds one cleanup function to the current installation record. `provide()` stores a service and its source plugin. `use()` reads the service and records the relationship between its consumer and provider during installation. **Registering a capability also registers its cleanup action and source.** A tool's dependency is then more than an object reference: the runtime can say which installation supplied it.\n\nThis implementation handles the collaboration boundary between plugins. During setup, `clock-tools` retrieves `clock` through `use()` and then registers its tool. On unloading, later cleanup actions run first, so the related tool and dependency records leave together. The order comes from recorded facts; each capability type does not need its own exit procedure.\n\n<!-- fill {\"index\":2} -->\n\n## 3.3 One Record Covers Tools, Prompts, and Listeners\n\nTools, prompts, and listeners have different parameters, yet the runtime registers them in the same way: write an entry with its owner, then pass a removal action to `effect()`. Read methods derive the current tool list, compiled prompt, and capability snapshot from these containers. `#requireMount()` and `#rollback()` protect the installation and cleanup boundaries.\n\nThis source segment keeps contribution registration, state reads, and rollback helpers inside `Context`. Every plugin can enter the runtime through one rule set, and `inspect()` can expose the current plugins, services, tools, prompts, listeners, and dependencies to the model or the page.\n\n<!-- fill {\"index\":3} -->\n\n## 3.4 Where DSH Separates from Many Harnesses\n\nTool loops and context projection are common in many Harnesses. **DSH also puts the capabilities that compose those mechanisms into the plugin tree.** Configuration describes the current assembly, plugin lifecycles manage every contribution entering and leaving, and runtime inspection can trace ownership and dependencies. Model adapters, tools, sessions, and the Agent Loop can then be assembled again for a task or environment.\n\nNano DSH shows service relationships, ownership, and reversible effects. Full DSH also includes dependency injection, waiting for services to become ready, layered configuration, and isolated scopes. The small implementation still demonstrates the architecture's central relationship: capabilities enter `Context` through plugins and leave through the same lifecycle.\n",
      "source": {
        "path": "src/runtime.ts",
        "content": "import type { ToolDefinition } from \"./protocol.js\";\n\nexport class ServiceToken<T> {\n  readonly key = Symbol();\n  constructor(readonly name: string) {}\n}\n\nexport interface Plugin {\n  name: string;\n  setup(context: Context): void | Promise<void>;\n}\n\nexport interface RuntimeInspection {\n  plugins: string[];\n  services: Array<{ name: string; provider: string }>;\n  tools: Array<{ name: string; plugin: string }>;\n  prompts: Array<{ id: string; plugin: string; text: string }>;\n  listeners: Array<{ event: string; plugin: string }>;\n  relations: Array<{ consumer: string; service: string; provider: string }>;\n}\n\ntype Disposer = () => void;\ntype Listener = (payload: unknown) => void;\n\ninterface MountRecord {\n  name: string;\n  effects: Disposer[];\n  state: \"installing\" | \"mounted\" | \"unmounted\";\n}\n\ninterface Owned<T> {\n  owner: string;\n  value: T;\n}\n\nexport class Context {\n  readonly #mounts = new Map<string, MountRecord>();\n  readonly #services = new Map<symbol, Owned<unknown> & { token: ServiceToken<unknown> }>();\n  readonly #tools = new Map<string, Owned<ToolDefinition>>();\n  readonly #prompts = new Map<string, Owned<string>>();\n  readonly #listeners = new Map<string, Set<Owned<Listener>>>();\n  readonly #relations: RuntimeInspection[\"relations\"] = [];\n  #activeMount: MountRecord | null = null;\n\n  async mount(plugin: Plugin): Promise<Disposer> {\n    if (this.#mounts.has(plugin.name)) throw new Error(`Plugin already mounted: ${plugin.name}`);\n    if (this.#activeMount) throw new Error(\"Nested plugin mounting is not supported during setup.\");\n    const record: MountRecord = { name: plugin.name, effects: [], state: \"installing\" };\n    this.#mounts.set(plugin.name, record);\n    this.#activeMount = record;\n    try {\n      await plugin.setup(this);\n      record.state = \"mounted\";\n    } catch (error) {\n      this.#rollback(record);\n      this.#mounts.delete(plugin.name);\n      throw error;\n    } finally {\n      this.#activeMount = null;\n    }\n\n    this.emit(\"runtime/plugin-mounted\", { plugin: plugin.name });\n    return once(() => {\n      if (record.state === \"unmounted\") return;\n      this.emit(\"runtime/plugin-unmounting\", { plugin: plugin.name });\n      this.#rollback(record);\n      this.#mounts.delete(plugin.name);\n      this.#relations.splice(\n        0,\n        this.#relations.length,\n        ...this.#relations.filter(\n          (relation) => relation.consumer !== plugin.name && relation.provider !== plugin.name,\n        ),\n      );\n      this.emit(\"runtime/plugin-unmounted\", { plugin: plugin.name });\n    });\n  }\n\n  effect(cleanup: Disposer): Disposer {\n    const mount = this.#requireMount();\n    const dispose = once(cleanup);\n    mount.effects.push(dispose);\n    return dispose;\n  }\n\n  provide<T>(token: ServiceToken<T>, value: T): Disposer {\n    const owner = this.#requireMount().name;\n    if (this.#services.has(token.key)) throw new Error(`Service already provided: ${token.name}`);\n    this.#services.set(token.key, { owner, value, token: token as ServiceToken<unknown> });\n    return this.effect(() => {\n      const service = this.#services.get(token.key);\n      if (service?.owner === owner) this.#services.delete(token.key);\n    });\n  }\n\n  use<T>(token: ServiceToken<T>): T {\n    const service = this.#services.get(token.key);\n    if (!service) throw new Error(`Missing service: ${token.name}`);\n    const consumer = this.#activeMount?.name;\n    if (consumer) {\n      const relation = { consumer, service: token.name, provider: service.owner };\n      if (!this.#relations.some((item) => sameRelation(item, relation))) this.#relations.push(relation);\n    }\n    return service.value as T;\n  }\n\n  registerTool(tool: ToolDefinition): Disposer {\n    const owner = this.#requireMount().name;\n    if (this.#tools.has(tool.name)) throw new Error(`Tool already registered: ${tool.name}`);\n    this.#tools.set(tool.name, { owner, value: tool });\n    return this.effect(() => {\n      if (this.#tools.get(tool.name)?.owner === owner) this.#tools.delete(tool.name);\n    });\n  }\n\n  contributePrompt(id: string, text: string): Disposer {\n    const owner = this.#requireMount().name;\n    if (this.#prompts.has(id)) throw new Error(`Prompt contribution already registered: ${id}`);\n    this.#prompts.set(id, { owner, value: text });\n    return this.effect(() => {\n      if (this.#prompts.get(id)?.owner === owner) this.#prompts.delete(id);\n    });\n  }\n\n  on(event: string, listener: Listener): Disposer {\n    const owner = this.#requireMount().name;\n    const entry = { owner, value: listener };\n    const listeners = this.#listeners.get(event) ?? new Set<Owned<Listener>>();\n    listeners.add(entry);\n    this.#listeners.set(event, listeners);\n    return this.effect(() => {\n      listeners.delete(entry);\n      if (listeners.size === 0) this.#listeners.delete(event);\n    });\n  }\n\n  emit(event: string, payload: unknown): void {\n    for (const listener of [...(this.#listeners.get(event) ?? [])]) listener.value(payload);\n  }\n\n  listTools(): ToolDefinition[] {\n    return [...this.#tools.values()].map((entry) => entry.value);\n  }\n\n  getTool(name: string): ToolDefinition | undefined {\n    return this.#tools.get(name)?.value;\n  }\n\n  compilePrompt(base = \"\"): string {\n    return [base, ...[...this.#prompts.values()].map((entry) => entry.value)]\n      .filter(Boolean)\n      .join(\"\\n\\n\");\n  }\n\n  inspect(): RuntimeInspection {\n    return {\n      plugins: [...this.#mounts.values()]\n        .filter((mount) => mount.state === \"mounted\")\n        .map((mount) => mount.name),\n      services: [...this.#services.values()].map((entry) => ({\n        name: entry.token.name,\n        provider: entry.owner,\n      })),\n      tools: [...this.#tools.entries()].map(([name, entry]) => ({ name, plugin: entry.owner })),\n      prompts: [...this.#prompts.entries()].map(([id, entry]) => ({\n        id,\n        plugin: entry.owner,\n        text: entry.value,\n      })),\n      listeners: [...this.#listeners.entries()].flatMap(([event, entries]) =>\n        [...entries].map((entry) => ({ event, plugin: entry.owner })),\n      ),\n      relations: structuredClone(this.#relations),\n    };\n  }\n\n  #requireMount(): MountRecord {\n    if (!this.#activeMount) throw new Error(\"Runtime contributions must be registered during plugin setup.\");\n    return this.#activeMount;\n  }\n\n  #rollback(record: MountRecord): void {\n    const failures: unknown[] = [];\n    for (const dispose of [...record.effects].reverse()) {\n      try {\n        dispose();\n      } catch (error) {\n        failures.push(error);\n      }\n    }\n    record.state = \"unmounted\";\n    if (failures.length > 0) throw new AggregateError(failures, `Failed to clean up ${record.name}`);\n  }\n}\n\nfunction once(action: Disposer): Disposer {\n  let done = false;\n  return () => {\n    if (done) return;\n    done = true;\n    action();\n  };\n}\n\nfunction sameRelation(\n  left: RuntimeInspection[\"relations\"][number],\n  right: RuntimeInspection[\"relations\"][number],\n): boolean {\n  return (\n    left.consumer === right.consumer &&\n    left.service === right.service &&\n    left.provider === right.provider\n  );\n}\n",
        "excerpt": "  async mount(plugin: Plugin): Promise<Disposer> {\n    if (this.#mounts.has(plugin.name)) throw new Error(`Plugin already mounted: ${plugin.name}`);\n    if (this.#activeMount) throw new Error(\"Nested plugin mounting is not supported during setup.\");\n    const record: MountRecord = { name: plugin.name, effects: [], state: \"installing\" };\n    this.#mounts.set(plugin.name, record);\n    this.#activeMount = record;\n    try {\n      await plugin.setup(this);\n      record.state = \"mounted\";\n    } catch (error) {\n      this.#rollback(record);\n      this.#mounts.delete(plugin.name);\n      throw error;\n    } finally {\n      this.#activeMount = null;\n    }\n\n    this.emit(\"runtime/plugin-mounted\", { plugin: plugin.name });\n    return once(() => {\n      if (record.state === \"unmounted\") return;\n      this.emit(\"runtime/plugin-unmounting\", { plugin: plugin.name });\n      this.#rollback(record);\n      this.#mounts.delete(plugin.name);\n      this.#relations.splice(\n        0,\n        this.#relations.length,\n        ...this.#relations.filter(\n          (relation) => relation.consumer !== plugin.name && relation.provider !== plugin.name,\n        ),\n      );\n      this.emit(\"runtime/plugin-unmounted\", { plugin: plugin.name });\n    });\n  }\n\n  effect(cleanup: Disposer): Disposer {\n    const mount = this.#requireMount();\n    const dispose = once(cleanup);\n    mount.effects.push(dispose);\n    return dispose;\n  }\n\n  provide<T>(token: ServiceToken<T>, value: T): Disposer {\n    const owner = this.#requireMount().name;\n    if (this.#services.has(token.key)) throw new Error(`Service already provided: ${token.name}`);\n    this.#services.set(token.key, { owner, value, token: token as ServiceToken<unknown> });\n    return this.effect(() => {\n      const service = this.#services.get(token.key);\n      if (service?.owner === owner) this.#services.delete(token.key);\n    });\n  }\n\n  use<T>(token: ServiceToken<T>): T {\n    const service = this.#services.get(token.key);\n    if (!service) throw new Error(`Missing service: ${token.name}`);\n    const consumer = this.#activeMount?.name;\n    if (consumer) {\n      const relation = { consumer, service: token.name, provider: service.owner };\n      if (!this.#relations.some((item) => sameRelation(item, relation))) this.#relations.push(relation);\n    }\n    return service.value as T;\n  }\n\n  registerTool(tool: ToolDefinition): Disposer {\n    const owner = this.#requireMount().name;\n    if (this.#tools.has(tool.name)) throw new Error(`Tool already registered: ${tool.name}`);\n    this.#tools.set(tool.name, { owner, value: tool });\n    return this.effect(() => {\n      if (this.#tools.get(tool.name)?.owner === owner) this.#tools.delete(tool.name);\n    });\n  }\n\n  contributePrompt(id: string, text: string): Disposer {\n    const owner = this.#requireMount().name;\n    if (this.#prompts.has(id)) throw new Error(`Prompt contribution already registered: ${id}`);\n    this.#prompts.set(id, { owner, value: text });\n    return this.effect(() => {\n      if (this.#prompts.get(id)?.owner === owner) this.#prompts.delete(id);\n    });\n  }",
        "startLine": 45,
        "endLine": 123
      },
      "extraFiles": [
        {
          "path": "src/agent.ts",
          "content": "import { Ajv, type ValidateFunction } from \"ajv\";\nimport {\n  DEFAULT_PROJECTION,\n  projectMessages,\n  toolSchemas,\n  type ProjectionSettings,\n} from \"./context.js\";\nimport type {\n  AssistantMessage,\n  JsonValue,\n  Llm,\n  ModelMessage,\n  ToolDefinition,\n  UnifiedRequest,\n} from \"./protocol.js\";\nimport type { Context } from \"./runtime.js\";\n\nexport interface AgentTraceEntry {\n  kind: \"turn\" | \"step\" | \"request\" | \"assistant\" | \"tool\";\n  label: string;\n  detail?: string;\n}\n\nexport interface RunTurnResult {\n  finalMessage: AssistantMessage;\n  steps: number;\n  trace: AgentTraceEntry[];\n  requests: UnifiedRequest[];\n}\n\nexport interface AgentOptions {\n  llm: Llm;\n  context: Context;\n  systemPrompt: string;\n  dynamicContext?: (step: number) => string;\n  projection?: ProjectionSettings;\n  maxSteps?: number;\n}\n\nexport class Agent {\n  readonly #llm: Llm;\n  readonly #context: Context;\n  readonly #validators = new Map<string, { schema: string; validate: ValidateFunction }>();\n  readonly #ajv = new Ajv({ allErrors: true, strict: false });\n  readonly #systemPrompt: string;\n  readonly #dynamicContext: (step: number) => string;\n  readonly #maxSteps: number;\n  readonly #projection: ProjectionSettings;\n  readonly #messages: ModelMessage[] = [];\n\n  constructor(options: AgentOptions) {\n    this.#llm = options.llm;\n    this.#context = options.context;\n    this.#systemPrompt = options.systemPrompt;\n    this.#dynamicContext = options.dynamicContext ?? ((step) => `Current step: ${step}`);\n    this.#maxSteps = options.maxSteps ?? 8;\n    this.#projection = options.projection ?? DEFAULT_PROJECTION;\n  }\n\n  async runTurn(userInput: string): Promise<RunTurnResult> {\n    this.#messages.push({ role: \"user\", content: userInput });\n    const trace: AgentTraceEntry[] = [{ kind: \"turn\", label: \"turn/start\", detail: userInput }];\n    const requests: UnifiedRequest[] = [];\n\n    for (let step = 1; step <= this.#maxSteps; step += 1) {\n      trace.push({ kind: \"step\", label: `step/${step}/start` });\n      const request: UnifiedRequest = {\n        system: this.#context.compilePrompt(this.#systemPrompt),\n        tools: toolSchemas(this.#context.listTools()),\n        messages: projectMessages(this.#messages, this.#projection),\n        dynamicContext: this.#dynamicContext(step),\n      };\n      requests.push(structuredClone(request));\n      trace.push({ kind: \"request\", label: \"llm/request\", detail: `${request.tools.length} tools` });\n\n      const response = await this.#llm.complete(request);\n      const assistant = response.message;\n      this.#messages.push(assistant);\n      trace.push({ kind: \"assistant\", label: \"assistant/message\", detail: assistant.content });\n\n      if (assistant.toolCalls.length === 0) {\n        trace.push({ kind: \"turn\", label: \"turn/end\", detail: \"completed\" });\n        return { finalMessage: assistant, steps: step, trace, requests };\n      }\n\n      for (const call of assistant.toolCalls) {\n        const result = await this.#executeTool(call.name, call.arguments);\n        this.#messages.push({\n          role: \"tool\",\n          toolCallId: call.id,\n          name: call.name,\n          content: JSON.stringify(result),\n        });\n        trace.push({ kind: \"tool\", label: call.name, detail: JSON.stringify(result) });\n      }\n    }\n\n    throw new Error(`Agent exceeded maxSteps (${this.#maxSteps}) before finishing the turn.`);\n  }\n\n  async #executeTool(name: string, input: JsonValue): Promise<JsonValue> {\n    const tool = this.#context.getTool(name);\n    if (!tool) return { ok: false, error: `Unknown tool: ${name}` };\n    const schema = JSON.stringify(tool.inputSchema);\n    const cached = this.#validators.get(name);\n    const validate = cached?.schema === schema ? cached.validate : this.#ajv.compile(tool.inputSchema);\n    this.#validators.set(name, { schema, validate });\n    if (!validate?.(input)) {\n      return {\n        ok: false,\n        error: \"Invalid tool arguments\",\n        issues: (validate?.errors ?? []).map((error) => `${error.instancePath || \"/\"} ${error.message}`),\n      };\n    }\n    try {\n      const result = await tool.execute(input);\n      this.#context.emit(\"tool/executed\", { name, input, result });\n      return result;\n    } catch (error) {\n      return { ok: false, error: error instanceof Error ? error.message : String(error) };\n    }\n  }\n}\n"
        },
        {
          "path": "src/context.ts",
          "content": "import type { ModelMessage, ToolMessage, ToolSchema, UnifiedRequest } from \"./protocol.js\";\n\nexport interface ProjectionSettings {\n  maxToolResultChars: number;\n  toolResultHeadChars: number;\n  toolResultTailChars: number;\n}\n\nexport const DEFAULT_PROJECTION: ProjectionSettings = {\n  maxToolResultChars: 720,\n  toolResultHeadChars: 430,\n  toolResultTailChars: 170,\n};\n\nexport interface RequestPart {\n  id: string;\n  kind: \"system\" | \"tools\" | \"message\" | \"dynamic\";\n  stability: \"stable\" | \"append-only\" | \"step-variable\";\n  label: string;\n  value: unknown;\n  approximateTokens: number;\n}\n\nexport interface PrefixComparison {\n  sharedParts: number;\n  sharedApproximateTokens: number;\n  previousParts: number;\n  currentParts: number;\n  firstInvalidation: string | null;\n  note: string;\n}\n\nexport interface ClippedToolResult {\n  content: string;\n  omittedCharacters: number;\n}\n\nexport function projectMessages(\n  messages: ModelMessage[],\n  settings: ProjectionSettings = DEFAULT_PROJECTION,\n): ModelMessage[] {\n  return messages.map((message) => {\n    if (message.role !== \"tool\") return structuredClone(message);\n    const clipped = clipToolResult(message.content, settings);\n    return { ...message, content: clipped.content } satisfies ToolMessage;\n  });\n}\n\nexport function clipToolResult(\n  content: string,\n  settings: ProjectionSettings = DEFAULT_PROJECTION,\n): ClippedToolResult {\n  if (content.length <= settings.maxToolResultChars) {\n    return { content, omittedCharacters: 0 };\n  }\n  const omittedCharacters = Math.max(\n    0,\n    content.length - settings.toolResultHeadChars - settings.toolResultTailChars,\n  );\n  const marker = `\\n[… ${omittedCharacters} characters omitted from the model projection …]\\n`;\n  return {\n    content:\n      content.slice(0, settings.toolResultHeadChars) +\n      marker +\n      content.slice(content.length - settings.toolResultTailChars),\n    omittedCharacters,\n  };\n}\n\nexport function describeRequest(request: UnifiedRequest): RequestPart[] {\n  const parts: RequestPart[] = [\n    part(\"system\", \"system\", \"stable\", \"System prompt\", request.system),\n    part(\"tools\", \"tools\", \"stable\", \"Tool schemas\", request.tools),\n  ];\n  for (const [index, message] of request.messages.entries()) {\n    const suffix = message.role === \"tool\" ? ` · ${message.name}` : \"\";\n    parts.push(\n      part(\n        `message-${index}`,\n        \"message\",\n        \"append-only\",\n        `${message.role}${suffix}`,\n        message,\n      ),\n    );\n  }\n  if (request.dynamicContext) {\n    parts.push(\n      part(\n        \"dynamic\",\n        \"dynamic\",\n        \"step-variable\",\n        \"Step context\",\n        request.dynamicContext,\n      ),\n    );\n  }\n  return parts;\n}\n\nexport function compareRequestPrefix(\n  previous: UnifiedRequest | undefined,\n  current: UnifiedRequest,\n): PrefixComparison {\n  const currentParts = describeRequest(current);\n  if (!previous) {\n    return {\n      sharedParts: 0,\n      sharedApproximateTokens: 0,\n      previousParts: 0,\n      currentParts: currentParts.length,\n      firstInvalidation: \"First request has no predecessor.\",\n      note: \"Teaching estimate only; no provider cache was queried.\",\n    };\n  }\n  const previousParts = describeRequest(previous);\n  let sharedParts = 0;\n  while (\n    sharedParts < previousParts.length &&\n    sharedParts < currentParts.length &&\n    canonical(previousParts[sharedParts]?.value) === canonical(currentParts[sharedParts]?.value)\n  ) {\n    sharedParts += 1;\n  }\n  return {\n    sharedParts,\n    sharedApproximateTokens: currentParts\n      .slice(0, sharedParts)\n      .reduce((total, item) => total + item.approximateTokens, 0),\n    previousParts: previousParts.length,\n    currentParts: currentParts.length,\n    firstInvalidation:\n      sharedParts === Math.min(previousParts.length, currentParts.length)\n        ? null\n        : currentParts[sharedParts]?.label ?? \"Request end\",\n    note: \"Longest identical canonical prefix; approximate tokens, not provider cache usage.\",\n  };\n}\n\nexport function approximateTokens(value: unknown): number {\n  const text = typeof value === \"string\" ? value : canonical(value);\n  return Math.max(1, Math.ceil([...text].length / 4));\n}\n\nexport function toolSchemas(tools: Array<{ name: string; description: string; inputSchema: Record<string, unknown> }>): ToolSchema[] {\n  return tools.map(({ name, description, inputSchema }) => ({ name, description, inputSchema }));\n}\n\nfunction part(\n  id: string,\n  kind: RequestPart[\"kind\"],\n  stability: RequestPart[\"stability\"],\n  label: string,\n  value: unknown,\n): RequestPart {\n  return { id, kind, stability, label, value, approximateTokens: approximateTokens(value) };\n}\n\nfunction canonical(value: unknown): string {\n  return JSON.stringify(sortObject(value));\n}\n\nfunction sortObject(value: unknown): unknown {\n  if (Array.isArray(value)) return value.map(sortObject);\n  if (value !== null && typeof value === \"object\") {\n    return Object.fromEntries(\n      Object.entries(value as Record<string, unknown>)\n        .sort(([left], [right]) => left.localeCompare(right))\n        .map(([key, item]) => [key, sortObject(item)]),\n    );\n  }\n  return value;\n}\n"
        }
      ],
      "diff": "diff --git a/src/runtime.ts b/src/runtime.ts\nnew file mode 100644\nindex 0000000..f0871eb\n--- /dev/null\n+++ b/src/runtime.ts\n@@ -0,0 +1,214 @@\n+import type { ToolDefinition } from \"./protocol.js\";\n+\n+export class ServiceToken<T> {\n+  readonly key = Symbol();\n+  constructor(readonly name: string) {}\n+}\n+\n+export interface Plugin {\n+  name: string;\n+  setup(context: Context): void | Promise<void>;\n+}\n+\n+export interface RuntimeInspection {\n+  plugins: string[];\n+  services: Array<{ name: string; provider: string }>;\n+  tools: Array<{ name: string; plugin: string }>;\n+  prompts: Array<{ id: string; plugin: string; text: string }>;\n+  listeners: Array<{ event: string; plugin: string }>;\n+  relations: Array<{ consumer: string; service: string; provider: string }>;\n+}\n+\n+type Disposer = () => void;\n+type Listener = (payload: unknown) => void;\n+\n+interface MountRecord {\n+  name: string;\n+  effects: Disposer[];\n+  state: \"installing\" | \"mounted\" | \"unmounted\";\n+}\n+\n+interface Owned<T> {\n+  owner: string;\n+  value: T;\n+}\n+\n+export class Context {\n+  readonly #mounts = new Map<string, MountRecord>();\n+  readonly #services = new Map<symbol, Owned<unknown> & { token: ServiceToken<unknown> }>();\n+  readonly #tools = new Map<string, Owned<ToolDefinition>>();\n+  readonly #prompts = new Map<string, Owned<string>>();\n+  readonly #listeners = new Map<string, Set<Owned<Listener>>>();\n+  readonly #relations: RuntimeInspection[\"relations\"] = [];\n+  #activeMount: MountRecord | null = null;\n+\n+  async mount(plugin: Plugin): Promise<Disposer> {\n+    if (this.#mounts.has(plugin.name)) throw new Error(`Plugin already mounted: ${plugin.name}`);\n+    if (this.#activeMount) throw new Error(\"Nested plugin mounting is not supported during setup.\");\n+    const record: MountRecord = { name: plugin.name, effects: [], state: \"installing\" };\n+    this.#mounts.set(plugin.name, record);\n+    this.#activeMount = record;\n+    try {\n+      await plugin.setup(this);\n+      record.state = \"mounted\";\n+    } catch (error) {\n+      this.#rollback(record);\n+      this.#mounts.delete(plugin.name);\n+      throw error;\n+    } finally {\n+      this.#activeMount = null;\n+    }\n+\n+    this.emit(\"runtime/plugin-mounted\", { plugin: plugin.name });\n+    return once(() => {\n+      if (record.state === \"unmounted\") return;\n+      this.emit(\"runtime/plugin-unmounting\", { plugin: plugin.name });\n+      this.#rollback(record);\n+      this.#mounts.delete(plugin.name);\n+      this.#relations.splice(\n+        0,\n+        this.#relations.length,\n+        ...this.#relations.filter(\n+          (relation) => relation.consumer !== plugin.name && relation.provider !== plugin.name,\n+        ),\n+      );\n+      this.emit(\"runtime/plugin-unmounted\", { plugin: plugin.name });\n+    });\n+  }\n+\n+  effect(cleanup: Disposer): Disposer {\n+    const mount = this.#requireMount();\n+    const dispose = once(cleanup);\n+    mount.effects.push(dispose);\n+    return dispose;\n+  }\n+\n+  provide<T>(token: ServiceToken<T>, value: T): Disposer {\n+    const owner = this.#requireMount().name;\n+    if (this.#services.has(token.key)) throw new Error(`Service already provided: ${token.name}`);\n+    this.#services.set(token.key, { owner, value, token: token as ServiceToken<unknown> });\n+    return this.effect(() => {\n+      const service = this.#services.get(token.key);\n+      if (service?.owner === owner) this.#services.delete(token.key);\n+    });\n+  }\n+\n+  use<T>(token: ServiceToken<T>): T {\n+    const service = this.#services.get(token.key);\n+    if (!service) throw new Error(`Missing service: ${token.name}`);\n+    const consumer = this.#activeMount?.name;\n+    if (consumer) {\n+      const relation = { consumer, service: token.name, provider: service.owner };\n+      if (!this.#relations.some((item) => sameRelation(item, relation))) this.#relations.push(relation);\n+    }\n+    return service.value as T;\n+  }\n+\n+  registerTool(tool: ToolDefinition): Disposer {\n+    const owner = this.#requireMount().name;\n+    if (this.#tools.has(tool.name)) throw new Error(`Tool already registered: ${tool.name}`);\n+    this.#tools.set(tool.name, { owner, value: tool });\n+    return this.effect(() => {\n+      if (this.#tools.get(tool.name)?.owner === owner) this.#tools.delete(tool.name);\n+    });\n+  }\n+\n+  contributePrompt(id: string, text: string): Disposer {\n+    const owner = this.#requireMount().name;\n+    if (this.#prompts.has(id)) throw new Error(`Prompt contribution already registered: ${id}`);\n+    this.#prompts.set(id, { owner, value: text });\n+    return this.effect(() => {\n+      if (this.#prompts.get(id)?.owner === owner) this.#prompts.delete(id);\n+    });\n+  }\n+\n+  on(event: string, listener: Listener): Disposer {\n+    const owner = this.#requireMount().name;\n+    const entry = { owner, value: listener };\n+    const listeners = this.#listeners.get(event) ?? new Set<Owned<Listener>>();\n+    listeners.add(entry);\n+    this.#listeners.set(event, listeners);\n+    return this.effect(() => {\n+      listeners.delete(entry);\n+      if (listeners.size === 0) this.#listeners.delete(event);\n+    });\n+  }\n+\n+  emit(event: string, payload: unknown): void {\n+    for (const listener of [...(this.#listeners.get(event) ?? [])]) listener.value(payload);\n+  }\n+\n+  listTools(): ToolDefinition[] {\n+    return [...this.#tools.values()].map((entry) => entry.value);\n+  }\n+\n+  getTool(name: string): ToolDefinition | undefined {\n+    return this.#tools.get(name)?.value;\n+  }\n+\n+  compilePrompt(base = \"\"): string {\n+    return [base, ...[...this.#prompts.values()].map((entry) => entry.value)]\n+      .filter(Boolean)\n+      .join(\"\\n\\n\");\n+  }\n+\n+  inspect(): RuntimeInspection {\n+    return {\n+      plugins: [...this.#mounts.values()]\n+        .filter((mount) => mount.state === \"mounted\")\n+        .map((mount) => mount.name),\n+      services: [...this.#services.values()].map((entry) => ({\n+        name: entry.token.name,\n+        provider: entry.owner,\n+      })),\n+      tools: [...this.#tools.entries()].map(([name, entry]) => ({ name, plugin: entry.owner })),\n+      prompts: [...this.#prompts.entries()].map(([id, entry]) => ({\n+        id,\n+        plugin: entry.owner,\n+        text: entry.value,\n+      })),\n+      listeners: [...this.#listeners.entries()].flatMap(([event, entries]) =>\n+        [...entries].map((entry) => ({ event, plugin: entry.owner })),\n+      ),\n+      relations: structuredClone(this.#relations),\n+    };\n+  }\n+\n+  #requireMount(): MountRecord {\n+    if (!this.#activeMount) throw new Error(\"Runtime contributions must be registered during plugin setup.\");\n+    return this.#activeMount;\n+  }\n+\n+  #rollback(record: MountRecord): void {\n+    const failures: unknown[] = [];\n+    for (const dispose of [...record.effects].reverse()) {\n+      try {\n+        dispose();\n+      } catch (error) {\n+        failures.push(error);\n+      }\n+    }\n+    record.state = \"unmounted\";\n+    if (failures.length > 0) throw new AggregateError(failures, `Failed to clean up ${record.name}`);\n+  }\n+}\n+\n+function once(action: Disposer): Disposer {\n+  let done = false;\n+  return () => {\n+    if (done) return;\n+    done = true;\n+    action();\n+  };\n+}\n+\n+function sameRelation(\n+  left: RuntimeInspection[\"relations\"][number],\n+  right: RuntimeInspection[\"relations\"][number],\n+): boolean {\n+  return (\n+    left.consumer === right.consumer &&\n+    left.service === right.service &&\n+    left.provider === right.provider\n+  );\n+}\n",
      "diffStats": {
        "filesChanged": 1,
        "additions": 214,
        "deletions": 0,
        "files": [
          {
            "path": "src/runtime.ts",
            "additions": 214,
            "deletions": 0
          }
        ]
      },
      "requests": [
        {
          "step": 1,
          "request": {
            "system": "Use a tool when the answer requires external facts. Answer directly after receiving the result.",
            "tools": [
              {
                "name": "get_time",
                "description": "Get the current time in a city.",
                "inputSchema": {
                  "type": "object",
                  "properties": {
                    "city": {
                      "type": "string"
                    }
                  },
                  "required": [
                    "city"
                  ],
                  "additionalProperties": false
                }
              }
            ],
            "messages": [
              {
                "role": "user",
                "content": "What time is it in Beijing?"
              }
            ],
            "dynamicContext": "Clock plugin ready."
          },
          "parts": [
            {
              "id": "system",
              "kind": "system",
              "stability": "stable",
              "label": "System prompt",
              "value": "Use a tool when the answer requires external facts. Answer directly after receiving the result.",
              "approximateTokens": 5
            },
            {
              "id": "tools",
              "kind": "tools",
              "stability": "stable",
              "label": "Tool schemas",
              "value": [
                {
                  "name": "get_time",
                  "description": "Get the current time in a city.",
                  "inputSchema": {
                    "type": "object",
                    "properties": {
                      "city": {
                        "type": "string"
                      }
                    },
                    "required": [
                      "city"
                    ],
                    "additionalProperties": false
                  }
                }
              ],
              "approximateTokens": 43
            },
            {
              "id": "message-0",
              "kind": "message",
              "stability": "append-only",
              "label": "user",
              "value": {
                "role": "user",
                "content": "What time is it in Beijing?"
              },
              "approximateTokens": 9
            },
            {
              "id": "dynamic",
              "kind": "dynamic",
              "stability": "step-variable",
              "label": "Step context",
              "value": "Clock plugin ready.",
              "approximateTokens": 5
            }
          ],
          "totalApproximateTokens": 62,
          "prefix": {
            "sharedParts": 0,
            "sharedApproximateTokens": 0,
            "previousParts": 0,
            "currentParts": 4,
            "firstInvalidation": "First request has no predecessor.",
            "note": "Teaching estimate only; no provider cache was queried."
          }
        }
      ],
      "events": [],
      "trace": [],
      "graphs": [
        {
          "stepId": "plugin-ready",
          "eventId": 1,
          "plugins": [
            "clock-core",
            "clock-tools"
          ],
          "tools": [
            {
              "name": "get_time",
              "owner": "clock-tools"
            }
          ],
          "prompts": [
            {
              "id": "clock-rule",
              "text": "Call get_time before answering a question about the current time.",
              "owner": "clock-tools"
            }
          ],
          "services": [
            {
              "name": "clock",
              "provider": "clock-core"
            }
          ],
          "relations": [
            {
              "consumer": "clock-tools",
              "service": "clock",
              "provider": "clock-core"
            }
          ],
          "nodes": [
            {
              "id": "clock-core",
              "label": "clock-core",
              "kind": "plugin"
            },
            {
              "id": "clock-tools",
              "label": "clock-tools",
              "kind": "plugin"
            },
            {
              "id": "tool:get_time",
              "label": "get_time",
              "kind": "tool"
            },
            {
              "id": "clock-rule",
              "label": "Prompt",
              "kind": "prompt"
            }
          ],
          "edges": [
            {
              "from": "clock-tools",
              "to": "tool:get_time",
              "label": "contributes"
            },
            {
              "from": "clock-tools",
              "to": "clock-rule",
              "label": "contributes"
            },
            {
              "from": "clock-core",
              "to": "clock-tools",
              "label": "clock"
            }
          ]
        }
      ]
    },
    {
      "id": "chapter-4",
      "number": "04",
      "shortTitle": "Making Every Run Traceable",
      "title": "Reconstruct execution from the log",
      "question": "How does DSH record and preserve an Agent run?",
      "codeGuide": {
        "title": "Use one event log to support requests and replay",
        "description": "Nano DSH appends numbered SessionEvents, then uses the target Step's request header, context checkpoints, and prior events to rebuild model input and the execution trace.",
        "observations": [
          {
            "title": "Events are appended and never rewritten in place",
            "text": "append copies an event, assigns an increasing sequence number, and places it at the end. A context checkpoint also enters the log as a new event and references the existing events that it summarizes.",
            "lines": [
              84,
              108
            ]
          },
          {
            "title": "The request header defines the reconstruction range",
            "text": "Nano DSH uses stepId to locate the corresponding complete request/header event. Content that occurred before this event forms the history for the current Step. The latest context checkpoint determines where the model projection resumes.",
            "lines": [
              130,
              153
            ]
          },
          {
            "title": "Events are projected into model messages",
            "text": "Tool calls are first grouped by Step and paired with the assistant message. User messages, model messages, and tool results are restored in their original order, then the projection settings from that point in time are applied.",
            "lines": [
              155,
              194
            ]
          }
        ],
        "fills": [
          {
            "label": "Complete SessionEvent types and the log shape",
            "kind": "skeleton",
            "ranges": [
              [
                1,
                92
              ]
            ]
          },
          {
            "label": "Append facts, assign identifiers, and add context checkpoints",
            "kind": "body",
            "ranges": [
              [
                93,
                109
              ]
            ]
          },
          {
            "label": "Connect the Session Log to the runtime plugin",
            "kind": "body",
            "ranges": [
              [
                111,
                129
              ]
            ]
          },
          {
            "label": "Use events to define a request boundary and rebuild messages",
            "kind": "body",
            "ranges": [
              [
                130,
                194
              ]
            ]
          },
          {
            "label": "Derive a Trace from the same event stream",
            "kind": "body",
            "ranges": [
              [
                196,
                267
              ]
            ]
          }
        ]
      },
      "changeStory": {
        "title": "Model input, replay, and display share one source of facts",
        "summary": "Full DSH appends interaction facts as SessionEvents and persists them through SessionPersistence to JSON Lines (JSONL) or SQLite. Nano DSH uses one Session Log for messages, tool round-trips, request headers, and runtime changes; buildRequest() and replayTrace() derive model input and execution history from those events.",
        "harnessRole": "Record layer: provide a shared source for request reconstruction, execution replay, and session recovery",
        "connection": "Chapter 1 produces tool exchanges, Chapter 2 defines message projection, and Chapter 3 produces runtime changes. This chapter writes all of them to one Session Log. The plugin experiments in Chapter 5 and Goal Rounds in Chapter 6 continue to append SessionEvents.",
        "outcomes": [
          "Explain why Nano DSH uses a complete request/header event to reconstruct a particular Step",
          "Explain how a context/checkpoint changes only the model projection while the original SessionEvent records remain in the log",
          "Distinguish among the Session Log, model requests, and Traces"
        ]
      },
      "lesson": "# What the Model Sees Is What Gets Logged\n\nThe first three chapters produce several kinds of state: user messages and Tool Results make up model history, plugins change the available tools and system rules, and the page needs to show the execution. If these details live separately in message arrays, debug output, and UI state, one run can quickly acquire conflicting versions of its own history.\n\n**DSH writes runtime facts to one append-only Session Log first, then derives model history, replay, UI, and persisted data from that log.** This order establishes a checkable constraint: **anything visible to the model must be reconstructable from the record.** This chapter joins Chapter 2's model view and Chapter 3's runtime changes to one source of facts.\n\n`session.ts` begins by listing the `SessionEvent` values a run can produce and defining `SessionLog` state. The event types are more precise than a generic log table: request headers, Tool Calls, Tool Results, plugin changes, and Goal status each carry the data reconstruction needs.\n\n<!-- fill {\"index\":0} -->\n\n## 4.1 Save Facts in the Order They Occur\n\nOne tool round-trip contains at least a user message, a request header, a model reply, a Tool Call, and a Tool Result. `SessionLog.append()` copies the incoming event, assigns a growing identifier, and appends it at the end. The identifier lets the page point at a definite event. **Append order preserves causality: a Tool Result follows its Tool Call and precedes the next request.**\n\nA context checkpoint is also an event. It records a summary and the last event identifier the summary covers, while original events remain in the log. Later requests can build a shorter view from the summary and newer events; review and replay still return to the complete facts. Keeping identifiers, appending, and checkpoints in one small class prepares dependable input for the reconstruction functions that follow.\n\n<!-- fill {\"index\":1} -->\n\n<!-- evidence {\"id\":\"m04-events\",\"tab\":\"events\",\"eventType\":\"tool/result\",\"eventOccurrence\":1,\"label\":\"One shared record for a tool round-trip\",\"description\":\"Read user input, request header, Tool Call, Tool Result, and the following request by event identifier to confirm their order.\"} -->\n\n## 4.2 The Log Must Follow Runtime Capability Changes\n\nRecording messages alone is insufficient. Plugin mounting and unloading from Chapter 3 change the tool list and prompts. Without those changes in the same record, a later reader cannot explain why a particular Step saw a particular tool. `sessionPlugin()` provides the Session Log through `Context`, listens to plugin mount and unload events, and appends them as session facts.\n\nThis layer connects the plugin runtime and the session record. It does not build model requests or execute tools. Its job is to keep capability changes on the same timeline as messages and Tool Results.\n\n<!-- fill {\"index\":2} -->\n\n## 4.3 A Request Header Marks the Boundary at That Time\n\nMessage history alone cannot describe a complete request. System rules, the Tool Schema, model configuration, Step instructions, and projection settings can change over time. **A `request/header` stores the full input boundary for one Step at the time it ran.** Nano DSH records one for each Step. Reconstruction first locates that header by `stepId` and then reads only the events before it.\n\n`buildRequest()` then finds the newest context checkpoint to determine which older events its summary already covers. It projects remaining user messages, model replies, and Tool Results into model messages. Tool Calls are recorded separately and later attached to the reply that issued them. Finally, it calls Chapter 2's `projectMessages()` to create the current model view. This array work remains dependable because the first two sections recorded facts at a sufficiently fine granularity.\n\n<!-- fill {\"index\":3} -->\n\n## 4.4 Replay and the UI Reuse the Same Facts\n\n`replayTrace()` reads the same `SessionEvent` values and creates the titles and details shown by the page. **Model requests, replay, and the UI share one event-based source of facts.** `replayTrace()` and `buildRequest()` serve different purposes without maintaining a second execution history. New display or debugging views can likewise derive from the log without rewriting model input.\n\n<!-- fill {\"index\":4} -->\n\nFull DSH writes SessionEvents to JSON Lines, or JSONL, where each line is one JSON record, or to SQLite. Nano DSH keeps the log in memory and serializes teaching samples for the site. Both retain the important relationship: requests, replay, and display all derive from one event stream.\n",
      "source": {
        "path": "src/session.ts",
        "content": "import { projectMessages, type ProjectionSettings } from \"./context.js\";\nimport type {\n  AssistantMessage,\n  ModelMessage,\n  ToolCall,\n  ToolSchema,\n  UnifiedRequest,\n} from \"./protocol.js\";\nimport { ServiceToken, type Plugin } from \"./runtime.js\";\n\ninterface EventBase {\n  id: number;\n  type: string;\n}\n\nexport type SessionEvent =\n  | (EventBase & { type: \"turn/start\"; turnId: string })\n  | (EventBase & { type: \"user/message\"; turnId: string; content: string })\n  | (EventBase & { type: \"step/start\"; turnId: string; stepId: string; ordinal: number })\n  | (EventBase & {\n      type: \"request/header\";\n      stepId: string;\n      provider: string;\n      model: string;\n      system: string;\n      tools: ToolSchema[];\n      dynamicContext: string;\n      projection: ProjectionSettings;\n    })\n  | (EventBase & { type: \"assistant/message\"; stepId: string; content: string })\n  | (EventBase & { type: \"tool/call\"; stepId: string; call: ToolCall })\n  | (EventBase & {\n      type: \"tool/result\";\n      stepId: string;\n      toolCallId: string;\n      name: string;\n      content: string;\n    })\n  | (EventBase & {\n      type: \"context/checkpoint\";\n      summary: string;\n      coveredThroughEventId: number;\n    })\n  | (EventBase & { type: \"runtime/plugin-mounted\"; plugin: string })\n  | (EventBase & { type: \"runtime/plugin-unmounted\"; plugin: string })\n  | (EventBase & { type: \"step/end\"; stepId: string; outcome: \"tool-calls\" | \"complete\" })\n  | (EventBase & {\n      type: \"turn/end\";\n      turnId: string;\n      outcome: \"completed\" | \"max-steps\" | \"failed\";\n    })\n  | (EventBase & {\n      type: \"goal/created\";\n      goalId: string;\n      objective: string;\n      maxRounds: number;\n    })\n  | (EventBase & {\n      type: \"goal/round-started\";\n      goalId: string;\n      round: number;\n      label: string;\n    })\n  | (EventBase & {\n      type: \"goal/status-changed\";\n      goalId: string;\n      status: \"active\" | \"completed\" | \"blocked\" | \"max-rounds\";\n      reason: string;\n    });\n\nexport type NewSessionEvent = SessionEvent extends infer Event\n  ? Event extends SessionEvent\n    ? Omit<Event, \"id\">\n    : never\n  : never;\n\nexport interface TraceItem {\n  eventId: number;\n  type: SessionEvent[\"type\"];\n  title: string;\n  detail: string;\n}\n\nexport class SessionLog {\n  readonly #events: SessionEvent[] = [];\n  #nextId = 1;\n  #nextTurn = 1;\n\n  get events(): readonly SessionEvent[] {\n    return this.#events;\n  }\n\n  append(event: NewSessionEvent): SessionEvent {\n    const stored = { ...structuredClone(event), id: this.#nextId++ } as SessionEvent;\n    this.#events.push(stored);\n    return stored;\n  }\n\n  nextTurnId(): string {\n    return `turn-${this.#nextTurn++}`;\n  }\n\n  checkpoint(summary: string, coveredThroughEventId: number): SessionEvent {\n    if (coveredThroughEventId >= this.#nextId) {\n      throw new Error(\"A checkpoint may cover only events that already exist.\");\n    }\n    return this.append({ type: \"context/checkpoint\", summary, coveredThroughEventId });\n  }\n}\n\nexport const SESSION_LOG = new ServiceToken<SessionLog>(\"session-log\");\n\nexport function sessionPlugin(log: SessionLog): Plugin {\n  return {\n    name: \"session-log\",\n    setup(context) {\n      context.provide(SESSION_LOG, log);\n      context.on(\"runtime/plugin-mounted\", (payload) => {\n        const plugin = pluginName(payload);\n        log.append({ type: \"runtime/plugin-mounted\", plugin });\n      });\n      context.on(\"runtime/plugin-unmounting\", (payload) => {\n        const plugin = pluginName(payload);\n        log.append({ type: \"runtime/plugin-unmounted\", plugin });\n      });\n    },\n  };\n}\n\nexport function buildRequest(events: readonly SessionEvent[], stepId: string): UnifiedRequest {\n  const headerIndex = events.findIndex(\n    (event) => event.type === \"request/header\" && event.stepId === stepId,\n  );\n  if (headerIndex === -1) throw new Error(`Missing request/header for ${stepId}.`);\n  const header = events[headerIndex];\n  if (header?.type !== \"request/header\") throw new Error(`Invalid request header for ${stepId}.`);\n\n  const prefix = events.slice(0, headerIndex);\n  const checkpoint = [...prefix]\n    .reverse()\n    .find((event): event is Extract<SessionEvent, { type: \"context/checkpoint\" }> =>\n      event.type === \"context/checkpoint\",\n    );\n  const projectedEvents = checkpoint\n    ? prefix.filter((event) => event.id > checkpoint.coveredThroughEventId)\n    : prefix;\n  const messages: ModelMessage[] = [];\n  if (checkpoint) {\n    messages.push({\n      role: \"system\",\n      content: `[Context checkpoint through event ${checkpoint.coveredThroughEventId}]\\n${checkpoint.summary}`,\n    });\n  }\n\n  const callsByStep = new Map<string, ToolCall[]>();\n  for (const event of projectedEvents) {\n    if (event.type === \"tool/call\") {\n      const calls = callsByStep.get(event.stepId) ?? [];\n      calls.push(structuredClone(event.call));\n      callsByStep.set(event.stepId, calls);\n    }\n  }\n  for (const event of projectedEvents) {\n    switch (event.type) {\n      case \"user/message\":\n        messages.push({ role: \"user\", content: event.content });\n        break;\n      case \"assistant/message\":\n        messages.push({\n          role: \"assistant\",\n          content: event.content,\n          toolCalls: callsByStep.get(event.stepId) ?? [],\n        });\n        break;\n      case \"tool/result\":\n        messages.push({\n          role: \"tool\",\n          toolCallId: event.toolCallId,\n          name: event.name,\n          content: event.content,\n        });\n        break;\n      default:\n        break;\n    }\n  }\n\n  return {\n    system: header.system,\n    tools: structuredClone(header.tools),\n    messages: projectMessages(messages, header.projection),\n    dynamicContext: header.dynamicContext,\n  };\n}\n\nexport function requestStepIds(events: readonly SessionEvent[]): string[] {\n  return events\n    .filter((event): event is Extract<SessionEvent, { type: \"request/header\" }> =>\n      event.type === \"request/header\",\n    )\n    .map((event) => event.stepId);\n}\n\nexport function replayTrace(events: readonly SessionEvent[]): TraceItem[] {\n  return events.map((event) => ({\n    eventId: event.id,\n    type: event.type,\n    title: traceTitle(event),\n    detail: traceDetail(event),\n  }));\n}\n\nfunction traceTitle(event: SessionEvent): string {\n  switch (event.type) {\n    case \"request/header\":\n      return `${event.stepId} → ${event.provider}/${event.model}`;\n    case \"tool/call\":\n      return `call ${event.call.name}`;\n    case \"tool/result\":\n      return `result ${event.name}`;\n    case \"runtime/plugin-mounted\":\n    case \"runtime/plugin-unmounted\":\n      return event.plugin;\n    case \"goal/round-started\":\n      return `round ${event.round} · ${event.label}`;\n    default:\n      return event.type;\n  }\n}\n\nfunction traceDetail(event: SessionEvent): string {\n  switch (event.type) {\n    case \"user/message\":\n    case \"assistant/message\":\n      return event.content;\n    case \"tool/call\":\n      return JSON.stringify(event.call.arguments);\n    case \"tool/result\":\n      return event.content;\n    case \"context/checkpoint\":\n      return `${event.summary} (covers through #${event.coveredThroughEventId})`;\n    case \"request/header\":\n      return `${event.tools.length} tools · dynamic=${event.dynamicContext}`;\n    case \"turn/end\":\n    case \"step/end\":\n      return event.outcome;\n    case \"goal/created\":\n      return event.objective;\n    case \"goal/status-changed\":\n      return `${event.status}: ${event.reason}`;\n    default:\n      return \"\";\n  }\n}\n\nfunction pluginName(payload: unknown): string {\n  if (\n    typeof payload === \"object\" &&\n    payload !== null &&\n    \"plugin\" in payload &&\n    typeof payload.plugin === \"string\"\n  ) {\n    return payload.plugin;\n  }\n  throw new Error(\"Runtime plugin event is missing its plugin name.\");\n}\n",
        "excerpt": "export class SessionLog {\n  readonly #events: SessionEvent[] = [];\n  #nextId = 1;\n  #nextTurn = 1;\n\n  get events(): readonly SessionEvent[] {\n    return this.#events;\n  }\n\n  append(event: NewSessionEvent): SessionEvent {\n    const stored = { ...structuredClone(event), id: this.#nextId++ } as SessionEvent;\n    this.#events.push(stored);\n    return stored;\n  }\n\n  nextTurnId(): string {\n    return `turn-${this.#nextTurn++}`;\n  }\n\n  checkpoint(summary: string, coveredThroughEventId: number): SessionEvent {\n    if (coveredThroughEventId >= this.#nextId) {\n      throw new Error(\"A checkpoint may cover only events that already exist.\");\n    }\n    return this.append({ type: \"context/checkpoint\", summary, coveredThroughEventId });\n  }\n}\n\nexport const SESSION_LOG = new ServiceToken<SessionLog>(\"session-log\");\n\nexport function sessionPlugin(log: SessionLog): Plugin {\n  return {\n    name: \"session-log\",\n    setup(context) {\n      context.provide(SESSION_LOG, log);\n      context.on(\"runtime/plugin-mounted\", (payload) => {\n        const plugin = pluginName(payload);\n        log.append({ type: \"runtime/plugin-mounted\", plugin });\n      });\n      context.on(\"runtime/plugin-unmounting\", (payload) => {\n        const plugin = pluginName(payload);\n        log.append({ type: \"runtime/plugin-unmounted\", plugin });\n      });\n    },\n  };\n}\n\nexport function buildRequest(events: readonly SessionEvent[], stepId: string): UnifiedRequest {\n  const headerIndex = events.findIndex(\n    (event) => event.type === \"request/header\" && event.stepId === stepId,\n  );\n  if (headerIndex === -1) throw new Error(`Missing request/header for ${stepId}.`);\n  const header = events[headerIndex];\n  if (header?.type !== \"request/header\") throw new Error(`Invalid request header for ${stepId}.`);\n\n  const prefix = events.slice(0, headerIndex);\n  const checkpoint = [...prefix]\n    .reverse()\n    .find((event): event is Extract<SessionEvent, { type: \"context/checkpoint\" }> =>\n      event.type === \"context/checkpoint\",\n    );\n  const projectedEvents = checkpoint\n    ? prefix.filter((event) => event.id > checkpoint.coveredThroughEventId)\n    : prefix;\n  const messages: ModelMessage[] = [];\n  if (checkpoint) {\n    messages.push({\n      role: \"system\",\n      content: `[Context checkpoint through event ${checkpoint.coveredThroughEventId}]\\n${checkpoint.summary}`,\n    });\n  }\n\n  const callsByStep = new Map<string, ToolCall[]>();\n  for (const event of projectedEvents) {\n    if (event.type === \"tool/call\") {\n      const calls = callsByStep.get(event.stepId) ?? [];\n      calls.push(structuredClone(event.call));\n      callsByStep.set(event.stepId, calls);\n    }\n  }\n  for (const event of projectedEvents) {\n    switch (event.type) {\n      case \"user/message\":\n        messages.push({ role: \"user\", content: event.content });\n        break;\n      case \"assistant/message\":\n        messages.push({\n          role: \"assistant\",\n          content: event.content,\n          toolCalls: callsByStep.get(event.stepId) ?? [],\n        });\n        break;\n      case \"tool/result\":\n        messages.push({\n          role: \"tool\",\n          toolCallId: event.toolCallId,\n          name: event.name,\n          content: event.content,\n        });\n        break;\n      default:\n        break;\n    }\n  }\n\n  return {\n    system: header.system,\n    tools: structuredClone(header.tools),\n    messages: projectMessages(messages, header.projection),\n    dynamicContext: header.dynamicContext,\n  };\n}",
        "startLine": 84,
        "endLine": 194
      },
      "extraFiles": [
        {
          "path": "src/agent.ts",
          "content": "import { Ajv, type ValidateFunction } from \"ajv\";\nimport {\n  DEFAULT_PROJECTION,\n  toolSchemas,\n  type ProjectionSettings,\n} from \"./context.js\";\nimport type {\n  AssistantMessage,\n  JsonValue,\n  Llm,\n  UnifiedRequest,\n} from \"./protocol.js\";\nimport type { Context } from \"./runtime.js\";\nimport {\n  buildRequest,\n  replayTrace,\n  SESSION_LOG,\n  type SessionEvent,\n  type SessionLog,\n  type TraceItem,\n} from \"./session.js\";\n\nexport interface RunTurnResult {\n  finalMessage: AssistantMessage;\n  steps: number;\n  turnId: string;\n  stepIds: string[];\n  trace: TraceItem[];\n  requests: UnifiedRequest[];\n}\n\nexport interface CheckpointProposal {\n  summary: string;\n  coveredThroughEventId: number;\n}\n\nexport interface AgentOptions {\n  llm: Llm;\n  context: Context;\n  systemPrompt: string;\n  dynamicContext?: (step: number) => string;\n  projection?: ProjectionSettings;\n  checkpointBeforeStep?: (input: {\n    step: number;\n    events: readonly SessionEvent[];\n  }) => CheckpointProposal | undefined;\n  maxSteps?: number;\n}\n\nexport class Agent {\n  readonly #llm: Llm;\n  readonly #context: Context;\n  readonly #validators = new Map<string, { schema: string; validate: ValidateFunction }>();\n  readonly #ajv = new Ajv({ allErrors: true, strict: false });\n  readonly #systemPrompt: string;\n  readonly #dynamicContext: (step: number) => string;\n  readonly #maxSteps: number;\n  readonly #projection: ProjectionSettings;\n  readonly #session: SessionLog;\n  readonly #checkpointBeforeStep?: AgentOptions[\"checkpointBeforeStep\"];\n\n  constructor(options: AgentOptions) {\n    this.#llm = options.llm;\n    this.#context = options.context;\n    this.#systemPrompt = options.systemPrompt;\n    this.#dynamicContext = options.dynamicContext ?? ((step) => `Current step: ${step}`);\n    this.#maxSteps = options.maxSteps ?? 8;\n    this.#projection = options.projection ?? DEFAULT_PROJECTION;\n    this.#session = this.#context.use(SESSION_LOG);\n    this.#checkpointBeforeStep = options.checkpointBeforeStep;\n  }\n\n  async runTurn(userInput: string): Promise<RunTurnResult> {\n    const eventStart = this.#session.events.length;\n    const turnId = this.#session.nextTurnId();\n    this.#session.append({ type: \"turn/start\", turnId });\n    this.#session.append({ type: \"user/message\", turnId, content: userInput });\n    const requests: UnifiedRequest[] = [];\n    const stepIds: string[] = [];\n\n    for (let step = 1; step <= this.#maxSteps; step += 1) {\n      const checkpoint = this.#checkpointBeforeStep?.({ step, events: this.#session.events });\n      if (checkpoint) {\n        this.#session.checkpoint(checkpoint.summary, checkpoint.coveredThroughEventId);\n      }\n      const stepId = `${turnId}-step-${step}`;\n      stepIds.push(stepId);\n      this.#session.append({ type: \"step/start\", turnId, stepId, ordinal: step });\n      this.#session.append({\n        type: \"request/header\",\n        stepId,\n        provider: this.#llm.provider,\n        model: this.#llm.model,\n        system: this.#context.compilePrompt(this.#systemPrompt),\n        tools: toolSchemas(this.#context.listTools()),\n        dynamicContext: this.#dynamicContext(step),\n        projection: this.#projection,\n      });\n      const request = buildRequest(this.#session.events, stepId);\n      requests.push(structuredClone(request));\n\n      const response = await this.#llm.complete(request);\n      const assistant = response.message;\n      this.#session.append({ type: \"assistant/message\", stepId, content: assistant.content });\n      for (const call of assistant.toolCalls) {\n        this.#session.append({ type: \"tool/call\", stepId, call });\n      }\n\n      if (assistant.toolCalls.length === 0) {\n        this.#session.append({ type: \"step/end\", stepId, outcome: \"complete\" });\n        this.#session.append({ type: \"turn/end\", turnId, outcome: \"completed\" });\n        return {\n          finalMessage: assistant,\n          steps: step,\n          turnId,\n          stepIds,\n          trace: replayTrace(this.#session.events.slice(eventStart)),\n          requests,\n        };\n      }\n\n      for (const call of assistant.toolCalls) {\n        const result = await this.#executeTool(call.name, call.arguments);\n        this.#session.append({\n          type: \"tool/result\",\n          stepId,\n          toolCallId: call.id,\n          name: call.name,\n          content: JSON.stringify(result),\n        });\n      }\n      this.#session.append({ type: \"step/end\", stepId, outcome: \"tool-calls\" });\n    }\n\n    this.#session.append({ type: \"turn/end\", turnId, outcome: \"max-steps\" });\n    throw new Error(`Agent exceeded maxSteps (${this.#maxSteps}) before finishing the turn.`);\n  }\n\n  async #executeTool(name: string, input: JsonValue): Promise<JsonValue> {\n    const tool = this.#context.getTool(name);\n    if (!tool) return { ok: false, error: `Unknown tool: ${name}` };\n    const schema = JSON.stringify(tool.inputSchema);\n    const cached = this.#validators.get(name);\n    const validate = cached?.schema === schema ? cached.validate : this.#ajv.compile(tool.inputSchema);\n    this.#validators.set(name, { schema, validate });\n    if (!validate?.(input)) {\n      return {\n        ok: false,\n        error: \"Invalid tool arguments\",\n        issues: (validate?.errors ?? []).map((error) => `${error.instancePath || \"/\"} ${error.message}`),\n      };\n    }\n    try {\n      const result = await tool.execute(input);\n      this.#context.emit(\"tool/executed\", { name, input, result });\n      return result;\n    } catch (error) {\n      return { ok: false, error: error instanceof Error ? error.message : String(error) };\n    }\n  }\n}\n"
        },
        {
          "path": "src/context.ts",
          "content": "import type { ModelMessage, ToolMessage, ToolSchema, UnifiedRequest } from \"./protocol.js\";\n\nexport interface ProjectionSettings {\n  maxToolResultChars: number;\n  toolResultHeadChars: number;\n  toolResultTailChars: number;\n}\n\nexport const DEFAULT_PROJECTION: ProjectionSettings = {\n  maxToolResultChars: 720,\n  toolResultHeadChars: 430,\n  toolResultTailChars: 170,\n};\n\nexport interface RequestPart {\n  id: string;\n  kind: \"system\" | \"tools\" | \"message\" | \"dynamic\";\n  stability: \"stable\" | \"append-only\" | \"step-variable\";\n  label: string;\n  value: unknown;\n  approximateTokens: number;\n}\n\nexport interface PrefixComparison {\n  sharedParts: number;\n  sharedApproximateTokens: number;\n  previousParts: number;\n  currentParts: number;\n  firstInvalidation: string | null;\n  note: string;\n}\n\nexport interface ClippedToolResult {\n  content: string;\n  omittedCharacters: number;\n}\n\nexport function projectMessages(\n  messages: ModelMessage[],\n  settings: ProjectionSettings = DEFAULT_PROJECTION,\n): ModelMessage[] {\n  return messages.map((message) => {\n    if (message.role !== \"tool\") return structuredClone(message);\n    const clipped = clipToolResult(message.content, settings);\n    return { ...message, content: clipped.content } satisfies ToolMessage;\n  });\n}\n\nexport function clipToolResult(\n  content: string,\n  settings: ProjectionSettings = DEFAULT_PROJECTION,\n): ClippedToolResult {\n  if (content.length <= settings.maxToolResultChars) {\n    return { content, omittedCharacters: 0 };\n  }\n  const omittedCharacters = Math.max(\n    0,\n    content.length - settings.toolResultHeadChars - settings.toolResultTailChars,\n  );\n  const marker = `\\n[… ${omittedCharacters} characters omitted from the model projection …]\\n`;\n  return {\n    content:\n      content.slice(0, settings.toolResultHeadChars) +\n      marker +\n      content.slice(content.length - settings.toolResultTailChars),\n    omittedCharacters,\n  };\n}\n\nexport function describeRequest(request: UnifiedRequest): RequestPart[] {\n  const parts: RequestPart[] = [\n    part(\"system\", \"system\", \"stable\", \"System prompt\", request.system),\n    part(\"tools\", \"tools\", \"stable\", \"Tool schemas\", request.tools),\n  ];\n  for (const [index, message] of request.messages.entries()) {\n    const suffix = message.role === \"tool\" ? ` · ${message.name}` : \"\";\n    parts.push(\n      part(\n        `message-${index}`,\n        \"message\",\n        \"append-only\",\n        `${message.role}${suffix}`,\n        message,\n      ),\n    );\n  }\n  if (request.dynamicContext) {\n    parts.push(\n      part(\n        \"dynamic\",\n        \"dynamic\",\n        \"step-variable\",\n        \"Step context\",\n        request.dynamicContext,\n      ),\n    );\n  }\n  return parts;\n}\n\nexport function compareRequestPrefix(\n  previous: UnifiedRequest | undefined,\n  current: UnifiedRequest,\n): PrefixComparison {\n  const currentParts = describeRequest(current);\n  if (!previous) {\n    return {\n      sharedParts: 0,\n      sharedApproximateTokens: 0,\n      previousParts: 0,\n      currentParts: currentParts.length,\n      firstInvalidation: \"First request has no predecessor.\",\n      note: \"Teaching estimate only; no provider cache was queried.\",\n    };\n  }\n  const previousParts = describeRequest(previous);\n  let sharedParts = 0;\n  while (\n    sharedParts < previousParts.length &&\n    sharedParts < currentParts.length &&\n    canonical(previousParts[sharedParts]?.value) === canonical(currentParts[sharedParts]?.value)\n  ) {\n    sharedParts += 1;\n  }\n  return {\n    sharedParts,\n    sharedApproximateTokens: currentParts\n      .slice(0, sharedParts)\n      .reduce((total, item) => total + item.approximateTokens, 0),\n    previousParts: previousParts.length,\n    currentParts: currentParts.length,\n    firstInvalidation:\n      sharedParts === Math.min(previousParts.length, currentParts.length)\n        ? null\n        : currentParts[sharedParts]?.label ?? \"Request end\",\n    note: \"Longest identical canonical prefix; approximate tokens, not provider cache usage.\",\n  };\n}\n\nexport function approximateTokens(value: unknown): number {\n  const text = typeof value === \"string\" ? value : canonical(value);\n  return Math.max(1, Math.ceil([...text].length / 4));\n}\n\nexport function toolSchemas(tools: Array<{ name: string; description: string; inputSchema: Record<string, unknown> }>): ToolSchema[] {\n  return tools.map(({ name, description, inputSchema }) => ({ name, description, inputSchema }));\n}\n\nfunction part(\n  id: string,\n  kind: RequestPart[\"kind\"],\n  stability: RequestPart[\"stability\"],\n  label: string,\n  value: unknown,\n): RequestPart {\n  return { id, kind, stability, label, value, approximateTokens: approximateTokens(value) };\n}\n\nfunction canonical(value: unknown): string {\n  return JSON.stringify(sortObject(value));\n}\n\nfunction sortObject(value: unknown): unknown {\n  if (Array.isArray(value)) return value.map(sortObject);\n  if (value !== null && typeof value === \"object\") {\n    return Object.fromEntries(\n      Object.entries(value as Record<string, unknown>)\n        .sort(([left], [right]) => left.localeCompare(right))\n        .map(([key, item]) => [key, sortObject(item)]),\n    );\n  }\n  return value;\n}\n"
        },
        {
          "path": "src/runtime.ts",
          "content": "import type { ToolDefinition } from \"./protocol.js\";\n\nexport class ServiceToken<T> {\n  readonly key = Symbol();\n  constructor(readonly name: string) {}\n}\n\nexport interface Plugin {\n  name: string;\n  setup(context: Context): void | Promise<void>;\n}\n\nexport interface RuntimeInspection {\n  plugins: string[];\n  services: Array<{ name: string; provider: string }>;\n  tools: Array<{ name: string; plugin: string }>;\n  prompts: Array<{ id: string; plugin: string; text: string }>;\n  listeners: Array<{ event: string; plugin: string }>;\n  relations: Array<{ consumer: string; service: string; provider: string }>;\n}\n\ntype Disposer = () => void;\ntype Listener = (payload: unknown) => void;\n\ninterface MountRecord {\n  name: string;\n  effects: Disposer[];\n  state: \"installing\" | \"mounted\" | \"unmounted\";\n}\n\ninterface Owned<T> {\n  owner: string;\n  value: T;\n}\n\nexport class Context {\n  readonly #mounts = new Map<string, MountRecord>();\n  readonly #services = new Map<symbol, Owned<unknown> & { token: ServiceToken<unknown> }>();\n  readonly #tools = new Map<string, Owned<ToolDefinition>>();\n  readonly #prompts = new Map<string, Owned<string>>();\n  readonly #listeners = new Map<string, Set<Owned<Listener>>>();\n  readonly #relations: RuntimeInspection[\"relations\"] = [];\n  #activeMount: MountRecord | null = null;\n\n  async mount(plugin: Plugin): Promise<Disposer> {\n    if (this.#mounts.has(plugin.name)) throw new Error(`Plugin already mounted: ${plugin.name}`);\n    if (this.#activeMount) throw new Error(\"Nested plugin mounting is not supported during setup.\");\n    const record: MountRecord = { name: plugin.name, effects: [], state: \"installing\" };\n    this.#mounts.set(plugin.name, record);\n    this.#activeMount = record;\n    try {\n      await plugin.setup(this);\n      record.state = \"mounted\";\n    } catch (error) {\n      this.#rollback(record);\n      this.#mounts.delete(plugin.name);\n      throw error;\n    } finally {\n      this.#activeMount = null;\n    }\n\n    this.emit(\"runtime/plugin-mounted\", { plugin: plugin.name });\n    return once(() => {\n      if (record.state === \"unmounted\") return;\n      this.emit(\"runtime/plugin-unmounting\", { plugin: plugin.name });\n      this.#rollback(record);\n      this.#mounts.delete(plugin.name);\n      this.#relations.splice(\n        0,\n        this.#relations.length,\n        ...this.#relations.filter(\n          (relation) => relation.consumer !== plugin.name && relation.provider !== plugin.name,\n        ),\n      );\n      this.emit(\"runtime/plugin-unmounted\", { plugin: plugin.name });\n    });\n  }\n\n  effect(cleanup: Disposer): Disposer {\n    const mount = this.#requireMount();\n    const dispose = once(cleanup);\n    mount.effects.push(dispose);\n    return dispose;\n  }\n\n  provide<T>(token: ServiceToken<T>, value: T): Disposer {\n    const owner = this.#requireMount().name;\n    if (this.#services.has(token.key)) throw new Error(`Service already provided: ${token.name}`);\n    this.#services.set(token.key, { owner, value, token: token as ServiceToken<unknown> });\n    return this.effect(() => {\n      const service = this.#services.get(token.key);\n      if (service?.owner === owner) this.#services.delete(token.key);\n    });\n  }\n\n  use<T>(token: ServiceToken<T>): T {\n    const service = this.#services.get(token.key);\n    if (!service) throw new Error(`Missing service: ${token.name}`);\n    const consumer = this.#activeMount?.name;\n    if (consumer) {\n      const relation = { consumer, service: token.name, provider: service.owner };\n      if (!this.#relations.some((item) => sameRelation(item, relation))) this.#relations.push(relation);\n    }\n    return service.value as T;\n  }\n\n  registerTool(tool: ToolDefinition): Disposer {\n    const owner = this.#requireMount().name;\n    if (this.#tools.has(tool.name)) throw new Error(`Tool already registered: ${tool.name}`);\n    this.#tools.set(tool.name, { owner, value: tool });\n    return this.effect(() => {\n      if (this.#tools.get(tool.name)?.owner === owner) this.#tools.delete(tool.name);\n    });\n  }\n\n  contributePrompt(id: string, text: string): Disposer {\n    const owner = this.#requireMount().name;\n    if (this.#prompts.has(id)) throw new Error(`Prompt contribution already registered: ${id}`);\n    this.#prompts.set(id, { owner, value: text });\n    return this.effect(() => {\n      if (this.#prompts.get(id)?.owner === owner) this.#prompts.delete(id);\n    });\n  }\n\n  on(event: string, listener: Listener): Disposer {\n    const owner = this.#requireMount().name;\n    const entry = { owner, value: listener };\n    const listeners = this.#listeners.get(event) ?? new Set<Owned<Listener>>();\n    listeners.add(entry);\n    this.#listeners.set(event, listeners);\n    return this.effect(() => {\n      listeners.delete(entry);\n      if (listeners.size === 0) this.#listeners.delete(event);\n    });\n  }\n\n  emit(event: string, payload: unknown): void {\n    for (const listener of [...(this.#listeners.get(event) ?? [])]) listener.value(payload);\n  }\n\n  listTools(): ToolDefinition[] {\n    return [...this.#tools.values()].map((entry) => entry.value);\n  }\n\n  getTool(name: string): ToolDefinition | undefined {\n    return this.#tools.get(name)?.value;\n  }\n\n  compilePrompt(base = \"\"): string {\n    return [base, ...[...this.#prompts.values()].map((entry) => entry.value)]\n      .filter(Boolean)\n      .join(\"\\n\\n\");\n  }\n\n  inspect(): RuntimeInspection {\n    return {\n      plugins: [...this.#mounts.values()]\n        .filter((mount) => mount.state === \"mounted\")\n        .map((mount) => mount.name),\n      services: [...this.#services.values()].map((entry) => ({\n        name: entry.token.name,\n        provider: entry.owner,\n      })),\n      tools: [...this.#tools.entries()].map(([name, entry]) => ({ name, plugin: entry.owner })),\n      prompts: [...this.#prompts.entries()].map(([id, entry]) => ({\n        id,\n        plugin: entry.owner,\n        text: entry.value,\n      })),\n      listeners: [...this.#listeners.entries()].flatMap(([event, entries]) =>\n        [...entries].map((entry) => ({ event, plugin: entry.owner })),\n      ),\n      relations: structuredClone(this.#relations),\n    };\n  }\n\n  #requireMount(): MountRecord {\n    if (!this.#activeMount) throw new Error(\"Runtime contributions must be registered during plugin setup.\");\n    return this.#activeMount;\n  }\n\n  #rollback(record: MountRecord): void {\n    const failures: unknown[] = [];\n    for (const dispose of [...record.effects].reverse()) {\n      try {\n        dispose();\n      } catch (error) {\n        failures.push(error);\n      }\n    }\n    record.state = \"unmounted\";\n    if (failures.length > 0) throw new AggregateError(failures, `Failed to clean up ${record.name}`);\n  }\n}\n\nfunction once(action: Disposer): Disposer {\n  let done = false;\n  return () => {\n    if (done) return;\n    done = true;\n    action();\n  };\n}\n\nfunction sameRelation(\n  left: RuntimeInspection[\"relations\"][number],\n  right: RuntimeInspection[\"relations\"][number],\n): boolean {\n  return (\n    left.consumer === right.consumer &&\n    left.service === right.service &&\n    left.provider === right.provider\n  );\n}\n"
        }
      ],
      "diff": "diff --git a/src/session.ts b/src/session.ts\nnew file mode 100644\nindex 0000000..03733f4\n--- /dev/null\n+++ b/src/session.ts\n@@ -0,0 +1,266 @@\n+import { projectMessages, type ProjectionSettings } from \"./context.js\";\n+import type {\n+  AssistantMessage,\n+  ModelMessage,\n+  ToolCall,\n+  ToolSchema,\n+  UnifiedRequest,\n+} from \"./protocol.js\";\n+import { ServiceToken, type Plugin } from \"./runtime.js\";\n+\n+interface EventBase {\n+  id: number;\n+  type: string;\n+}\n+\n+export type SessionEvent =\n+  | (EventBase & { type: \"turn/start\"; turnId: string })\n+  | (EventBase & { type: \"user/message\"; turnId: string; content: string })\n+  | (EventBase & { type: \"step/start\"; turnId: string; stepId: string; ordinal: number })\n+  | (EventBase & {\n+      type: \"request/header\";\n+      stepId: string;\n+      provider: string;\n+      model: string;\n+      system: string;\n+      tools: ToolSchema[];\n+      dynamicContext: string;\n+      projection: ProjectionSettings;\n+    })\n+  | (EventBase & { type: \"assistant/message\"; stepId: string; content: string })\n+  | (EventBase & { type: \"tool/call\"; stepId: string; call: ToolCall })\n+  | (EventBase & {\n+      type: \"tool/result\";\n+      stepId: string;\n+      toolCallId: string;\n+      name: string;\n+      content: string;\n+    })\n+  | (EventBase & {\n+      type: \"context/checkpoint\";\n+      summary: string;\n+      coveredThroughEventId: number;\n+    })\n+  | (EventBase & { type: \"runtime/plugin-mounted\"; plugin: string })\n+  | (EventBase & { type: \"runtime/plugin-unmounted\"; plugin: string })\n+  | (EventBase & { type: \"step/end\"; stepId: string; outcome: \"tool-calls\" | \"complete\" })\n+  | (EventBase & {\n+      type: \"turn/end\";\n+      turnId: string;\n+      outcome: \"completed\" | \"max-steps\" | \"failed\";\n+    })\n+  | (EventBase & {\n+      type: \"goal/created\";\n+      goalId: string;\n+      objective: string;\n+      maxRounds: number;\n+    })\n+  | (EventBase & {\n+      type: \"goal/round-started\";\n+      goalId: string;\n+      round: number;\n+      label: string;\n+    })\n+  | (EventBase & {\n+      type: \"goal/status-changed\";\n+      goalId: string;\n+      status: \"active\" | \"completed\" | \"blocked\" | \"max-rounds\";\n+      reason: string;\n+    });\n+\n+export type NewSessionEvent = SessionEvent extends infer Event\n+  ? Event extends SessionEvent\n+    ? Omit<Event, \"id\">\n+    : never\n+  : never;\n+\n+export interface TraceItem {\n+  eventId: number;\n+  type: SessionEvent[\"type\"];\n+  title: string;\n+  detail: string;\n+}\n+\n+export class SessionLog {\n+  readonly #events: SessionEvent[] = [];\n+  #nextId = 1;\n+  #nextTurn = 1;\n+\n+  get events(): readonly SessionEvent[] {\n+    return this.#events;\n+  }\n+\n+  append(event: NewSessionEvent): SessionEvent {\n+    const stored = { ...structuredClone(event), id: this.#nextId++ } as SessionEvent;\n+    this.#events.push(stored);\n+    return stored;\n+  }\n+\n+  nextTurnId(): string {\n+    return `turn-${this.#nextTurn++}`;\n+  }\n+\n+  checkpoint(summary: string, coveredThroughEventId: number): SessionEvent {\n+    if (coveredThroughEventId >= this.#nextId) {\n+      throw new Error(\"A checkpoint may cover only events that already exist.\");\n+    }\n+    return this.append({ type: \"context/checkpoint\", summary, coveredThroughEventId });\n+  }\n+}\n+\n+export const SESSION_LOG = new ServiceToken<SessionLog>(\"session-log\");\n+\n+export function sessionPlugin(log: SessionLog): Plugin {\n+  return {\n+    name: \"session-log\",\n+    setup(context) {\n+      context.provide(SESSION_LOG, log);\n+      context.on(\"runtime/plugin-mounted\", (payload) => {\n+        const plugin = pluginName(payload);\n+        log.append({ type: \"runtime/plugin-mounted\", plugin });\n+      });\n+      context.on(\"runtime/plugin-unmounting\", (payload) => {\n+        const plugin = pluginName(payload);\n+        log.append({ type: \"runtime/plugin-unmounted\", plugin });\n+      });\n+    },\n+  };\n+}\n+\n+export function buildRequest(events: readonly SessionEvent[], stepId: string): UnifiedRequest {\n+  const headerIndex = events.findIndex(\n+    (event) => event.type === \"request/header\" && event.stepId === stepId,\n+  );\n+  if (headerIndex === -1) throw new Error(`Missing request/header for ${stepId}.`);\n+  const header = events[headerIndex];\n+  if (header?.type !== \"request/header\") throw new Error(`Invalid request header for ${stepId}.`);\n+\n+  const prefix = events.slice(0, headerIndex);\n+  const checkpoint = [...prefix]\n+    .reverse()\n+    .find((event): event is Extract<SessionEvent, { type: \"context/checkpoint\" }> =>\n+      event.type === \"context/checkpoint\",\n+    );\n+  const projectedEvents = checkpoint\n+    ? prefix.filter((event) => event.id > checkpoint.coveredThroughEventId)\n+    : prefix;\n+  const messages: ModelMessage[] = [];\n+  if (checkpoint) {\n+    messages.push({\n+      role: \"system\",\n+      content: `[Context checkpoint through event ${checkpoint.coveredThroughEventId}]\\n${checkpoint.summary}`,\n+    });\n+  }\n+\n+  const callsByStep = new Map<string, ToolCall[]>();\n+  for (const event of projectedEvents) {\n+    if (event.type === \"tool/call\") {\n+      const calls = callsByStep.get(event.stepId) ?? [];\n+      calls.push(structuredClone(event.call));\n+      callsByStep.set(event.stepId, calls);\n+    }\n+  }\n+  for (const event of projectedEvents) {\n+    switch (event.type) {\n+      case \"user/message\":\n+        messages.push({ role: \"user\", content: event.content });\n+        break;\n+      case \"assistant/message\":\n+        messages.push({\n+          role: \"assistant\",\n+          content: event.content,\n+          toolCalls: callsByStep.get(event.stepId) ?? [],\n+        });\n+        break;\n+      case \"tool/result\":\n+        messages.push({\n+          role: \"tool\",\n+          toolCallId: event.toolCallId,\n+          name: event.name,\n+          content: event.content,\n+        });\n+        break;\n+      default:\n+        break;\n+    }\n+  }\n+\n+  return {\n+    system: header.system,\n+    tools: structuredClone(header.tools),\n+    messages: projectMessages(messages, header.projection),\n+    dynamicContext: header.dynamicContext,\n+  };\n+}\n+\n+export function requestStepIds(events: readonly SessionEvent[]): string[] {\n+  return events\n+    .filter((event): event is Extract<SessionEvent, { type: \"request/header\" }> =>\n+      event.type === \"request/header\",\n+    )\n+    .map((event) => event.stepId);\n+}\n+\n+export function replayTrace(events: readonly SessionEvent[]): TraceItem[] {\n+  return events.map((event) => ({\n+    eventId: event.id,\n+    type: event.type,\n+    title: traceTitle(event),\n+    detail: traceDetail(event),\n+  }));\n+}\n+\n+function traceTitle(event: SessionEvent): string {\n+  switch (event.type) {\n+    case \"request/header\":\n+      return `${event.stepId} → ${event.provider}/${event.model}`;\n+    case \"tool/call\":\n+      return `call ${event.call.name}`;\n+    case \"tool/result\":\n+      return `result ${event.name}`;\n+    case \"runtime/plugin-mounted\":\n+    case \"runtime/plugin-unmounted\":\n+      return event.plugin;\n+    case \"goal/round-started\":\n+      return `round ${event.round} · ${event.label}`;\n+    default:\n+      return event.type;\n+  }\n+}\n+\n+function traceDetail(event: SessionEvent): string {\n+  switch (event.type) {\n+    case \"user/message\":\n+    case \"assistant/message\":\n+      return event.content;\n+    case \"tool/call\":\n+      return JSON.stringify(event.call.arguments);\n+    case \"tool/result\":\n+      return event.content;\n+    case \"context/checkpoint\":\n+      return `${event.summary} (covers through #${event.coveredThroughEventId})`;\n+    case \"request/header\":\n+      return `${event.tools.length} tools · dynamic=${event.dynamicContext}`;\n+    case \"turn/end\":\n+    case \"step/end\":\n+      return event.outcome;\n+    case \"goal/created\":\n+      return event.objective;\n+    case \"goal/status-changed\":\n+      return `${event.status}: ${event.reason}`;\n+    default:\n+      return \"\";\n+  }\n+}\n+\n+function pluginName(payload: unknown): string {\n+  if (\n+    typeof payload === \"object\" &&\n+    payload !== null &&\n+    \"plugin\" in payload &&\n+    typeof payload.plugin === \"string\"\n+  ) {\n+    return payload.plugin;\n+  }\n+  throw new Error(\"Runtime plugin event is missing its plugin name.\");\n+}\n",
      "diffStats": {
        "filesChanged": 1,
        "additions": 266,
        "deletions": 0,
        "files": [
          {
            "path": "src/session.ts",
            "additions": 266,
            "deletions": 0
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      },
      "requests": [
        {
          "step": 1,
          "stepId": "turn-1-step-1",
          "request": {
            "system": "Use a tool when the answer requires external facts.",
            "tools": [
              {
                "name": "get_time",
                "description": "Get the current time in a city.",
                "inputSchema": {
                  "type": "object",
                  "properties": {
                    "city": {
                      "type": "string"
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                  "required": [
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                "role": "user",
                "content": "What time is it in Beijing?"
              }
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                  }
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              "approximateTokens": 43
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            {
              "id": "message-0",
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            {
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              "value": "Step 1",
              "approximateTokens": 2
            }
          ],
          "totalApproximateTokens": 57,
          "prefix": {
            "sharedParts": 0,
            "sharedApproximateTokens": 0,
            "previousParts": 0,
            "currentParts": 4,
            "firstInvalidation": "First request has no predecessor.",
            "note": "Teaching estimate only; no provider cache was queried."
          }
        },
        {
          "step": 2,
          "stepId": "turn-1-step-2",
          "request": {
            "system": "Use a tool when the answer requires external facts.",
            "tools": [
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                "name": "get_time",
                "description": "Get the current time in a city.",
                "inputSchema": {
                  "type": "object",
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                  },
                  "required": [
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                  ],
                  "additionalProperties": false
                }
              }
            ],
            "messages": [
              {
                "role": "user",
                "content": "What time is it in Beijing?"
              },
              {
                "role": "assistant",
                "content": "I will check the time first.",
                "toolCalls": [
                  {
                    "id": "call-time",
                    "name": "get_time",
                    "arguments": {
                      "city": "Beijing"
                    }
                  }
                ]
              },
              {
                "role": "tool",
                "toolCallId": "call-time",
                "name": "get_time",
                "content": "{\"time\":\"14:30\"}"
              }
            ],
            "dynamicContext": "Step 2"
          },
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                  }
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              "stability": "append-only",
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                "content": "What time is it in Beijing?"
              },
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            },
            {
              "id": "message-1",
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              "stability": "append-only",
              "label": "assistant",
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                "role": "assistant",
                "content": "I will check the time first.",
                "toolCalls": [
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                    "id": "call-time",
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                    "arguments": {
                      "city": "Beijing"
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            },
            {
              "id": "message-2",
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              "stability": "append-only",
              "label": "tool · get_time",
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                "toolCallId": "call-time",
                "name": "get_time",
                "content": "{\"time\":\"14:30\"}"
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              "id": "dynamic",
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              "stability": "step-variable",
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            }
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          "prefix": {
            "sharedParts": 3,
            "sharedApproximateTokens": 55,
            "previousParts": 4,
            "currentParts": 6,
            "firstInvalidation": "assistant",
            "note": "Longest identical canonical prefix; approximate tokens, not provider cache usage."
          }
        }
      ],
      "events": [
        {
          "id": 1,
          "type": "user/message",
          "turnId": "turn-1",
          "content": "What time is it in Beijing?"
        },
        {
          "id": 2,
          "type": "request/header",
          "stepId": "turn-1-step-1",
          "provider": "demo",
          "model": "tiny-model",
          "system": "Use a tool when the answer requires external facts.",
          "tools": [
            {
              "name": "get_time",
              "description": "Get the current time in a city.",
              "inputSchema": {
                "type": "object",
                "properties": {
                  "city": {
                    "type": "string"
                  }
                },
                "required": [
                  "city"
                ],
                "additionalProperties": false
              }
            }
          ],
          "dynamicContext": "Step 1",
          "projection": {
            "maxToolResultChars": 720,
            "toolResultHeadChars": 430,
            "toolResultTailChars": 170
          }
        },
        {
          "id": 3,
          "type": "assistant/message",
          "stepId": "turn-1-step-1",
          "content": "I will check the time first."
        },
        {
          "id": 4,
          "type": "tool/call",
          "stepId": "turn-1-step-1",
          "call": {
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            "arguments": {
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        },
        {
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          "stepId": "turn-1-step-1",
          "toolCallId": "call-time",
          "name": "get_time",
          "content": "{\"time\":\"14:30\"}"
        },
        {
          "id": 6,
          "type": "request/header",
          "stepId": "turn-1-step-2",
          "provider": "demo",
          "model": "tiny-model",
          "system": "Use a tool when the answer requires external facts.",
          "tools": [
            {
              "name": "get_time",
              "description": "Get the current time in a city.",
              "inputSchema": {
                "type": "object",
                "properties": {
                  "city": {
                    "type": "string"
                  }
                },
                "required": [
                  "city"
                ],
                "additionalProperties": false
              }
            }
          ],
          "dynamicContext": "Step 2",
          "projection": {
            "maxToolResultChars": 720,
            "toolResultHeadChars": 430,
            "toolResultTailChars": 170
          }
        }
      ],
      "trace": [
        {
          "eventId": 1,
          "type": "user/message",
          "title": "user/message",
          "detail": "What time is it in Beijing?"
        },
        {
          "eventId": 2,
          "type": "request/header",
          "title": "turn-1-step-1 → demo/tiny-model",
          "detail": "1 tools · dynamic=Step 1"
        },
        {
          "eventId": 3,
          "type": "assistant/message",
          "title": "assistant/message",
          "detail": "I will check the time first."
        },
        {
          "eventId": 4,
          "type": "tool/call",
          "title": "call get_time",
          "detail": "{\"city\":\"Beijing\"}"
        },
        {
          "eventId": 5,
          "type": "tool/result",
          "title": "result get_time",
          "detail": "{\"time\":\"14:30\"}"
        },
        {
          "eventId": 6,
          "type": "request/header",
          "title": "turn-1-step-2 → demo/tiny-model",
          "detail": "1 tools · dynamic=Step 2"
        }
      ],
      "graphs": []
    },
    {
      "id": "chapter-5",
      "number": "05",
      "shortTitle": "Runtime Self-Evolution",
      "title": "Write and run a new Cordis plugin",
      "question": "How does DSH continuously evolve at runtime?",
      "codeGuide": {
        "title": "Close a capability gap through inspection, mounting, and release",
        "description": "Runtime Tools first lets the Agent inspect the current Context, then define and run a Cordis plugin. Dynamic plugins contribute tools and prompts through the same Context mount and unload flow as ordinary plugins.",
        "observations": [
          {
            "title": "The definition table stores code and runtime state",
            "text": "Runtime Tools stores each dynamic plugin's identifier, purpose, code, and removal function. When Runtime Tools itself is removed, it stops any plugins that are still running and clears their definitions.",
            "lines": [
              13,
              28
            ]
          },
          {
            "title": "Definition and execution are separate actions",
            "text": "cordis_define checks and stores the JavaScript plugin code submitted by the Agent. cordis_run retrieves the code by pluginId, obtains a Plugin, and mounts it through context.mount().",
            "lines": [
              29,
              75
            ]
          },
          {
            "title": "Stopping and deletion use the same removal function",
            "text": "cordis_stop runs the removal function but retains the definition. cordis_undefine stops the plugin and deletes its definition. Context withdraws the plugin's contributions as part of the same lifecycle.",
            "lines": [
              76,
              102
            ]
          }
        ],
        "fills": [
          {
            "label": "Dynamic plugin definitions and the Runtime Tools entry point",
            "kind": "skeleton",
            "ranges": [
              [
                1,
                16
              ],
              [
                104,
                105
              ]
            ]
          },
          {
            "label": "Store definitions, cleanup actions, and the inspection interface",
            "kind": "body",
            "ranges": [
              [
                17,
                34
              ]
            ]
          },
          {
            "label": "Define and mount a new Cordis plugin",
            "kind": "body",
            "ranges": [
              [
                35,
                75
              ]
            ]
          },
          {
            "label": "Stop, remove, and load plugin code",
            "kind": "body",
            "ranges": [
              [
                76,
                145
              ]
            ]
          }
        ]
      },
      "changeStory": {
        "title": "Inspect first, mount next, and verify before release",
        "summary": "Nano DSH exposes cordis_inspect, cordis_define, cordis_run, cordis_stop, and cordis_undefine as ordinary Tools. The Agent confirms a missing capability, defines and mounts a Cordis plugin, calls its new tool to validate the result, then stops or removes the definition. Tools and prompts take effect in subsequent requests.",
        "harnessRole": "Capability evolution layer: let the Agent write, run, and validate new plugins while it works",
        "connection": "This process uses the mechanisms from the first four chapters. The Agent Loop runs Cordis tools, request projection includes new Tools and Prompts, the Plugin supplies a reversible lifecycle, and the Session Log records definition, mounting, invocation, and removal. With these mechanisms, the Agent can write new capabilities needed for its current task.",
        "outcomes": [
          "Describe a capability experiment in this order: cordis_inspect, cordis_define, cordis_run, call the new tool, and cordis_stop or cordis_undefine",
          "Explain the difference between mounting a plugin successfully and confirming that its new tool works",
          "Identify the first request that can see a new Tool and Prompt, and explain how to verify that their contributions are gone after the plugin stops"
        ]
      },
      "lesson": "# The Agent Installs Its Own Tool\n\nChapter 4's log can explain what happened, yet it does not add a capability. Suppose an Agent needs to count the words in a piece of text and the current `Context` has no `word_count` tool. Guessing tool names or repeatedly calling unrelated tools consumes Steps without closing that capability gap.\n\nDSH treats this situation as a controlled runtime change: **inspect the current capabilities and interfaces, mount a temporary plugin, call its new tool to verify the result, and release the plugin when the task ends.** This path brings the first four chapters together: the Agent Loop starts the action, request projection carries the current tools, the plugin lifecycle manages entry and exit, and the Session Log records the process.\n\n`runtime-tools.ts` begins with the shape of a dynamic plugin and the entry point for `runtimeToolsPlugin()`. The following implementation works from one definition table, making three states easy to distinguish: plugin code is registered, the plugin is running, or the plugin has been released.\n\n<!-- fill {\"index\":0} -->\n\n## 5.1 Confirm the Gap Before Deciding What to Add\n\n`cordis_inspect` returns the plugins, services, tools, prompts, and dependencies in the current `Context`. **Confirm what is missing before deciding what to add.** The Agent uses this result to establish that `word_count` is absent, then decides what the plugin must provide. The baseline capability graph matters too: it gives a comparable state from before the change, so the model does not mistake an existing capability for a gap.\n\nRuntime Tools also needs its own lifecycle. Its definition table holds dynamic plugins. When Runtime Tools unloads, it stops any still-running plugins in reverse order and clears the definitions. It also registers the inspection tool and a Prompt that tells the Agent when to inspect, define, and run a plugin. These preparations give a temporary extension a definite entry and exit.\n\n<!-- fill {\"index\":1} -->\n\n<!-- evidence {\"id\":\"m05-before\",\"tab\":\"graph\",\"requestStep\":1,\"label\":\"Confirm the capability gap before changing it\",\"description\":\"Inspect the current Context: it contains neither a dynamic:word_count plugin nor a word_count tool.\"} -->\n\n## 5.2 Defining Code and Mounting a Capability Are Different Moments\n\nAfter inspection confirms the gap, the Agent submits a plugin name, purpose, and JavaScript code through `cordis_define`. **Defining code and mounting a capability happen at different moments.** The function compiles the code and saves its definition in the table. The tool catalog does not change yet. `cordis_run` retrieves the definition, instantiates a Plugin, and calls Chapter 3's `context.mount()`. Only after mounting succeeds does the new tool join the current `Context`.\n\nThe distinction has a practical purpose. Definition can report whether code is loadable; mounting can report whether the plugin actually changed the runtime. A failure therefore has a precise location. The full DSH offers `cordis_inspect`, `cordis_mount`, and `cordis_unmount`; temporary plugins act on the current process and disappear when they unload or the process ends.\n\n<!-- fill {\"index\":2} -->\n\n## 5.3 Verification and Release Close the Change Loop\n\nAfter the plugin mounts, `dynamic:word_count` appears in the plugin list and `word_count` in the tool list. **The new tool becomes visible from a later model request onward.** A new request reads `Context` again, so its Tool Schema gains the tool name and argument rules; a request already sent remains unchanged.\n\n<!-- evidence {\"id\":\"m05-installed\",\"tab\":\"graph\",\"requestStep\":2,\"label\":\"The new tool has joined the current Context\",\"description\":\"Compare this graph with the baseline: dynamic:word_count registers word_count, whose argument rules can enter later requests.\"} -->\n\nThe Agent calls `word_count` and receives `words: 3`. That result proves that the new capability has passed through the Agent Loop and completed the task.\n\n<!-- evidence {\"id\":\"m05-verified\",\"tab\":\"events\",\"eventType\":\"tool/result\",\"eventOccurrence\":4,\"label\":\"A tool result verifies the new capability\",\"description\":\"Inspect the word_count result to confirm that the new tool completed one real call.\"} -->\n\nAfter verification, `cordis_stop` calls the unload function saved during mounting but retains the definition for a later rerun. `cordis_undefine` first stops a running plugin, then removes its definition. Supporting helpers compile code, instantiate plugins, and read arguments so the Tool bodies can focus on state transitions. `node:vm` loads only trusted teaching samples here; a deployment still needs explicit permissions and an enablement boundary.\n\n<!-- evidence {\"id\":\"m05-removed\",\"tab\":\"graph\",\"requestStep\":3,\"label\":\"Release returns to the baseline\",\"description\":\"Inspect the final graph to confirm that dynamic:word_count and word_count have both left the Context.\"} -->\n\n<!-- fill {\"index\":3} -->\n",
      "source": {
        "path": "src/runtime-tools.ts",
        "content": "import { Script, createContext } from \"node:vm\";\nimport type { JsonValue } from \"./protocol.js\";\nimport type { Context, Plugin } from \"./runtime.js\";\n\ninterface DynamicPluginDefinition {\n  pluginId: string;\n  name: string;\n  purpose: string;\n  code: string;\n  dispose?: () => void;\n}\n\nexport function runtimeToolsPlugin(): Plugin {\n  return {\n    name: \"runtime-tools\",\n    setup(context) {\n      const definitions = new Map<string, DynamicPluginDefinition>();\n      let nextPluginId = 1;\n\n      context.effect(() => {\n        for (const definition of [...definitions.values()].reverse()) definition.dispose?.();\n        definitions.clear();\n      });\n\n      context.contributePrompt(\n        \"runtime-evolution-guide\",\n        \"Inspect the current Context, define a small Cordis Plugin when the task needs a new capability, run it, use its tools, then stop or undefine it after the experiment.\",\n      );\n      context.registerTool({\n        name: \"cordis_inspect\",\n        description: \"Inspect mounted plugins, services, tools, Prompt contributions, listeners, and relations.\",\n        inputSchema: { type: \"object\", properties: {}, additionalProperties: false },\n        execute: () => context.inspect() as unknown as JsonValue,\n      });\n      context.registerTool({\n        name: \"cordis_define\",\n        description: \"Define a new Cordis Plugin from a JavaScript function body that returns the Plugin.\",\n        inputSchema: {\n          type: \"object\",\n          properties: {\n            name: { type: \"string\", minLength: 1 },\n            purpose: { type: \"string\", minLength: 1 },\n            code: { type: \"string\", minLength: 1 },\n          },\n          required: [\"name\", \"purpose\", \"code\"],\n          additionalProperties: false,\n        },\n        execute(input) {\n          const { name, purpose, code } = readDefinition(input);\n          compilePlugin(code);\n          const pluginId = `dyn-${nextPluginId}`;\n          nextPluginId += 1;\n          definitions.set(pluginId, { pluginId, name, purpose, code });\n          return { ok: true, pluginId, name, status: \"defined\" };\n        },\n      });\n      context.registerTool({\n        name: \"cordis_run\",\n        description: \"Run one previously defined Cordis Plugin in the current Context.\",\n        inputSchema: pluginIdSchema(),\n        async execute(input) {\n          const pluginId = readPluginId(input);\n          const definition = definitions.get(pluginId);\n          if (!definition) return { ok: false, error: `Unknown dynamic Plugin: ${pluginId}` };\n          if (definition.dispose) return { ok: false, error: `${pluginId} is already running.` };\n          const plugin = instantiatePlugin(definition);\n          definition.dispose = await context.mount(plugin);\n          return {\n            ok: true,\n            pluginId,\n            status: \"running\",\n            tools: context.inspect().tools.map((tool) => tool.name),\n          };\n        },\n      });\n      context.registerTool({\n        name: \"cordis_stop\",\n        description: \"Stop a running dynamic Plugin while retaining its definition.\",\n        inputSchema: pluginIdSchema(),\n        execute(input) {\n          const pluginId = readPluginId(input);\n          const definition = definitions.get(pluginId);\n          if (!definition) return { ok: false, error: `Unknown dynamic Plugin: ${pluginId}` };\n          if (!definition.dispose) return { ok: false, error: `${pluginId} is not running.` };\n          definition.dispose();\n          delete definition.dispose;\n          return { ok: true, pluginId, status: \"stopped\" };\n        },\n      });\n      context.registerTool({\n        name: \"cordis_undefine\",\n        description: \"Stop a dynamic Plugin if needed and remove its definition.\",\n        inputSchema: pluginIdSchema(),\n        execute(input) {\n          const pluginId = readPluginId(input);\n          const definition = definitions.get(pluginId);\n          if (!definition) return { ok: false, error: `Unknown dynamic Plugin: ${pluginId}` };\n          definition.dispose?.();\n          definitions.delete(pluginId);\n          return { ok: true, pluginId, status: \"undefined\" };\n        },\n      });\n    },\n  };\n}\n\nfunction compilePlugin(code: string): Script {\n  return new Script(`(() => {\\n\"use strict\";\\n${code}\\n})()`);\n}\n\nfunction instantiatePlugin(definition: DynamicPluginDefinition): Plugin {\n  const value = compilePlugin(definition.code).runInContext(createContext(Object.create(null)), {\n    timeout: 100,\n  }) as unknown;\n  if (\n    typeof value !== \"object\" ||\n    value === null ||\n    typeof (value as { apply?: unknown }).apply !== \"function\"\n  ) {\n    throw new Error(\"Dynamic code must return a Cordis Plugin with apply(context).\");\n  }\n  const dynamic = value as { apply(context: Context): void | Promise<void> };\n  return {\n    name: `dynamic:${definition.name}`,\n    setup: (context) => dynamic.apply(context),\n  };\n}\n\nfunction pluginIdSchema(): Record<string, unknown> {\n  return {\n    type: \"object\",\n    properties: { pluginId: { type: \"string\", pattern: \"^dyn-[1-9][0-9]*$\" } },\n    required: [\"pluginId\"],\n    additionalProperties: false,\n  };\n}\n\nfunction readDefinition(input: JsonValue): { name: string; purpose: string; code: string } {\n  return input as { name: string; purpose: string; code: string };\n}\n\nfunction readPluginId(input: JsonValue): string {\n  return (input as { pluginId: string }).pluginId;\n}\n",
        "excerpt": "export function runtimeToolsPlugin(): Plugin {\n  return {\n    name: \"runtime-tools\",\n    setup(context) {\n      const definitions = new Map<string, DynamicPluginDefinition>();\n      let nextPluginId = 1;\n\n      context.effect(() => {\n        for (const definition of [...definitions.values()].reverse()) definition.dispose?.();\n        definitions.clear();\n      });\n\n      context.contributePrompt(\n        \"runtime-evolution-guide\",\n        \"Inspect the current Context, define a small Cordis Plugin when the task needs a new capability, run it, use its tools, then stop or undefine it after the experiment.\",\n      );\n      context.registerTool({\n        name: \"cordis_inspect\",\n        description: \"Inspect mounted plugins, services, tools, Prompt contributions, listeners, and relations.\",\n        inputSchema: { type: \"object\", properties: {}, additionalProperties: false },\n        execute: () => context.inspect() as unknown as JsonValue,\n      });\n      context.registerTool({\n        name: \"cordis_define\",\n        description: \"Define a new Cordis Plugin from a JavaScript function body that returns the Plugin.\",\n        inputSchema: {\n          type: \"object\",\n          properties: {\n            name: { type: \"string\", minLength: 1 },\n            purpose: { type: \"string\", minLength: 1 },\n            code: { type: \"string\", minLength: 1 },\n          },\n          required: [\"name\", \"purpose\", \"code\"],\n          additionalProperties: false,\n        },\n        execute(input) {\n          const { name, purpose, code } = readDefinition(input);\n          compilePlugin(code);\n          const pluginId = `dyn-${nextPluginId}`;\n          nextPluginId += 1;\n          definitions.set(pluginId, { pluginId, name, purpose, code });\n          return { ok: true, pluginId, name, status: \"defined\" };\n        },\n      });\n      context.registerTool({\n        name: \"cordis_run\",\n        description: \"Run one previously defined Cordis Plugin in the current Context.\",\n        inputSchema: pluginIdSchema(),\n        async execute(input) {\n          const pluginId = readPluginId(input);\n          const definition = definitions.get(pluginId);\n          if (!definition) return { ok: false, error: `Unknown dynamic Plugin: ${pluginId}` };\n          if (definition.dispose) return { ok: false, error: `${pluginId} is already running.` };\n          const plugin = instantiatePlugin(definition);\n          definition.dispose = await context.mount(plugin);\n          return {\n            ok: true,\n            pluginId,\n            status: \"running\",\n            tools: context.inspect().tools.map((tool) => tool.name),\n          };\n        },\n      });\n      context.registerTool({\n        name: \"cordis_stop\",\n        description: \"Stop a running dynamic Plugin while retaining its definition.\",\n        inputSchema: pluginIdSchema(),\n        execute(input) {\n          const pluginId = readPluginId(input);\n          const definition = definitions.get(pluginId);\n          if (!definition) return { ok: false, error: `Unknown dynamic Plugin: ${pluginId}` };\n          if (!definition.dispose) return { ok: false, error: `${pluginId} is not running.` };\n          definition.dispose();\n          delete definition.dispose;\n          return { ok: true, pluginId, status: \"stopped\" };\n        },\n      });\n      context.registerTool({\n        name: \"cordis_undefine\",\n        description: \"Stop a dynamic Plugin if needed and remove its definition.\",\n        inputSchema: pluginIdSchema(),\n        execute(input) {\n          const pluginId = readPluginId(input);\n          const definition = definitions.get(pluginId);\n          if (!definition) return { ok: false, error: `Unknown dynamic Plugin: ${pluginId}` };\n          definition.dispose?.();\n          definitions.delete(pluginId);\n          return { ok: true, pluginId, status: \"undefined\" };\n        },\n      });\n    },\n  };\n}\n\nfunction compilePlugin(code: string): Script {\n  return new Script(`(() => {\\n\"use strict\";\\n${code}\\n})()`);\n}\n\nfunction instantiatePlugin(definition: DynamicPluginDefinition): Plugin {\n  const value = compilePlugin(definition.code).runInContext(createContext(Object.create(null)), {\n    timeout: 100,\n  }) as unknown;\n  if (\n    typeof value !== \"object\" ||\n    value === null ||\n    typeof (value as { apply?: unknown }).apply !== \"function\"\n  ) {\n    throw new Error(\"Dynamic code must return a Cordis Plugin with apply(context).\");\n  }\n  const dynamic = value as { apply(context: Context): void | Promise<void> };\n  return {\n    name: `dynamic:${definition.name}`,\n    setup: (context) => dynamic.apply(context),\n  };\n}",
        "startLine": 13,
        "endLine": 127
      },
      "extraFiles": [
        {
          "path": "src/agent.ts",
          "content": "import { Ajv, type ValidateFunction } from \"ajv\";\nimport {\n  DEFAULT_PROJECTION,\n  toolSchemas,\n  type ProjectionSettings,\n} from \"./context.js\";\nimport type {\n  AssistantMessage,\n  JsonValue,\n  Llm,\n  UnifiedRequest,\n} from \"./protocol.js\";\nimport type { Context } from \"./runtime.js\";\nimport {\n  buildRequest,\n  replayTrace,\n  SESSION_LOG,\n  type SessionEvent,\n  type SessionLog,\n  type TraceItem,\n} from \"./session.js\";\n\nexport interface RunTurnResult {\n  finalMessage: AssistantMessage;\n  steps: number;\n  turnId: string;\n  stepIds: string[];\n  trace: TraceItem[];\n  requests: UnifiedRequest[];\n}\n\nexport interface CheckpointProposal {\n  summary: string;\n  coveredThroughEventId: number;\n}\n\nexport interface AgentOptions {\n  llm: Llm;\n  context: Context;\n  systemPrompt: string;\n  dynamicContext?: (step: number) => string;\n  projection?: ProjectionSettings;\n  checkpointBeforeStep?: (input: {\n    step: number;\n    events: readonly SessionEvent[];\n  }) => CheckpointProposal | undefined;\n  maxSteps?: number;\n}\n\nexport class Agent {\n  readonly #llm: Llm;\n  readonly #context: Context;\n  readonly #validators = new Map<string, { schema: string; validate: ValidateFunction }>();\n  readonly #ajv = new Ajv({ allErrors: true, strict: false });\n  readonly #systemPrompt: string;\n  readonly #dynamicContext: (step: number) => string;\n  readonly #maxSteps: number;\n  readonly #projection: ProjectionSettings;\n  readonly #session: SessionLog;\n  readonly #checkpointBeforeStep?: AgentOptions[\"checkpointBeforeStep\"];\n\n  constructor(options: AgentOptions) {\n    this.#llm = options.llm;\n    this.#context = options.context;\n    this.#systemPrompt = options.systemPrompt;\n    this.#dynamicContext = options.dynamicContext ?? ((step) => `Current step: ${step}`);\n    this.#maxSteps = options.maxSteps ?? 8;\n    this.#projection = options.projection ?? DEFAULT_PROJECTION;\n    this.#session = this.#context.use(SESSION_LOG);\n    this.#checkpointBeforeStep = options.checkpointBeforeStep;\n  }\n\n  async runTurn(userInput: string): Promise<RunTurnResult> {\n    const eventStart = this.#session.events.length;\n    const turnId = this.#session.nextTurnId();\n    this.#session.append({ type: \"turn/start\", turnId });\n    this.#session.append({ type: \"user/message\", turnId, content: userInput });\n    const requests: UnifiedRequest[] = [];\n    const stepIds: string[] = [];\n\n    for (let step = 1; step <= this.#maxSteps; step += 1) {\n      const checkpoint = this.#checkpointBeforeStep?.({ step, events: this.#session.events });\n      if (checkpoint) {\n        this.#session.checkpoint(checkpoint.summary, checkpoint.coveredThroughEventId);\n      }\n      const stepId = `${turnId}-step-${step}`;\n      stepIds.push(stepId);\n      this.#session.append({ type: \"step/start\", turnId, stepId, ordinal: step });\n      this.#session.append({\n        type: \"request/header\",\n        stepId,\n        provider: this.#llm.provider,\n        model: this.#llm.model,\n        system: this.#context.compilePrompt(this.#systemPrompt),\n        tools: toolSchemas(this.#context.listTools()),\n        dynamicContext: this.#dynamicContext(step),\n        projection: this.#projection,\n      });\n      const request = buildRequest(this.#session.events, stepId);\n      requests.push(structuredClone(request));\n\n      const response = await this.#llm.complete(request);\n      const assistant = response.message;\n      this.#session.append({ type: \"assistant/message\", stepId, content: assistant.content });\n      for (const call of assistant.toolCalls) {\n        this.#session.append({ type: \"tool/call\", stepId, call });\n      }\n\n      if (assistant.toolCalls.length === 0) {\n        this.#session.append({ type: \"step/end\", stepId, outcome: \"complete\" });\n        this.#session.append({ type: \"turn/end\", turnId, outcome: \"completed\" });\n        return {\n          finalMessage: assistant,\n          steps: step,\n          turnId,\n          stepIds,\n          trace: replayTrace(this.#session.events.slice(eventStart)),\n          requests,\n        };\n      }\n\n      for (const call of assistant.toolCalls) {\n        const result = await this.#executeTool(call.name, call.arguments);\n        this.#session.append({\n          type: \"tool/result\",\n          stepId,\n          toolCallId: call.id,\n          name: call.name,\n          content: JSON.stringify(result),\n        });\n      }\n      this.#session.append({ type: \"step/end\", stepId, outcome: \"tool-calls\" });\n    }\n\n    this.#session.append({ type: \"turn/end\", turnId, outcome: \"max-steps\" });\n    throw new Error(`Agent exceeded maxSteps (${this.#maxSteps}) before finishing the turn.`);\n  }\n\n  async #executeTool(name: string, input: JsonValue): Promise<JsonValue> {\n    const tool = this.#context.getTool(name);\n    if (!tool) return { ok: false, error: `Unknown tool: ${name}` };\n    const schema = JSON.stringify(tool.inputSchema);\n    const cached = this.#validators.get(name);\n    const validate = cached?.schema === schema ? cached.validate : this.#ajv.compile(tool.inputSchema);\n    this.#validators.set(name, { schema, validate });\n    if (!validate?.(input)) {\n      return {\n        ok: false,\n        error: \"Invalid tool arguments\",\n        issues: (validate?.errors ?? []).map((error) => `${error.instancePath || \"/\"} ${error.message}`),\n      };\n    }\n    try {\n      const result = await tool.execute(input);\n      this.#context.emit(\"tool/executed\", { name, input, result });\n      return result;\n    } catch (error) {\n      return { ok: false, error: error instanceof Error ? error.message : String(error) };\n    }\n  }\n}\n"
        },
        {
          "path": "src/context.ts",
          "content": "import type { ModelMessage, ToolMessage, ToolSchema, UnifiedRequest } from \"./protocol.js\";\n\nexport interface ProjectionSettings {\n  maxToolResultChars: number;\n  toolResultHeadChars: number;\n  toolResultTailChars: number;\n}\n\nexport const DEFAULT_PROJECTION: ProjectionSettings = {\n  maxToolResultChars: 720,\n  toolResultHeadChars: 430,\n  toolResultTailChars: 170,\n};\n\nexport interface RequestPart {\n  id: string;\n  kind: \"system\" | \"tools\" | \"message\" | \"dynamic\";\n  stability: \"stable\" | \"append-only\" | \"step-variable\";\n  label: string;\n  value: unknown;\n  approximateTokens: number;\n}\n\nexport interface PrefixComparison {\n  sharedParts: number;\n  sharedApproximateTokens: number;\n  previousParts: number;\n  currentParts: number;\n  firstInvalidation: string | null;\n  note: string;\n}\n\nexport interface ClippedToolResult {\n  content: string;\n  omittedCharacters: number;\n}\n\nexport function projectMessages(\n  messages: ModelMessage[],\n  settings: ProjectionSettings = DEFAULT_PROJECTION,\n): ModelMessage[] {\n  return messages.map((message) => {\n    if (message.role !== \"tool\") return structuredClone(message);\n    const clipped = clipToolResult(message.content, settings);\n    return { ...message, content: clipped.content } satisfies ToolMessage;\n  });\n}\n\nexport function clipToolResult(\n  content: string,\n  settings: ProjectionSettings = DEFAULT_PROJECTION,\n): ClippedToolResult {\n  if (content.length <= settings.maxToolResultChars) {\n    return { content, omittedCharacters: 0 };\n  }\n  const omittedCharacters = Math.max(\n    0,\n    content.length - settings.toolResultHeadChars - settings.toolResultTailChars,\n  );\n  const marker = `\\n[… ${omittedCharacters} characters omitted from the model projection …]\\n`;\n  return {\n    content:\n      content.slice(0, settings.toolResultHeadChars) +\n      marker +\n      content.slice(content.length - settings.toolResultTailChars),\n    omittedCharacters,\n  };\n}\n\nexport function describeRequest(request: UnifiedRequest): RequestPart[] {\n  const parts: RequestPart[] = [\n    part(\"system\", \"system\", \"stable\", \"System prompt\", request.system),\n    part(\"tools\", \"tools\", \"stable\", \"Tool schemas\", request.tools),\n  ];\n  for (const [index, message] of request.messages.entries()) {\n    const suffix = message.role === \"tool\" ? ` · ${message.name}` : \"\";\n    parts.push(\n      part(\n        `message-${index}`,\n        \"message\",\n        \"append-only\",\n        `${message.role}${suffix}`,\n        message,\n      ),\n    );\n  }\n  if (request.dynamicContext) {\n    parts.push(\n      part(\n        \"dynamic\",\n        \"dynamic\",\n        \"step-variable\",\n        \"Step context\",\n        request.dynamicContext,\n      ),\n    );\n  }\n  return parts;\n}\n\nexport function compareRequestPrefix(\n  previous: UnifiedRequest | undefined,\n  current: UnifiedRequest,\n): PrefixComparison {\n  const currentParts = describeRequest(current);\n  if (!previous) {\n    return {\n      sharedParts: 0,\n      sharedApproximateTokens: 0,\n      previousParts: 0,\n      currentParts: currentParts.length,\n      firstInvalidation: \"First request has no predecessor.\",\n      note: \"Teaching estimate only; no provider cache was queried.\",\n    };\n  }\n  const previousParts = describeRequest(previous);\n  let sharedParts = 0;\n  while (\n    sharedParts < previousParts.length &&\n    sharedParts < currentParts.length &&\n    canonical(previousParts[sharedParts]?.value) === canonical(currentParts[sharedParts]?.value)\n  ) {\n    sharedParts += 1;\n  }\n  return {\n    sharedParts,\n    sharedApproximateTokens: currentParts\n      .slice(0, sharedParts)\n      .reduce((total, item) => total + item.approximateTokens, 0),\n    previousParts: previousParts.length,\n    currentParts: currentParts.length,\n    firstInvalidation:\n      sharedParts === Math.min(previousParts.length, currentParts.length)\n        ? null\n        : currentParts[sharedParts]?.label ?? \"Request end\",\n    note: \"Longest identical canonical prefix; approximate tokens, not provider cache usage.\",\n  };\n}\n\nexport function approximateTokens(value: unknown): number {\n  const text = typeof value === \"string\" ? value : canonical(value);\n  return Math.max(1, Math.ceil([...text].length / 4));\n}\n\nexport function toolSchemas(tools: Array<{ name: string; description: string; inputSchema: Record<string, unknown> }>): ToolSchema[] {\n  return tools.map(({ name, description, inputSchema }) => ({ name, description, inputSchema }));\n}\n\nfunction part(\n  id: string,\n  kind: RequestPart[\"kind\"],\n  stability: RequestPart[\"stability\"],\n  label: string,\n  value: unknown,\n): RequestPart {\n  return { id, kind, stability, label, value, approximateTokens: approximateTokens(value) };\n}\n\nfunction canonical(value: unknown): string {\n  return JSON.stringify(sortObject(value));\n}\n\nfunction sortObject(value: unknown): unknown {\n  if (Array.isArray(value)) return value.map(sortObject);\n  if (value !== null && typeof value === \"object\") {\n    return Object.fromEntries(\n      Object.entries(value as Record<string, unknown>)\n        .sort(([left], [right]) => left.localeCompare(right))\n        .map(([key, item]) => [key, sortObject(item)]),\n    );\n  }\n  return value;\n}\n"
        },
        {
          "path": "src/runtime.ts",
          "content": "import type { ToolDefinition } from \"./protocol.js\";\n\nexport class ServiceToken<T> {\n  readonly key = Symbol();\n  constructor(readonly name: string) {}\n}\n\nexport interface Plugin {\n  name: string;\n  setup(context: Context): void | Promise<void>;\n}\n\nexport interface RuntimeInspection {\n  plugins: string[];\n  services: Array<{ name: string; provider: string }>;\n  tools: Array<{ name: string; plugin: string }>;\n  prompts: Array<{ id: string; plugin: string; text: string }>;\n  listeners: Array<{ event: string; plugin: string }>;\n  relations: Array<{ consumer: string; service: string; provider: string }>;\n}\n\ntype Disposer = () => void;\ntype Listener = (payload: unknown) => void;\n\ninterface MountRecord {\n  name: string;\n  effects: Disposer[];\n  state: \"installing\" | \"mounted\" | \"unmounted\";\n}\n\ninterface Owned<T> {\n  owner: string;\n  value: T;\n}\n\nexport class Context {\n  readonly #mounts = new Map<string, MountRecord>();\n  readonly #services = new Map<symbol, Owned<unknown> & { token: ServiceToken<unknown> }>();\n  readonly #tools = new Map<string, Owned<ToolDefinition>>();\n  readonly #prompts = new Map<string, Owned<string>>();\n  readonly #listeners = new Map<string, Set<Owned<Listener>>>();\n  readonly #relations: RuntimeInspection[\"relations\"] = [];\n  #activeMount: MountRecord | null = null;\n\n  async mount(plugin: Plugin): Promise<Disposer> {\n    if (this.#mounts.has(plugin.name)) throw new Error(`Plugin already mounted: ${plugin.name}`);\n    if (this.#activeMount) throw new Error(\"Nested plugin mounting is not supported during setup.\");\n    const record: MountRecord = { name: plugin.name, effects: [], state: \"installing\" };\n    this.#mounts.set(plugin.name, record);\n    this.#activeMount = record;\n    try {\n      await plugin.setup(this);\n      record.state = \"mounted\";\n    } catch (error) {\n      this.#rollback(record);\n      this.#mounts.delete(plugin.name);\n      throw error;\n    } finally {\n      this.#activeMount = null;\n    }\n\n    this.emit(\"runtime/plugin-mounted\", { plugin: plugin.name });\n    return once(() => {\n      if (record.state === \"unmounted\") return;\n      this.emit(\"runtime/plugin-unmounting\", { plugin: plugin.name });\n      this.#rollback(record);\n      this.#mounts.delete(plugin.name);\n      this.#relations.splice(\n        0,\n        this.#relations.length,\n        ...this.#relations.filter(\n          (relation) => relation.consumer !== plugin.name && relation.provider !== plugin.name,\n        ),\n      );\n      this.emit(\"runtime/plugin-unmounted\", { plugin: plugin.name });\n    });\n  }\n\n  effect(cleanup: Disposer): Disposer {\n    const mount = this.#requireMount();\n    const dispose = once(cleanup);\n    mount.effects.push(dispose);\n    return dispose;\n  }\n\n  provide<T>(token: ServiceToken<T>, value: T): Disposer {\n    const owner = this.#requireMount().name;\n    if (this.#services.has(token.key)) throw new Error(`Service already provided: ${token.name}`);\n    this.#services.set(token.key, { owner, value, token: token as ServiceToken<unknown> });\n    return this.effect(() => {\n      const service = this.#services.get(token.key);\n      if (service?.owner === owner) this.#services.delete(token.key);\n    });\n  }\n\n  use<T>(token: ServiceToken<T>): T {\n    const service = this.#services.get(token.key);\n    if (!service) throw new Error(`Missing service: ${token.name}`);\n    const consumer = this.#activeMount?.name;\n    if (consumer) {\n      const relation = { consumer, service: token.name, provider: service.owner };\n      if (!this.#relations.some((item) => sameRelation(item, relation))) this.#relations.push(relation);\n    }\n    return service.value as T;\n  }\n\n  registerTool(tool: ToolDefinition): Disposer {\n    const owner = this.#requireMount().name;\n    if (this.#tools.has(tool.name)) throw new Error(`Tool already registered: ${tool.name}`);\n    this.#tools.set(tool.name, { owner, value: tool });\n    return this.effect(() => {\n      if (this.#tools.get(tool.name)?.owner === owner) this.#tools.delete(tool.name);\n    });\n  }\n\n  contributePrompt(id: string, text: string): Disposer {\n    const owner = this.#requireMount().name;\n    if (this.#prompts.has(id)) throw new Error(`Prompt contribution already registered: ${id}`);\n    this.#prompts.set(id, { owner, value: text });\n    return this.effect(() => {\n      if (this.#prompts.get(id)?.owner === owner) this.#prompts.delete(id);\n    });\n  }\n\n  on(event: string, listener: Listener): Disposer {\n    const owner = this.#requireMount().name;\n    const entry = { owner, value: listener };\n    const listeners = this.#listeners.get(event) ?? new Set<Owned<Listener>>();\n    listeners.add(entry);\n    this.#listeners.set(event, listeners);\n    return this.effect(() => {\n      listeners.delete(entry);\n      if (listeners.size === 0) this.#listeners.delete(event);\n    });\n  }\n\n  emit(event: string, payload: unknown): void {\n    for (const listener of [...(this.#listeners.get(event) ?? [])]) listener.value(payload);\n  }\n\n  listTools(): ToolDefinition[] {\n    return [...this.#tools.values()].map((entry) => entry.value);\n  }\n\n  getTool(name: string): ToolDefinition | undefined {\n    return this.#tools.get(name)?.value;\n  }\n\n  compilePrompt(base = \"\"): string {\n    return [base, ...[...this.#prompts.values()].map((entry) => entry.value)]\n      .filter(Boolean)\n      .join(\"\\n\\n\");\n  }\n\n  inspect(): RuntimeInspection {\n    return {\n      plugins: [...this.#mounts.values()]\n        .filter((mount) => mount.state === \"mounted\")\n        .map((mount) => mount.name),\n      services: [...this.#services.values()].map((entry) => ({\n        name: entry.token.name,\n        provider: entry.owner,\n      })),\n      tools: [...this.#tools.entries()].map(([name, entry]) => ({ name, plugin: entry.owner })),\n      prompts: [...this.#prompts.entries()].map(([id, entry]) => ({\n        id,\n        plugin: entry.owner,\n        text: entry.value,\n      })),\n      listeners: [...this.#listeners.entries()].flatMap(([event, entries]) =>\n        [...entries].map((entry) => ({ event, plugin: entry.owner })),\n      ),\n      relations: structuredClone(this.#relations),\n    };\n  }\n\n  #requireMount(): MountRecord {\n    if (!this.#activeMount) throw new Error(\"Runtime contributions must be registered during plugin setup.\");\n    return this.#activeMount;\n  }\n\n  #rollback(record: MountRecord): void {\n    const failures: unknown[] = [];\n    for (const dispose of [...record.effects].reverse()) {\n      try {\n        dispose();\n      } catch (error) {\n        failures.push(error);\n      }\n    }\n    record.state = \"unmounted\";\n    if (failures.length > 0) throw new AggregateError(failures, `Failed to clean up ${record.name}`);\n  }\n}\n\nfunction once(action: Disposer): Disposer {\n  let done = false;\n  return () => {\n    if (done) return;\n    done = true;\n    action();\n  };\n}\n\nfunction sameRelation(\n  left: RuntimeInspection[\"relations\"][number],\n  right: RuntimeInspection[\"relations\"][number],\n): boolean {\n  return (\n    left.consumer === right.consumer &&\n    left.service === right.service &&\n    left.provider === right.provider\n  );\n}\n"
        },
        {
          "path": "src/session.ts",
          "content": "import { projectMessages, type ProjectionSettings } from \"./context.js\";\nimport type {\n  AssistantMessage,\n  ModelMessage,\n  ToolCall,\n  ToolSchema,\n  UnifiedRequest,\n} from \"./protocol.js\";\nimport { ServiceToken, type Plugin } from \"./runtime.js\";\n\ninterface EventBase {\n  id: number;\n  type: string;\n}\n\nexport type SessionEvent =\n  | (EventBase & { type: \"turn/start\"; turnId: string })\n  | (EventBase & { type: \"user/message\"; turnId: string; content: string })\n  | (EventBase & { type: \"step/start\"; turnId: string; stepId: string; ordinal: number })\n  | (EventBase & {\n      type: \"request/header\";\n      stepId: string;\n      provider: string;\n      model: string;\n      system: string;\n      tools: ToolSchema[];\n      dynamicContext: string;\n      projection: ProjectionSettings;\n    })\n  | (EventBase & { type: \"assistant/message\"; stepId: string; content: string })\n  | (EventBase & { type: \"tool/call\"; stepId: string; call: ToolCall })\n  | (EventBase & {\n      type: \"tool/result\";\n      stepId: string;\n      toolCallId: string;\n      name: string;\n      content: string;\n    })\n  | (EventBase & {\n      type: \"context/checkpoint\";\n      summary: string;\n      coveredThroughEventId: number;\n    })\n  | (EventBase & { type: \"runtime/plugin-mounted\"; plugin: string })\n  | (EventBase & { type: \"runtime/plugin-unmounted\"; plugin: string })\n  | (EventBase & { type: \"step/end\"; stepId: string; outcome: \"tool-calls\" | \"complete\" })\n  | (EventBase & {\n      type: \"turn/end\";\n      turnId: string;\n      outcome: \"completed\" | \"max-steps\" | \"failed\";\n    })\n  | (EventBase & {\n      type: \"goal/created\";\n      goalId: string;\n      objective: string;\n      maxRounds: number;\n    })\n  | (EventBase & {\n      type: \"goal/round-started\";\n      goalId: string;\n      round: number;\n      label: string;\n    })\n  | (EventBase & {\n      type: \"goal/status-changed\";\n      goalId: string;\n      status: \"active\" | \"completed\" | \"blocked\" | \"max-rounds\";\n      reason: string;\n    });\n\nexport type NewSessionEvent = SessionEvent extends infer Event\n  ? Event extends SessionEvent\n    ? Omit<Event, \"id\">\n    : never\n  : never;\n\nexport interface TraceItem {\n  eventId: number;\n  type: SessionEvent[\"type\"];\n  title: string;\n  detail: string;\n}\n\nexport class SessionLog {\n  readonly #events: SessionEvent[] = [];\n  #nextId = 1;\n  #nextTurn = 1;\n\n  get events(): readonly SessionEvent[] {\n    return this.#events;\n  }\n\n  append(event: NewSessionEvent): SessionEvent {\n    const stored = { ...structuredClone(event), id: this.#nextId++ } as SessionEvent;\n    this.#events.push(stored);\n    return stored;\n  }\n\n  nextTurnId(): string {\n    return `turn-${this.#nextTurn++}`;\n  }\n\n  checkpoint(summary: string, coveredThroughEventId: number): SessionEvent {\n    if (coveredThroughEventId >= this.#nextId) {\n      throw new Error(\"A checkpoint may cover only events that already exist.\");\n    }\n    return this.append({ type: \"context/checkpoint\", summary, coveredThroughEventId });\n  }\n}\n\nexport const SESSION_LOG = new ServiceToken<SessionLog>(\"session-log\");\n\nexport function sessionPlugin(log: SessionLog): Plugin {\n  return {\n    name: \"session-log\",\n    setup(context) {\n      context.provide(SESSION_LOG, log);\n      context.on(\"runtime/plugin-mounted\", (payload) => {\n        const plugin = pluginName(payload);\n        log.append({ type: \"runtime/plugin-mounted\", plugin });\n      });\n      context.on(\"runtime/plugin-unmounting\", (payload) => {\n        const plugin = pluginName(payload);\n        log.append({ type: \"runtime/plugin-unmounted\", plugin });\n      });\n    },\n  };\n}\n\nexport function buildRequest(events: readonly SessionEvent[], stepId: string): UnifiedRequest {\n  const headerIndex = events.findIndex(\n    (event) => event.type === \"request/header\" && event.stepId === stepId,\n  );\n  if (headerIndex === -1) throw new Error(`Missing request/header for ${stepId}.`);\n  const header = events[headerIndex];\n  if (header?.type !== \"request/header\") throw new Error(`Invalid request header for ${stepId}.`);\n\n  const prefix = events.slice(0, headerIndex);\n  const checkpoint = [...prefix]\n    .reverse()\n    .find((event): event is Extract<SessionEvent, { type: \"context/checkpoint\" }> =>\n      event.type === \"context/checkpoint\",\n    );\n  const projectedEvents = checkpoint\n    ? prefix.filter((event) => event.id > checkpoint.coveredThroughEventId)\n    : prefix;\n  const messages: ModelMessage[] = [];\n  if (checkpoint) {\n    messages.push({\n      role: \"system\",\n      content: `[Context checkpoint through event ${checkpoint.coveredThroughEventId}]\\n${checkpoint.summary}`,\n    });\n  }\n\n  const callsByStep = new Map<string, ToolCall[]>();\n  for (const event of projectedEvents) {\n    if (event.type === \"tool/call\") {\n      const calls = callsByStep.get(event.stepId) ?? [];\n      calls.push(structuredClone(event.call));\n      callsByStep.set(event.stepId, calls);\n    }\n  }\n  for (const event of projectedEvents) {\n    switch (event.type) {\n      case \"user/message\":\n        messages.push({ role: \"user\", content: event.content });\n        break;\n      case \"assistant/message\":\n        messages.push({\n          role: \"assistant\",\n          content: event.content,\n          toolCalls: callsByStep.get(event.stepId) ?? [],\n        });\n        break;\n      case \"tool/result\":\n        messages.push({\n          role: \"tool\",\n          toolCallId: event.toolCallId,\n          name: event.name,\n          content: event.content,\n        });\n        break;\n      default:\n        break;\n    }\n  }\n\n  return {\n    system: header.system,\n    tools: structuredClone(header.tools),\n    messages: projectMessages(messages, header.projection),\n    dynamicContext: header.dynamicContext,\n  };\n}\n\nexport function requestStepIds(events: readonly SessionEvent[]): string[] {\n  return events\n    .filter((event): event is Extract<SessionEvent, { type: \"request/header\" }> =>\n      event.type === \"request/header\",\n    )\n    .map((event) => event.stepId);\n}\n\nexport function replayTrace(events: readonly SessionEvent[]): TraceItem[] {\n  return events.map((event) => ({\n    eventId: event.id,\n    type: event.type,\n    title: traceTitle(event),\n    detail: traceDetail(event),\n  }));\n}\n\nfunction traceTitle(event: SessionEvent): string {\n  switch (event.type) {\n    case \"request/header\":\n      return `${event.stepId} → ${event.provider}/${event.model}`;\n    case \"tool/call\":\n      return `call ${event.call.name}`;\n    case \"tool/result\":\n      return `result ${event.name}`;\n    case \"runtime/plugin-mounted\":\n    case \"runtime/plugin-unmounted\":\n      return event.plugin;\n    case \"goal/round-started\":\n      return `round ${event.round} · ${event.label}`;\n    default:\n      return event.type;\n  }\n}\n\nfunction traceDetail(event: SessionEvent): string {\n  switch (event.type) {\n    case \"user/message\":\n    case \"assistant/message\":\n      return event.content;\n    case \"tool/call\":\n      return JSON.stringify(event.call.arguments);\n    case \"tool/result\":\n      return event.content;\n    case \"context/checkpoint\":\n      return `${event.summary} (covers through #${event.coveredThroughEventId})`;\n    case \"request/header\":\n      return `${event.tools.length} tools · dynamic=${event.dynamicContext}`;\n    case \"turn/end\":\n    case \"step/end\":\n      return event.outcome;\n    case \"goal/created\":\n      return event.objective;\n    case \"goal/status-changed\":\n      return `${event.status}: ${event.reason}`;\n    default:\n      return \"\";\n  }\n}\n\nfunction pluginName(payload: unknown): string {\n  if (\n    typeof payload === \"object\" &&\n    payload !== null &&\n    \"plugin\" in payload &&\n    typeof payload.plugin === \"string\"\n  ) {\n    return payload.plugin;\n  }\n  throw new Error(\"Runtime plugin event is missing its plugin name.\");\n}\n"
        }
      ],
      "diff": "diff --git a/src/runtime-tools.ts b/src/runtime-tools.ts\nnew file mode 100644\nindex 0000000..c0cde24\n--- /dev/null\n+++ b/src/runtime-tools.ts\n@@ -0,0 +1,144 @@\n+import { Script, createContext } from \"node:vm\";\n+import type { JsonValue } from \"./protocol.js\";\n+import type { Context, Plugin } from \"./runtime.js\";\n+\n+interface DynamicPluginDefinition {\n+  pluginId: string;\n+  name: string;\n+  purpose: string;\n+  code: string;\n+  dispose?: () => void;\n+}\n+\n+export function runtimeToolsPlugin(): Plugin {\n+  return {\n+    name: \"runtime-tools\",\n+    setup(context) {\n+      const definitions = new Map<string, DynamicPluginDefinition>();\n+      let nextPluginId = 1;\n+\n+      context.effect(() => {\n+        for (const definition of [...definitions.values()].reverse()) definition.dispose?.();\n+        definitions.clear();\n+      });\n+\n+      context.contributePrompt(\n+        \"runtime-evolution-guide\",\n+        \"Inspect the current Context, define a small Cordis Plugin when the task needs a new capability, run it, use its tools, then stop or undefine it after the experiment.\",\n+      );\n+      context.registerTool({\n+        name: \"cordis_inspect\",\n+        description: \"Inspect mounted plugins, services, tools, Prompt contributions, listeners, and relations.\",\n+        inputSchema: { type: \"object\", properties: {}, additionalProperties: false },\n+        execute: () => context.inspect() as unknown as JsonValue,\n+      });\n+      context.registerTool({\n+        name: \"cordis_define\",\n+        description: \"Define a new Cordis Plugin from a JavaScript function body that returns the Plugin.\",\n+        inputSchema: {\n+          type: \"object\",\n+          properties: {\n+            name: { type: \"string\", minLength: 1 },\n+            purpose: { type: \"string\", minLength: 1 },\n+            code: { type: \"string\", minLength: 1 },\n+          },\n+          required: [\"name\", \"purpose\", \"code\"],\n+          additionalProperties: false,\n+        },\n+        execute(input) {\n+          const { name, purpose, code } = readDefinition(input);\n+          compilePlugin(code);\n+          const pluginId = `dyn-${nextPluginId}`;\n+          nextPluginId += 1;\n+          definitions.set(pluginId, { pluginId, name, purpose, code });\n+          return { ok: true, pluginId, name, status: \"defined\" };\n+        },\n+      });\n+      context.registerTool({\n+        name: \"cordis_run\",\n+        description: \"Run one previously defined Cordis Plugin in the current Context.\",\n+        inputSchema: pluginIdSchema(),\n+        async execute(input) {\n+          const pluginId = readPluginId(input);\n+          const definition = definitions.get(pluginId);\n+          if (!definition) return { ok: false, error: `Unknown dynamic Plugin: ${pluginId}` };\n+          if (definition.dispose) return { ok: false, error: `${pluginId} is already running.` };\n+          const plugin = instantiatePlugin(definition);\n+          definition.dispose = await context.mount(plugin);\n+          return {\n+            ok: true,\n+            pluginId,\n+            status: \"running\",\n+            tools: context.inspect().tools.map((tool) => tool.name),\n+          };\n+        },\n+      });\n+      context.registerTool({\n+        name: \"cordis_stop\",\n+        description: \"Stop a running dynamic Plugin while retaining its definition.\",\n+        inputSchema: pluginIdSchema(),\n+        execute(input) {\n+          const pluginId = readPluginId(input);\n+          const definition = definitions.get(pluginId);\n+          if (!definition) return { ok: false, error: `Unknown dynamic Plugin: ${pluginId}` };\n+          if (!definition.dispose) return { ok: false, error: `${pluginId} is not running.` };\n+          definition.dispose();\n+          delete definition.dispose;\n+          return { ok: true, pluginId, status: \"stopped\" };\n+        },\n+      });\n+      context.registerTool({\n+        name: \"cordis_undefine\",\n+        description: \"Stop a dynamic Plugin if needed and remove its definition.\",\n+        inputSchema: pluginIdSchema(),\n+        execute(input) {\n+          const pluginId = readPluginId(input);\n+          const definition = definitions.get(pluginId);\n+          if (!definition) return { ok: false, error: `Unknown dynamic Plugin: ${pluginId}` };\n+          definition.dispose?.();\n+          definitions.delete(pluginId);\n+          return { ok: true, pluginId, status: \"undefined\" };\n+        },\n+      });\n+    },\n+  };\n+}\n+\n+function compilePlugin(code: string): Script {\n+  return new Script(`(() => {\\n\"use strict\";\\n${code}\\n})()`);\n+}\n+\n+function instantiatePlugin(definition: DynamicPluginDefinition): Plugin {\n+  const value = compilePlugin(definition.code).runInContext(createContext(Object.create(null)), {\n+    timeout: 100,\n+  }) as unknown;\n+  if (\n+    typeof value !== \"object\" ||\n+    value === null ||\n+    typeof (value as { apply?: unknown }).apply !== \"function\"\n+  ) {\n+    throw new Error(\"Dynamic code must return a Cordis Plugin with apply(context).\");\n+  }\n+  const dynamic = value as { apply(context: Context): void | Promise<void> };\n+  return {\n+    name: `dynamic:${definition.name}`,\n+    setup: (context) => dynamic.apply(context),\n+  };\n+}\n+\n+function pluginIdSchema(): Record<string, unknown> {\n+  return {\n+    type: \"object\",\n+    properties: { pluginId: { type: \"string\", pattern: \"^dyn-[1-9][0-9]*$\" } },\n+    required: [\"pluginId\"],\n+    additionalProperties: false,\n+  };\n+}\n+\n+function readDefinition(input: JsonValue): { name: string; purpose: string; code: string } {\n+  return input as { name: string; purpose: string; code: string };\n+}\n+\n+function readPluginId(input: JsonValue): string {\n+  return (input as { pluginId: string }).pluginId;\n+}\n",
      "diffStats": {
        "filesChanged": 1,
        "additions": 144,
        "deletions": 0,
        "files": [
          {
            "path": "src/runtime-tools.ts",
            "additions": 144,
            "deletions": 0
          }
        ]
      },
      "requests": [
        {
          "step": 1,
          "request": {
            "system": "Inspect the current Context first. If the task needs a new capability, define and run a Cordis plugin, then stop or remove it after verification.",
            "tools": [
              {
                "name": "cordis_inspect",
                "description": "List the plugins and capabilities in the current Context.",
                "inputSchema": {
                  "type": "object",
                  "properties": {},
                  "additionalProperties": false
                }
              },
              {
                "name": "cordis_define",
                "description": "Register Cordis plugin code written by the Agent.",
                "inputSchema": {
                  "type": "object",
                  "properties": {},
                  "additionalProperties": false
                }
              },
              {
                "name": "cordis_run",
                "description": "Run a defined Cordis plugin.",
                "inputSchema": {
                  "type": "object",
                  "properties": {},
                  "additionalProperties": false
                }
              },
              {
                "name": "cordis_stop",
                "description": "Stop the plugin and retain its definition.",
                "inputSchema": {
                  "type": "object",
                  "properties": {},
                  "additionalProperties": false
                }
              },
              {
                "name": "cordis_undefine",
                "description": "Stop the plugin and delete its definition.",
                "inputSchema": {
                  "type": "object",
                  "properties": {},
                  "additionalProperties": false
                }
              }
            ],
            "messages": [
              {
                "role": "user",
                "content": "Count the words in 'one two three', then restore the original capabilities."
              }
            ],
            "dynamicContext": "Inspect the current capabilities and define the word_count plugin."
          },
          "parts": [
            {
              "id": "system",
              "kind": "system",
              "stability": "stable",
              "label": "System prompt",
              "value": "Inspect the current Context first. If the task needs a new capability, define and run a Cordis plugin, then stop or remove it after verification.",
              "approximateTokens": 12
            },
            {
              "id": "tools",
              "kind": "tools",
              "stability": "stable",
              "label": "Tool schemas",
              "value": [
                {
                  "name": "cordis_inspect",
                  "description": "List the plugins and capabilities in the current Context.",
                  "inputSchema": {
                    "type": "object",
                    "properties": {},
                    "additionalProperties": false
                  }
                },
                {
                  "name": "cordis_define",
                  "description": "Register Cordis plugin code written by the Agent.",
                  "inputSchema": {
                    "type": "object",
                    "properties": {},
                    "additionalProperties": false
                  }
                },
                {
                  "name": "cordis_run",
                  "description": "Run a defined Cordis plugin.",
                  "inputSchema": {
                    "type": "object",
                    "properties": {},
                    "additionalProperties": false
                  }
                },
                {
                  "name": "cordis_stop",
                  "description": "Stop the plugin and retain its definition.",
                  "inputSchema": {
                    "type": "object",
                    "properties": {},
                    "additionalProperties": false
                  }
                },
                {
                  "name": "cordis_undefine",
                  "description": "Stop the plugin and delete its definition.",
                  "inputSchema": {
                    "type": "object",
                    "properties": {},
                    "additionalProperties": false
                  }
                }
              ],
              "approximateTokens": 170
            },
            {
              "id": "message-0",
              "kind": "message",
              "stability": "append-only",
              "label": "user",
              "value": {
                "role": "user",
                "content": "Count the words in 'one two three', then restore the original capabilities."
              },
              "approximateTokens": 16
            },
            {
              "id": "dynamic",
              "kind": "dynamic",
              "stability": "step-variable",
              "label": "Step context",
              "value": "Inspect the current capabilities and define the word_count plugin.",
              "approximateTokens": 6
            }
          ],
          "totalApproximateTokens": 204,
          "prefix": {
            "sharedParts": 0,
            "sharedApproximateTokens": 0,
            "previousParts": 0,
            "currentParts": 4,
            "firstInvalidation": "First request has no predecessor.",
            "note": "Teaching estimate only; no provider cache was queried."
          }
        },
        {
          "step": 2,
          "request": {
            "system": "Inspect the current Context first. If the task needs a new capability, define and run a Cordis plugin, then stop or remove it after verification.",
            "tools": [
              {
                "name": "cordis_inspect",
                "description": "List the plugins and capabilities in the current Context.",
                "inputSchema": {
                  "type": "object",
                  "properties": {},
                  "additionalProperties": false
                }
              },
              {
                "name": "cordis_define",
                "description": "Register Cordis plugin code written by the Agent.",
                "inputSchema": {
                  "type": "object",
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                "name": "cordis_run",
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                  "type": "object",
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                }
              },
              {
                "name": "cordis_stop",
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              },
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                "name": "cordis_undefine",
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                  "type": "object",
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              },
              {
                "name": "word_count",
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            }
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    },
    {
      "id": "chapter-6",
      "number": "06",
      "shortTitle": "Continuing Long-Running Tasks",
      "title": "Advance a long-running task with Goals and Rounds",
      "question": "How does DSH keep long-running tasks moving to completion?",
      "codeGuide": {
        "title": "Let a Goal decide whether work continues outside a Turn",
        "description": "LongTaskRunner stores a Goal and starts Rounds in sequence. Each Round reuses the standard Agent Loop; the outer layer uses a structured result to decide whether work continues, completes, blocks, or reaches its limit.",
        "observations": [
          {
            "title": "Goal keeps cross-Turn state in one place",
            "text": "The constructor fixes the objective, status, Round count, and reason. When run begins, it first writes the Goal creation event to the Session Log.",
            "lines": [
              33,
              60
            ]
          },
          {
            "title": "Every Round has a number and explicit input",
            "text": "LongTaskRunner continues only while the Goal is active. It first checks the Round count and phase definition, then records the start of the Round, and finally calls the externally supplied runRound function. runRound executes the current Round; in this tutorial, it starts a standard Agent Turn.",
            "lines": [
              62,
              82
            ]
          },
          {
            "title": "Continuation requires observable progress",
            "text": "completed, blocked, no progress, and max-rounds are explicit exits. The status remains active and another Round begins only when the current Round makes progress without completing the Goal.",
            "lines": [
              83,
              114
            ]
          }
        ],
        "fills": [
          {
            "label": "Type declarations and the LongTaskRunner shape",
            "kind": "skeleton",
            "ranges": [
              [
                1,
                33
              ],
              [
                115,
                116
              ]
            ]
          },
          {
            "label": "Initialize the Goal and record its creation",
            "kind": "body",
            "ranges": [
              [
                34,
                61
              ]
            ]
          },
          {
            "label": "Start the next Round through the ordinary Agent Loop",
            "kind": "body",
            "ranges": [
              [
                62,
                82
              ]
            ]
          },
          {
            "label": "Finish or continue the Goal based on the result",
            "kind": "body",
            "ranges": [
              [
                83,
                114
              ]
            ]
          }
        ]
      },
      "changeStory": {
        "title": "A Goal, not an individual Turn, decides whether work continues",
        "summary": "Nano DSH stores a Goal outside the Agent Loop and starts ordinary Turns repeatedly, once per Round. After each Round, LongTaskRunner uses progressed, completed, and blockedReason to decide whether work continues, completes, blocks, or reaches its limit. Full DeepSeek Harness uses a persistent Goal and Goal Round Driver to continue work in the same Session.",
        "harnessRole": "Coordination layer: let finite Turns advance a long-running Goal together",
        "connection": "Each Round uses the Harness assembled in the first five chapters. Chapter 1 executes Steps, Chapter 2 builds requests, Chapter 3 provides the plugin lifecycle, Chapter 4 continues the record, and Chapter 5 lets the Agent write new capabilities. Chapter 6 coordinates multiple standard Turns through a Goal and its Rounds.",
        "outcomes": [
          "Explain the four control layers in order: Goal, Round, Turn, and Step",
          "Explain how the coordinator uses observable results to decide whether to continue",
          "Identify the explicit exits for completed, blocked, no progress, and max-rounds"
        ]
      },
      "lesson": "# Let the Task Continue Past a Turn\n\nChapter 5 can add a tool during a task. A Turn still ends when the current model stops calling tools. The task may remain incomplete: after one round of organizing three release notes, the Agent may have collected only two. A different layer has to decide whether work should continue.\n\nDSH manages long-term objectives outside the Agent Loop. **A Goal decides whether cross-Turn work continues; Turns and Steps continue to handle model requests and tool execution.** A Goal holds state that spans rounds, while a Round is one new attempt to advance it. These four layers check their own stop conditions. A lack of Tool Calls therefore does not directly mean the Goal is complete.\n\n`long-task.ts` defines Goal state, the input and result of each Round, and the outline of `LongTaskRunner`. It holds only an objective, session, and `runRound` callback. The caller keeps reusing the specific Agent, Context, and workspace on each Round. This boundary lets long-task coordination do just what it needs to do: decide whether to start another ordinary Turn.\n\n<!-- fill {\"index\":0} -->\n\n## 6.1 A Goal Needs State That Can Be Checked Across Rounds\n\nThe `LongTaskRunner` constructor fixes the objective, maximum Round count, initial status, and the definitions for later Rounds. At the start of `run()`, it writes `goal/created` to the Session Log. **The Goal enters the replayable flow of facts from the beginning.** No model request happens here; the code is establishing the shared boundary for work that spans several Rounds.\n\nKeeping this state in `LongTaskState` has two benefits. The outer layer can read a consistent status, reason, and number of started Rounds after each attempt. The page and recovery logic can also find the moment the Goal was created in the log. The source keeps initialization and the creation event together so the state object and recorded fact remain aligned.\n\n<!-- fill {\"index\":1} -->\n\n## 6.2 The Next Round Still Calls the Ordinary Agent Loop\n\nWhile the Goal is `active`, the runner checks the Round limit and whether another Round definition exists. If both checks pass, it increments the Round count, writes `goal/round-started`, and calls `runRound`. The callback can reuse the Agent, Context, workspace, and Session Log from earlier Rounds. Messages, Tool Results, and plugin changes recorded in previous chapters are therefore still available.\n\nThis layer does not duplicate Chapter 1's tool loop. **A Goal can start a new Turn without rewriting the Agent Loop.** It draws a Round boundary for the next Turn; the ordinary Agent Loop continues to handle model requests, Tool Calls, and feedback. Separating the loops lets the model's decision to stop calling tools and the Goal's decision to continue progress be evaluated independently.\n\n<!-- fill {\"index\":2} -->\n\n<!-- evidence {\"id\":\"m06-round-history\",\"tab\":\"request\",\"requestStep\":2,\"label\":\"How one Round's progress enters the next\",\"description\":\"Compare the two requests to confirm that the Goal, first-round result, and second-round input appear in order within one Session.\"} -->\n\n## 6.3 Continuation Must Use Observable Results\n\n`runRound` returns a `RoundResult`. `completed` reports that the Goal is achieved, `blockedReason` says why it cannot continue, and `progressed` reports whether the Round made the intended progress. The runner uses these values to finish the Goal, mark it blocked, enforce its Round limit, or retain `active` and begin another Round. Every exit writes `goal/status-changed`, so state transitions enter the same session record.\n\nThe source keeps these branches with `#finish()` so that ending a Goal always updates in-memory state and the Session Log together. **The loop continues only when the current Round made progress, the Goal remains incomplete, and the limit has not been reached.** That condition imposes a more useful constraint on long-running work than simply calling the model again.\n\n<!-- fill {\"index\":3} -->\n\n<!-- evidence {\"id\":\"m06-completed\",\"tab\":\"events\",\"eventType\":\"goal/status-changed\",\"eventOccurrence\":1,\"label\":\"Completion closes the Goal\",\"description\":\"Inspect Goal creation, the two Round starts, and the completed status event to see how active work becomes complete.\"} -->\n\n## 6.4 The Six Chapters on One DSH Runtime Path\n\nOne Step completes a model-and-tool round-trip in the Agent Loop. The context layer decides which information that Step sees. The plugin runtime decides what capabilities currently exist. The Session Log saves inputs, actions, and capability changes as shared facts. Runtime Tools add a needed capability during work. Goals and Rounds decide whether work needs another Turn after the current one ends.\n\nThis relationship shows that DSH reaches beyond a single model call. It organizes execution, capabilities, records, and Goal state into a runtime that can be inspected, replaced, and continued. Nano DSH retains the smallest set of these relationships; full DSH adds configuration layers, persistence, permissions, sandboxes, and more runtime providers.\n",
      "source": {
        "path": "src/long-task.ts",
        "content": "import type { SessionLog } from \"./session.js\";\n\nexport type LongTaskStatus = \"active\" | \"completed\" | \"blocked\" | \"max-rounds\";\n\nexport interface LongTaskState {\n  goalId: string;\n  objective: string;\n  status: LongTaskStatus;\n  roundsStarted: number;\n  maxRounds: number;\n  reason: string;\n}\n\nexport interface RoundDefinition {\n  label: string;\n  input: string;\n}\n\nexport interface RoundResult {\n  progressed: boolean;\n  completed?: boolean;\n  blockedReason?: string;\n}\n\nexport interface LongTaskOptions {\n  objective: string;\n  rounds: RoundDefinition[];\n  maxRounds: number;\n  session: SessionLog;\n  runRound(round: RoundDefinition, roundNumber: number): Promise<RoundResult>;\n}\n\nexport class LongTaskRunner {\n  readonly state: LongTaskState;\n  readonly #rounds: RoundDefinition[];\n  readonly #session: SessionLog;\n  readonly #runRound: LongTaskOptions[\"runRound\"];\n\n  constructor(options: LongTaskOptions) {\n    if (options.maxRounds < 1) throw new Error(\"maxRounds must be at least 1.\");\n    this.state = {\n      goalId: \"goal-1\",\n      objective: options.objective,\n      status: \"active\",\n      roundsStarted: 0,\n      maxRounds: options.maxRounds,\n      reason: \"Ready to start.\",\n    };\n    this.#rounds = options.rounds;\n    this.#session = options.session;\n    this.#runRound = options.runRound;\n  }\n\n  async run(): Promise<LongTaskState> {\n    this.#session.append({\n      type: \"goal/created\",\n      goalId: this.state.goalId,\n      objective: this.state.objective,\n      maxRounds: this.state.maxRounds,\n    });\n\n    while (this.state.status === \"active\") {\n      if (this.state.roundsStarted >= this.state.maxRounds) {\n        return this.#finish(\n          \"max-rounds\",\n          `Stopped after the configured ${this.state.maxRounds} rounds.`,\n        );\n      }\n      const definition = this.#rounds[this.state.roundsStarted];\n      if (!definition) {\n        return this.#finish(\"blocked\", \"No next round is defined for the active goal.\");\n      }\n\n      this.state.roundsStarted += 1;\n      this.#session.append({\n        type: \"goal/round-started\",\n        goalId: this.state.goalId,\n        round: this.state.roundsStarted,\n        label: definition.label,\n      });\n      const result = await this.#runRound(definition, this.state.roundsStarted);\n\n      if (result.completed) {\n        return this.#finish(\"completed\", `Completed in round ${this.state.roundsStarted}.`);\n      }\n      if (result.blockedReason) return this.#finish(\"blocked\", result.blockedReason);\n      if (!result.progressed) {\n        return this.#finish(\n          \"blocked\",\n          `Round ${this.state.roundsStarted} produced no observable progress.`,\n        );\n      }\n      if (this.state.roundsStarted >= this.state.maxRounds) {\n        return this.#finish(\n          \"max-rounds\",\n          `Stopped after the configured ${this.state.maxRounds} rounds.`,\n        );\n      }\n      this.state.reason = `Round ${this.state.roundsStarted} made progress; continuing.`;\n    }\n    return structuredClone(this.state);\n  }\n\n  #finish(status: Exclude<LongTaskStatus, \"active\">, reason: string): LongTaskState {\n    this.state.status = status;\n    this.state.reason = reason;\n    this.#session.append({\n      type: \"goal/status-changed\",\n      goalId: this.state.goalId,\n      status,\n      reason,\n    });\n    return structuredClone(this.state);\n  }\n}\n",
        "excerpt": "export class LongTaskRunner {\n  readonly state: LongTaskState;\n  readonly #rounds: RoundDefinition[];\n  readonly #session: SessionLog;\n  readonly #runRound: LongTaskOptions[\"runRound\"];\n\n  constructor(options: LongTaskOptions) {\n    if (options.maxRounds < 1) throw new Error(\"maxRounds must be at least 1.\");\n    this.state = {\n      goalId: \"goal-1\",\n      objective: options.objective,\n      status: \"active\",\n      roundsStarted: 0,\n      maxRounds: options.maxRounds,\n      reason: \"Ready to start.\",\n    };\n    this.#rounds = options.rounds;\n    this.#session = options.session;\n    this.#runRound = options.runRound;\n  }\n\n  async run(): Promise<LongTaskState> {\n    this.#session.append({\n      type: \"goal/created\",\n      goalId: this.state.goalId,\n      objective: this.state.objective,\n      maxRounds: this.state.maxRounds,\n    });\n\n    while (this.state.status === \"active\") {\n      if (this.state.roundsStarted >= this.state.maxRounds) {\n        return this.#finish(\n          \"max-rounds\",\n          `Stopped after the configured ${this.state.maxRounds} rounds.`,\n        );\n      }\n      const definition = this.#rounds[this.state.roundsStarted];\n      if (!definition) {\n        return this.#finish(\"blocked\", \"No next round is defined for the active goal.\");\n      }\n\n      this.state.roundsStarted += 1;\n      this.#session.append({\n        type: \"goal/round-started\",\n        goalId: this.state.goalId,\n        round: this.state.roundsStarted,\n        label: definition.label,\n      });\n      const result = await this.#runRound(definition, this.state.roundsStarted);\n\n      if (result.completed) {\n        return this.#finish(\"completed\", `Completed in round ${this.state.roundsStarted}.`);\n      }\n      if (result.blockedReason) return this.#finish(\"blocked\", result.blockedReason);\n      if (!result.progressed) {\n        return this.#finish(\n          \"blocked\",\n          `Round ${this.state.roundsStarted} produced no observable progress.`,\n        );\n      }\n      if (this.state.roundsStarted >= this.state.maxRounds) {\n        return this.#finish(\n          \"max-rounds\",\n          `Stopped after the configured ${this.state.maxRounds} rounds.`,\n        );\n      }\n      this.state.reason = `Round ${this.state.roundsStarted} made progress; continuing.`;\n    }\n    return structuredClone(this.state);\n  }\n\n  #finish(status: Exclude<LongTaskStatus, \"active\">, reason: string): LongTaskState {\n    this.state.status = status;\n    this.state.reason = reason;\n    this.#session.append({\n      type: \"goal/status-changed\",\n      goalId: this.state.goalId,\n      status,\n      reason,\n    });\n    return structuredClone(this.state);\n  }",
        "startLine": 33,
        "endLine": 114
      },
      "extraFiles": [
        {
          "path": "src/agent.ts",
          "content": "import { Ajv, type ValidateFunction } from \"ajv\";\nimport {\n  DEFAULT_PROJECTION,\n  toolSchemas,\n  type ProjectionSettings,\n} from \"./context.js\";\nimport type {\n  AssistantMessage,\n  JsonValue,\n  Llm,\n  UnifiedRequest,\n} from \"./protocol.js\";\nimport type { Context } from \"./runtime.js\";\nimport {\n  buildRequest,\n  replayTrace,\n  SESSION_LOG,\n  type SessionEvent,\n  type SessionLog,\n  type TraceItem,\n} from \"./session.js\";\n\nexport interface RunTurnResult {\n  finalMessage: AssistantMessage;\n  steps: number;\n  turnId: string;\n  stepIds: string[];\n  trace: TraceItem[];\n  requests: UnifiedRequest[];\n}\n\nexport interface CheckpointProposal {\n  summary: string;\n  coveredThroughEventId: number;\n}\n\nexport interface AgentOptions {\n  llm: Llm;\n  context: Context;\n  systemPrompt: string;\n  dynamicContext?: (step: number) => string;\n  projection?: ProjectionSettings;\n  checkpointBeforeStep?: (input: {\n    step: number;\n    events: readonly SessionEvent[];\n  }) => CheckpointProposal | undefined;\n  maxSteps?: number;\n}\n\nexport class Agent {\n  readonly #llm: Llm;\n  readonly #context: Context;\n  readonly #validators = new Map<string, { schema: string; validate: ValidateFunction }>();\n  readonly #ajv = new Ajv({ allErrors: true, strict: false });\n  readonly #systemPrompt: string;\n  readonly #dynamicContext: (step: number) => string;\n  readonly #maxSteps: number;\n  readonly #projection: ProjectionSettings;\n  readonly #session: SessionLog;\n  readonly #checkpointBeforeStep?: AgentOptions[\"checkpointBeforeStep\"];\n\n  constructor(options: AgentOptions) {\n    this.#llm = options.llm;\n    this.#context = options.context;\n    this.#systemPrompt = options.systemPrompt;\n    this.#dynamicContext = options.dynamicContext ?? ((step) => `Current step: ${step}`);\n    this.#maxSteps = options.maxSteps ?? 8;\n    this.#projection = options.projection ?? DEFAULT_PROJECTION;\n    this.#session = this.#context.use(SESSION_LOG);\n    this.#checkpointBeforeStep = options.checkpointBeforeStep;\n  }\n\n  async runTurn(userInput: string): Promise<RunTurnResult> {\n    const eventStart = this.#session.events.length;\n    const turnId = this.#session.nextTurnId();\n    this.#session.append({ type: \"turn/start\", turnId });\n    this.#session.append({ type: \"user/message\", turnId, content: userInput });\n    const requests: UnifiedRequest[] = [];\n    const stepIds: string[] = [];\n\n    for (let step = 1; step <= this.#maxSteps; step += 1) {\n      const checkpoint = this.#checkpointBeforeStep?.({ step, events: this.#session.events });\n      if (checkpoint) {\n        this.#session.checkpoint(checkpoint.summary, checkpoint.coveredThroughEventId);\n      }\n      const stepId = `${turnId}-step-${step}`;\n      stepIds.push(stepId);\n      this.#session.append({ type: \"step/start\", turnId, stepId, ordinal: step });\n      this.#session.append({\n        type: \"request/header\",\n        stepId,\n        provider: this.#llm.provider,\n        model: this.#llm.model,\n        system: this.#context.compilePrompt(this.#systemPrompt),\n        tools: toolSchemas(this.#context.listTools()),\n        dynamicContext: this.#dynamicContext(step),\n        projection: this.#projection,\n      });\n      const request = buildRequest(this.#session.events, stepId);\n      requests.push(structuredClone(request));\n\n      const response = await this.#llm.complete(request);\n      const assistant = response.message;\n      this.#session.append({ type: \"assistant/message\", stepId, content: assistant.content });\n      for (const call of assistant.toolCalls) {\n        this.#session.append({ type: \"tool/call\", stepId, call });\n      }\n\n      if (assistant.toolCalls.length === 0) {\n        this.#session.append({ type: \"step/end\", stepId, outcome: \"complete\" });\n        this.#session.append({ type: \"turn/end\", turnId, outcome: \"completed\" });\n        return {\n          finalMessage: assistant,\n          steps: step,\n          turnId,\n          stepIds,\n          trace: replayTrace(this.#session.events.slice(eventStart)),\n          requests,\n        };\n      }\n\n      for (const call of assistant.toolCalls) {\n        const result = await this.#executeTool(call.name, call.arguments);\n        this.#session.append({\n          type: \"tool/result\",\n          stepId,\n          toolCallId: call.id,\n          name: call.name,\n          content: JSON.stringify(result),\n        });\n      }\n      this.#session.append({ type: \"step/end\", stepId, outcome: \"tool-calls\" });\n    }\n\n    this.#session.append({ type: \"turn/end\", turnId, outcome: \"max-steps\" });\n    throw new Error(`Agent exceeded maxSteps (${this.#maxSteps}) before finishing the turn.`);\n  }\n\n  async #executeTool(name: string, input: JsonValue): Promise<JsonValue> {\n    const tool = this.#context.getTool(name);\n    if (!tool) return { ok: false, error: `Unknown tool: ${name}` };\n    const schema = JSON.stringify(tool.inputSchema);\n    const cached = this.#validators.get(name);\n    const validate = cached?.schema === schema ? cached.validate : this.#ajv.compile(tool.inputSchema);\n    this.#validators.set(name, { schema, validate });\n    if (!validate?.(input)) {\n      return {\n        ok: false,\n        error: \"Invalid tool arguments\",\n        issues: (validate?.errors ?? []).map((error) => `${error.instancePath || \"/\"} ${error.message}`),\n      };\n    }\n    try {\n      const result = await tool.execute(input);\n      this.#context.emit(\"tool/executed\", { name, input, result });\n      return result;\n    } catch (error) {\n      return { ok: false, error: error instanceof Error ? error.message : String(error) };\n    }\n  }\n}\n"
        },
        {
          "path": "src/context.ts",
          "content": "import type { ModelMessage, ToolMessage, ToolSchema, UnifiedRequest } from \"./protocol.js\";\n\nexport interface ProjectionSettings {\n  maxToolResultChars: number;\n  toolResultHeadChars: number;\n  toolResultTailChars: number;\n}\n\nexport const DEFAULT_PROJECTION: ProjectionSettings = {\n  maxToolResultChars: 720,\n  toolResultHeadChars: 430,\n  toolResultTailChars: 170,\n};\n\nexport interface RequestPart {\n  id: string;\n  kind: \"system\" | \"tools\" | \"message\" | \"dynamic\";\n  stability: \"stable\" | \"append-only\" | \"step-variable\";\n  label: string;\n  value: unknown;\n  approximateTokens: number;\n}\n\nexport interface PrefixComparison {\n  sharedParts: number;\n  sharedApproximateTokens: number;\n  previousParts: number;\n  currentParts: number;\n  firstInvalidation: string | null;\n  note: string;\n}\n\nexport interface ClippedToolResult {\n  content: string;\n  omittedCharacters: number;\n}\n\nexport function projectMessages(\n  messages: ModelMessage[],\n  settings: ProjectionSettings = DEFAULT_PROJECTION,\n): ModelMessage[] {\n  return messages.map((message) => {\n    if (message.role !== \"tool\") return structuredClone(message);\n    const clipped = clipToolResult(message.content, settings);\n    return { ...message, content: clipped.content } satisfies ToolMessage;\n  });\n}\n\nexport function clipToolResult(\n  content: string,\n  settings: ProjectionSettings = DEFAULT_PROJECTION,\n): ClippedToolResult {\n  if (content.length <= settings.maxToolResultChars) {\n    return { content, omittedCharacters: 0 };\n  }\n  const omittedCharacters = Math.max(\n    0,\n    content.length - settings.toolResultHeadChars - settings.toolResultTailChars,\n  );\n  const marker = `\\n[… ${omittedCharacters} characters omitted from the model projection …]\\n`;\n  return {\n    content:\n      content.slice(0, settings.toolResultHeadChars) +\n      marker +\n      content.slice(content.length - settings.toolResultTailChars),\n    omittedCharacters,\n  };\n}\n\nexport function describeRequest(request: UnifiedRequest): RequestPart[] {\n  const parts: RequestPart[] = [\n    part(\"system\", \"system\", \"stable\", \"System prompt\", request.system),\n    part(\"tools\", \"tools\", \"stable\", \"Tool schemas\", request.tools),\n  ];\n  for (const [index, message] of request.messages.entries()) {\n    const suffix = message.role === \"tool\" ? ` · ${message.name}` : \"\";\n    parts.push(\n      part(\n        `message-${index}`,\n        \"message\",\n        \"append-only\",\n        `${message.role}${suffix}`,\n        message,\n      ),\n    );\n  }\n  if (request.dynamicContext) {\n    parts.push(\n      part(\n        \"dynamic\",\n        \"dynamic\",\n        \"step-variable\",\n        \"Step context\",\n        request.dynamicContext,\n      ),\n    );\n  }\n  return parts;\n}\n\nexport function compareRequestPrefix(\n  previous: UnifiedRequest | undefined,\n  current: UnifiedRequest,\n): PrefixComparison {\n  const currentParts = describeRequest(current);\n  if (!previous) {\n    return {\n      sharedParts: 0,\n      sharedApproximateTokens: 0,\n      previousParts: 0,\n      currentParts: currentParts.length,\n      firstInvalidation: \"First request has no predecessor.\",\n      note: \"Teaching estimate only; no provider cache was queried.\",\n    };\n  }\n  const previousParts = describeRequest(previous);\n  let sharedParts = 0;\n  while (\n    sharedParts < previousParts.length &&\n    sharedParts < currentParts.length &&\n    canonical(previousParts[sharedParts]?.value) === canonical(currentParts[sharedParts]?.value)\n  ) {\n    sharedParts += 1;\n  }\n  return {\n    sharedParts,\n    sharedApproximateTokens: currentParts\n      .slice(0, sharedParts)\n      .reduce((total, item) => total + item.approximateTokens, 0),\n    previousParts: previousParts.length,\n    currentParts: currentParts.length,\n    firstInvalidation:\n      sharedParts === Math.min(previousParts.length, currentParts.length)\n        ? null\n        : currentParts[sharedParts]?.label ?? \"Request end\",\n    note: \"Longest identical canonical prefix; approximate tokens, not provider cache usage.\",\n  };\n}\n\nexport function approximateTokens(value: unknown): number {\n  const text = typeof value === \"string\" ? value : canonical(value);\n  return Math.max(1, Math.ceil([...text].length / 4));\n}\n\nexport function toolSchemas(tools: Array<{ name: string; description: string; inputSchema: Record<string, unknown> }>): ToolSchema[] {\n  return tools.map(({ name, description, inputSchema }) => ({ name, description, inputSchema }));\n}\n\nfunction part(\n  id: string,\n  kind: RequestPart[\"kind\"],\n  stability: RequestPart[\"stability\"],\n  label: string,\n  value: unknown,\n): RequestPart {\n  return { id, kind, stability, label, value, approximateTokens: approximateTokens(value) };\n}\n\nfunction canonical(value: unknown): string {\n  return JSON.stringify(sortObject(value));\n}\n\nfunction sortObject(value: unknown): unknown {\n  if (Array.isArray(value)) return value.map(sortObject);\n  if (value !== null && typeof value === \"object\") {\n    return Object.fromEntries(\n      Object.entries(value as Record<string, unknown>)\n        .sort(([left], [right]) => left.localeCompare(right))\n        .map(([key, item]) => [key, sortObject(item)]),\n    );\n  }\n  return value;\n}\n"
        },
        {
          "path": "src/runtime.ts",
          "content": "import type { ToolDefinition } from \"./protocol.js\";\n\nexport class ServiceToken<T> {\n  readonly key = Symbol();\n  constructor(readonly name: string) {}\n}\n\nexport interface Plugin {\n  name: string;\n  setup(context: Context): void | Promise<void>;\n}\n\nexport interface RuntimeInspection {\n  plugins: string[];\n  services: Array<{ name: string; provider: string }>;\n  tools: Array<{ name: string; plugin: string }>;\n  prompts: Array<{ id: string; plugin: string; text: string }>;\n  listeners: Array<{ event: string; plugin: string }>;\n  relations: Array<{ consumer: string; service: string; provider: string }>;\n}\n\ntype Disposer = () => void;\ntype Listener = (payload: unknown) => void;\n\ninterface MountRecord {\n  name: string;\n  effects: Disposer[];\n  state: \"installing\" | \"mounted\" | \"unmounted\";\n}\n\ninterface Owned<T> {\n  owner: string;\n  value: T;\n}\n\nexport class Context {\n  readonly #mounts = new Map<string, MountRecord>();\n  readonly #services = new Map<symbol, Owned<unknown> & { token: ServiceToken<unknown> }>();\n  readonly #tools = new Map<string, Owned<ToolDefinition>>();\n  readonly #prompts = new Map<string, Owned<string>>();\n  readonly #listeners = new Map<string, Set<Owned<Listener>>>();\n  readonly #relations: RuntimeInspection[\"relations\"] = [];\n  #activeMount: MountRecord | null = null;\n\n  async mount(plugin: Plugin): Promise<Disposer> {\n    if (this.#mounts.has(plugin.name)) throw new Error(`Plugin already mounted: ${plugin.name}`);\n    if (this.#activeMount) throw new Error(\"Nested plugin mounting is not supported during setup.\");\n    const record: MountRecord = { name: plugin.name, effects: [], state: \"installing\" };\n    this.#mounts.set(plugin.name, record);\n    this.#activeMount = record;\n    try {\n      await plugin.setup(this);\n      record.state = \"mounted\";\n    } catch (error) {\n      this.#rollback(record);\n      this.#mounts.delete(plugin.name);\n      throw error;\n    } finally {\n      this.#activeMount = null;\n    }\n\n    this.emit(\"runtime/plugin-mounted\", { plugin: plugin.name });\n    return once(() => {\n      if (record.state === \"unmounted\") return;\n      this.emit(\"runtime/plugin-unmounting\", { plugin: plugin.name });\n      this.#rollback(record);\n      this.#mounts.delete(plugin.name);\n      this.#relations.splice(\n        0,\n        this.#relations.length,\n        ...this.#relations.filter(\n          (relation) => relation.consumer !== plugin.name && relation.provider !== plugin.name,\n        ),\n      );\n      this.emit(\"runtime/plugin-unmounted\", { plugin: plugin.name });\n    });\n  }\n\n  effect(cleanup: Disposer): Disposer {\n    const mount = this.#requireMount();\n    const dispose = once(cleanup);\n    mount.effects.push(dispose);\n    return dispose;\n  }\n\n  provide<T>(token: ServiceToken<T>, value: T): Disposer {\n    const owner = this.#requireMount().name;\n    if (this.#services.has(token.key)) throw new Error(`Service already provided: ${token.name}`);\n    this.#services.set(token.key, { owner, value, token: token as ServiceToken<unknown> });\n    return this.effect(() => {\n      const service = this.#services.get(token.key);\n      if (service?.owner === owner) this.#services.delete(token.key);\n    });\n  }\n\n  use<T>(token: ServiceToken<T>): T {\n    const service = this.#services.get(token.key);\n    if (!service) throw new Error(`Missing service: ${token.name}`);\n    const consumer = this.#activeMount?.name;\n    if (consumer) {\n      const relation = { consumer, service: token.name, provider: service.owner };\n      if (!this.#relations.some((item) => sameRelation(item, relation))) this.#relations.push(relation);\n    }\n    return service.value as T;\n  }\n\n  registerTool(tool: ToolDefinition): Disposer {\n    const owner = this.#requireMount().name;\n    if (this.#tools.has(tool.name)) throw new Error(`Tool already registered: ${tool.name}`);\n    this.#tools.set(tool.name, { owner, value: tool });\n    return this.effect(() => {\n      if (this.#tools.get(tool.name)?.owner === owner) this.#tools.delete(tool.name);\n    });\n  }\n\n  contributePrompt(id: string, text: string): Disposer {\n    const owner = this.#requireMount().name;\n    if (this.#prompts.has(id)) throw new Error(`Prompt contribution already registered: ${id}`);\n    this.#prompts.set(id, { owner, value: text });\n    return this.effect(() => {\n      if (this.#prompts.get(id)?.owner === owner) this.#prompts.delete(id);\n    });\n  }\n\n  on(event: string, listener: Listener): Disposer {\n    const owner = this.#requireMount().name;\n    const entry = { owner, value: listener };\n    const listeners = this.#listeners.get(event) ?? new Set<Owned<Listener>>();\n    listeners.add(entry);\n    this.#listeners.set(event, listeners);\n    return this.effect(() => {\n      listeners.delete(entry);\n      if (listeners.size === 0) this.#listeners.delete(event);\n    });\n  }\n\n  emit(event: string, payload: unknown): void {\n    for (const listener of [...(this.#listeners.get(event) ?? [])]) listener.value(payload);\n  }\n\n  listTools(): ToolDefinition[] {\n    return [...this.#tools.values()].map((entry) => entry.value);\n  }\n\n  getTool(name: string): ToolDefinition | undefined {\n    return this.#tools.get(name)?.value;\n  }\n\n  compilePrompt(base = \"\"): string {\n    return [base, ...[...this.#prompts.values()].map((entry) => entry.value)]\n      .filter(Boolean)\n      .join(\"\\n\\n\");\n  }\n\n  inspect(): RuntimeInspection {\n    return {\n      plugins: [...this.#mounts.values()]\n        .filter((mount) => mount.state === \"mounted\")\n        .map((mount) => mount.name),\n      services: [...this.#services.values()].map((entry) => ({\n        name: entry.token.name,\n        provider: entry.owner,\n      })),\n      tools: [...this.#tools.entries()].map(([name, entry]) => ({ name, plugin: entry.owner })),\n      prompts: [...this.#prompts.entries()].map(([id, entry]) => ({\n        id,\n        plugin: entry.owner,\n        text: entry.value,\n      })),\n      listeners: [...this.#listeners.entries()].flatMap(([event, entries]) =>\n        [...entries].map((entry) => ({ event, plugin: entry.owner })),\n      ),\n      relations: structuredClone(this.#relations),\n    };\n  }\n\n  #requireMount(): MountRecord {\n    if (!this.#activeMount) throw new Error(\"Runtime contributions must be registered during plugin setup.\");\n    return this.#activeMount;\n  }\n\n  #rollback(record: MountRecord): void {\n    const failures: unknown[] = [];\n    for (const dispose of [...record.effects].reverse()) {\n      try {\n        dispose();\n      } catch (error) {\n        failures.push(error);\n      }\n    }\n    record.state = \"unmounted\";\n    if (failures.length > 0) throw new AggregateError(failures, `Failed to clean up ${record.name}`);\n  }\n}\n\nfunction once(action: Disposer): Disposer {\n  let done = false;\n  return () => {\n    if (done) return;\n    done = true;\n    action();\n  };\n}\n\nfunction sameRelation(\n  left: RuntimeInspection[\"relations\"][number],\n  right: RuntimeInspection[\"relations\"][number],\n): boolean {\n  return (\n    left.consumer === right.consumer &&\n    left.service === right.service &&\n    left.provider === right.provider\n  );\n}\n"
        },
        {
          "path": "src/session.ts",
          "content": "import { projectMessages, type ProjectionSettings } from \"./context.js\";\nimport type {\n  AssistantMessage,\n  ModelMessage,\n  ToolCall,\n  ToolSchema,\n  UnifiedRequest,\n} from \"./protocol.js\";\nimport { ServiceToken, type Plugin } from \"./runtime.js\";\n\ninterface EventBase {\n  id: number;\n  type: string;\n}\n\nexport type SessionEvent =\n  | (EventBase & { type: \"turn/start\"; turnId: string })\n  | (EventBase & { type: \"user/message\"; turnId: string; content: string })\n  | (EventBase & { type: \"step/start\"; turnId: string; stepId: string; ordinal: number })\n  | (EventBase & {\n      type: \"request/header\";\n      stepId: string;\n      provider: string;\n      model: string;\n      system: string;\n      tools: ToolSchema[];\n      dynamicContext: string;\n      projection: ProjectionSettings;\n    })\n  | (EventBase & { type: \"assistant/message\"; stepId: string; content: string })\n  | (EventBase & { type: \"tool/call\"; stepId: string; call: ToolCall })\n  | (EventBase & {\n      type: \"tool/result\";\n      stepId: string;\n      toolCallId: string;\n      name: string;\n      content: string;\n    })\n  | (EventBase & {\n      type: \"context/checkpoint\";\n      summary: string;\n      coveredThroughEventId: number;\n    })\n  | (EventBase & { type: \"runtime/plugin-mounted\"; plugin: string })\n  | (EventBase & { type: \"runtime/plugin-unmounted\"; plugin: string })\n  | (EventBase & { type: \"step/end\"; stepId: string; outcome: \"tool-calls\" | \"complete\" })\n  | (EventBase & {\n      type: \"turn/end\";\n      turnId: string;\n      outcome: \"completed\" | \"max-steps\" | \"failed\";\n    })\n  | (EventBase & {\n      type: \"goal/created\";\n      goalId: string;\n      objective: string;\n      maxRounds: number;\n    })\n  | (EventBase & {\n      type: \"goal/round-started\";\n      goalId: string;\n      round: number;\n      label: string;\n    })\n  | (EventBase & {\n      type: \"goal/status-changed\";\n      goalId: string;\n      status: \"active\" | \"completed\" | \"blocked\" | \"max-rounds\";\n      reason: string;\n    });\n\nexport type NewSessionEvent = SessionEvent extends infer Event\n  ? Event extends SessionEvent\n    ? Omit<Event, \"id\">\n    : never\n  : never;\n\nexport interface TraceItem {\n  eventId: number;\n  type: SessionEvent[\"type\"];\n  title: string;\n  detail: string;\n}\n\nexport class SessionLog {\n  readonly #events: SessionEvent[] = [];\n  #nextId = 1;\n  #nextTurn = 1;\n\n  get events(): readonly SessionEvent[] {\n    return this.#events;\n  }\n\n  append(event: NewSessionEvent): SessionEvent {\n    const stored = { ...structuredClone(event), id: this.#nextId++ } as SessionEvent;\n    this.#events.push(stored);\n    return stored;\n  }\n\n  nextTurnId(): string {\n    return `turn-${this.#nextTurn++}`;\n  }\n\n  checkpoint(summary: string, coveredThroughEventId: number): SessionEvent {\n    if (coveredThroughEventId >= this.#nextId) {\n      throw new Error(\"A checkpoint may cover only events that already exist.\");\n    }\n    return this.append({ type: \"context/checkpoint\", summary, coveredThroughEventId });\n  }\n}\n\nexport const SESSION_LOG = new ServiceToken<SessionLog>(\"session-log\");\n\nexport function sessionPlugin(log: SessionLog): Plugin {\n  return {\n    name: \"session-log\",\n    setup(context) {\n      context.provide(SESSION_LOG, log);\n      context.on(\"runtime/plugin-mounted\", (payload) => {\n        const plugin = pluginName(payload);\n        log.append({ type: \"runtime/plugin-mounted\", plugin });\n      });\n      context.on(\"runtime/plugin-unmounting\", (payload) => {\n        const plugin = pluginName(payload);\n        log.append({ type: \"runtime/plugin-unmounted\", plugin });\n      });\n    },\n  };\n}\n\nexport function buildRequest(events: readonly SessionEvent[], stepId: string): UnifiedRequest {\n  const headerIndex = events.findIndex(\n    (event) => event.type === \"request/header\" && event.stepId === stepId,\n  );\n  if (headerIndex === -1) throw new Error(`Missing request/header for ${stepId}.`);\n  const header = events[headerIndex];\n  if (header?.type !== \"request/header\") throw new Error(`Invalid request header for ${stepId}.`);\n\n  const prefix = events.slice(0, headerIndex);\n  const checkpoint = [...prefix]\n    .reverse()\n    .find((event): event is Extract<SessionEvent, { type: \"context/checkpoint\" }> =>\n      event.type === \"context/checkpoint\",\n    );\n  const projectedEvents = checkpoint\n    ? prefix.filter((event) => event.id > checkpoint.coveredThroughEventId)\n    : prefix;\n  const messages: ModelMessage[] = [];\n  if (checkpoint) {\n    messages.push({\n      role: \"system\",\n      content: `[Context checkpoint through event ${checkpoint.coveredThroughEventId}]\\n${checkpoint.summary}`,\n    });\n  }\n\n  const callsByStep = new Map<string, ToolCall[]>();\n  for (const event of projectedEvents) {\n    if (event.type === \"tool/call\") {\n      const calls = callsByStep.get(event.stepId) ?? [];\n      calls.push(structuredClone(event.call));\n      callsByStep.set(event.stepId, calls);\n    }\n  }\n  for (const event of projectedEvents) {\n    switch (event.type) {\n      case \"user/message\":\n        messages.push({ role: \"user\", content: event.content });\n        break;\n      case \"assistant/message\":\n        messages.push({\n          role: \"assistant\",\n          content: event.content,\n          toolCalls: callsByStep.get(event.stepId) ?? [],\n        });\n        break;\n      case \"tool/result\":\n        messages.push({\n          role: \"tool\",\n          toolCallId: event.toolCallId,\n          name: event.name,\n          content: event.content,\n        });\n        break;\n      default:\n        break;\n    }\n  }\n\n  return {\n    system: header.system,\n    tools: structuredClone(header.tools),\n    messages: projectMessages(messages, header.projection),\n    dynamicContext: header.dynamicContext,\n  };\n}\n\nexport function requestStepIds(events: readonly SessionEvent[]): string[] {\n  return events\n    .filter((event): event is Extract<SessionEvent, { type: \"request/header\" }> =>\n      event.type === \"request/header\",\n    )\n    .map((event) => event.stepId);\n}\n\nexport function replayTrace(events: readonly SessionEvent[]): TraceItem[] {\n  return events.map((event) => ({\n    eventId: event.id,\n    type: event.type,\n    title: traceTitle(event),\n    detail: traceDetail(event),\n  }));\n}\n\nfunction traceTitle(event: SessionEvent): string {\n  switch (event.type) {\n    case \"request/header\":\n      return `${event.stepId} → ${event.provider}/${event.model}`;\n    case \"tool/call\":\n      return `call ${event.call.name}`;\n    case \"tool/result\":\n      return `result ${event.name}`;\n    case \"runtime/plugin-mounted\":\n    case \"runtime/plugin-unmounted\":\n      return event.plugin;\n    case \"goal/round-started\":\n      return `round ${event.round} · ${event.label}`;\n    default:\n      return event.type;\n  }\n}\n\nfunction traceDetail(event: SessionEvent): string {\n  switch (event.type) {\n    case \"user/message\":\n    case \"assistant/message\":\n      return event.content;\n    case \"tool/call\":\n      return JSON.stringify(event.call.arguments);\n    case \"tool/result\":\n      return event.content;\n    case \"context/checkpoint\":\n      return `${event.summary} (covers through #${event.coveredThroughEventId})`;\n    case \"request/header\":\n      return `${event.tools.length} tools · dynamic=${event.dynamicContext}`;\n    case \"turn/end\":\n    case \"step/end\":\n      return event.outcome;\n    case \"goal/created\":\n      return event.objective;\n    case \"goal/status-changed\":\n      return `${event.status}: ${event.reason}`;\n    default:\n      return \"\";\n  }\n}\n\nfunction pluginName(payload: unknown): string {\n  if (\n    typeof payload === \"object\" &&\n    payload !== null &&\n    \"plugin\" in payload &&\n    typeof payload.plugin === \"string\"\n  ) {\n    return payload.plugin;\n  }\n  throw new Error(\"Runtime plugin event is missing its plugin name.\");\n}\n"
        },
        {
          "path": "src/runtime-tools.ts",
          "content": "import { Script, createContext } from \"node:vm\";\nimport type { JsonValue } from \"./protocol.js\";\nimport type { Context, Plugin } from \"./runtime.js\";\n\ninterface DynamicPluginDefinition {\n  pluginId: string;\n  name: string;\n  purpose: string;\n  code: string;\n  dispose?: () => void;\n}\n\nexport function runtimeToolsPlugin(): Plugin {\n  return {\n    name: \"runtime-tools\",\n    setup(context) {\n      const definitions = new Map<string, DynamicPluginDefinition>();\n      let nextPluginId = 1;\n\n      context.effect(() => {\n        for (const definition of [...definitions.values()].reverse()) definition.dispose?.();\n        definitions.clear();\n      });\n\n      context.contributePrompt(\n        \"runtime-evolution-guide\",\n        \"Inspect the current Context, define a small Cordis Plugin when the task needs a new capability, run it, use its tools, then stop or undefine it after the experiment.\",\n      );\n      context.registerTool({\n        name: \"cordis_inspect\",\n        description: \"Inspect mounted plugins, services, tools, Prompt contributions, listeners, and relations.\",\n        inputSchema: { type: \"object\", properties: {}, additionalProperties: false },\n        execute: () => context.inspect() as unknown as JsonValue,\n      });\n      context.registerTool({\n        name: \"cordis_define\",\n        description: \"Define a new Cordis Plugin from a JavaScript function body that returns the Plugin.\",\n        inputSchema: {\n          type: \"object\",\n          properties: {\n            name: { type: \"string\", minLength: 1 },\n            purpose: { type: \"string\", minLength: 1 },\n            code: { type: \"string\", minLength: 1 },\n          },\n          required: [\"name\", \"purpose\", \"code\"],\n          additionalProperties: false,\n        },\n        execute(input) {\n          const { name, purpose, code } = readDefinition(input);\n          compilePlugin(code);\n          const pluginId = `dyn-${nextPluginId}`;\n          nextPluginId += 1;\n          definitions.set(pluginId, { pluginId, name, purpose, code });\n          return { ok: true, pluginId, name, status: \"defined\" };\n        },\n      });\n      context.registerTool({\n        name: \"cordis_run\",\n        description: \"Run one previously defined Cordis Plugin in the current Context.\",\n        inputSchema: pluginIdSchema(),\n        async execute(input) {\n          const pluginId = readPluginId(input);\n          const definition = definitions.get(pluginId);\n          if (!definition) return { ok: false, error: `Unknown dynamic Plugin: ${pluginId}` };\n          if (definition.dispose) return { ok: false, error: `${pluginId} is already running.` };\n          const plugin = instantiatePlugin(definition);\n          definition.dispose = await context.mount(plugin);\n          return {\n            ok: true,\n            pluginId,\n            status: \"running\",\n            tools: context.inspect().tools.map((tool) => tool.name),\n          };\n        },\n      });\n      context.registerTool({\n        name: \"cordis_stop\",\n        description: \"Stop a running dynamic Plugin while retaining its definition.\",\n        inputSchema: pluginIdSchema(),\n        execute(input) {\n          const pluginId = readPluginId(input);\n          const definition = definitions.get(pluginId);\n          if (!definition) return { ok: false, error: `Unknown dynamic Plugin: ${pluginId}` };\n          if (!definition.dispose) return { ok: false, error: `${pluginId} is not running.` };\n          definition.dispose();\n          delete definition.dispose;\n          return { ok: true, pluginId, status: \"stopped\" };\n        },\n      });\n      context.registerTool({\n        name: \"cordis_undefine\",\n        description: \"Stop a dynamic Plugin if needed and remove its definition.\",\n        inputSchema: pluginIdSchema(),\n        execute(input) {\n          const pluginId = readPluginId(input);\n          const definition = definitions.get(pluginId);\n          if (!definition) return { ok: false, error: `Unknown dynamic Plugin: ${pluginId}` };\n          definition.dispose?.();\n          definitions.delete(pluginId);\n          return { ok: true, pluginId, status: \"undefined\" };\n        },\n      });\n    },\n  };\n}\n\nfunction compilePlugin(code: string): Script {\n  return new Script(`(() => {\\n\"use strict\";\\n${code}\\n})()`);\n}\n\nfunction instantiatePlugin(definition: DynamicPluginDefinition): Plugin {\n  const value = compilePlugin(definition.code).runInContext(createContext(Object.create(null)), {\n    timeout: 100,\n  }) as unknown;\n  if (\n    typeof value !== \"object\" ||\n    value === null ||\n    typeof (value as { apply?: unknown }).apply !== \"function\"\n  ) {\n    throw new Error(\"Dynamic code must return a Cordis Plugin with apply(context).\");\n  }\n  const dynamic = value as { apply(context: Context): void | Promise<void> };\n  return {\n    name: `dynamic:${definition.name}`,\n    setup: (context) => dynamic.apply(context),\n  };\n}\n\nfunction pluginIdSchema(): Record<string, unknown> {\n  return {\n    type: \"object\",\n    properties: { pluginId: { type: \"string\", pattern: \"^dyn-[1-9][0-9]*$\" } },\n    required: [\"pluginId\"],\n    additionalProperties: false,\n  };\n}\n\nfunction readDefinition(input: JsonValue): { name: string; purpose: string; code: string } {\n  return input as { name: string; purpose: string; code: string };\n}\n\nfunction readPluginId(input: JsonValue): string {\n  return (input as { pluginId: string }).pluginId;\n}\n"
        }
      ],
      "diff": "diff --git a/src/long-task.ts b/src/long-task.ts\nnew file mode 100644\nindex 0000000..2a36c38\n--- /dev/null\n+++ b/src/long-task.ts\n@@ -0,0 +1,115 @@\n+import type { SessionLog } from \"./session.js\";\n+\n+export type LongTaskStatus = \"active\" | \"completed\" | \"blocked\" | \"max-rounds\";\n+\n+export interface LongTaskState {\n+  goalId: string;\n+  objective: string;\n+  status: LongTaskStatus;\n+  roundsStarted: number;\n+  maxRounds: number;\n+  reason: string;\n+}\n+\n+export interface RoundDefinition {\n+  label: string;\n+  input: string;\n+}\n+\n+export interface RoundResult {\n+  progressed: boolean;\n+  completed?: boolean;\n+  blockedReason?: string;\n+}\n+\n+export interface LongTaskOptions {\n+  objective: string;\n+  rounds: RoundDefinition[];\n+  maxRounds: number;\n+  session: SessionLog;\n+  runRound(round: RoundDefinition, roundNumber: number): Promise<RoundResult>;\n+}\n+\n+export class LongTaskRunner {\n+  readonly state: LongTaskState;\n+  readonly #rounds: RoundDefinition[];\n+  readonly #session: SessionLog;\n+  readonly #runRound: LongTaskOptions[\"runRound\"];\n+\n+  constructor(options: LongTaskOptions) {\n+    if (options.maxRounds < 1) throw new Error(\"maxRounds must be at least 1.\");\n+    this.state = {\n+      goalId: \"goal-1\",\n+      objective: options.objective,\n+      status: \"active\",\n+      roundsStarted: 0,\n+      maxRounds: options.maxRounds,\n+      reason: \"Ready to start.\",\n+    };\n+    this.#rounds = options.rounds;\n+    this.#session = options.session;\n+    this.#runRound = options.runRound;\n+  }\n+\n+  async run(): Promise<LongTaskState> {\n+    this.#session.append({\n+      type: \"goal/created\",\n+      goalId: this.state.goalId,\n+      objective: this.state.objective,\n+      maxRounds: this.state.maxRounds,\n+    });\n+\n+    while (this.state.status === \"active\") {\n+      if (this.state.roundsStarted >= this.state.maxRounds) {\n+        return this.#finish(\n+          \"max-rounds\",\n+          `Stopped after the configured ${this.state.maxRounds} rounds.`,\n+        );\n+      }\n+      const definition = this.#rounds[this.state.roundsStarted];\n+      if (!definition) {\n+        return this.#finish(\"blocked\", \"No next round is defined for the active goal.\");\n+      }\n+\n+      this.state.roundsStarted += 1;\n+      this.#session.append({\n+        type: \"goal/round-started\",\n+        goalId: this.state.goalId,\n+        round: this.state.roundsStarted,\n+        label: definition.label,\n+      });\n+      const result = await this.#runRound(definition, this.state.roundsStarted);\n+\n+      if (result.completed) {\n+        return this.#finish(\"completed\", `Completed in round ${this.state.roundsStarted}.`);\n+      }\n+      if (result.blockedReason) return this.#finish(\"blocked\", result.blockedReason);\n+      if (!result.progressed) {\n+        return this.#finish(\n+          \"blocked\",\n+          `Round ${this.state.roundsStarted} produced no observable progress.`,\n+        );\n+      }\n+      if (this.state.roundsStarted >= this.state.maxRounds) {\n+        return this.#finish(\n+          \"max-rounds\",\n+          `Stopped after the configured ${this.state.maxRounds} rounds.`,\n+        );\n+      }\n+      this.state.reason = `Round ${this.state.roundsStarted} made progress; continuing.`;\n+    }\n+    return structuredClone(this.state);\n+  }\n+\n+  #finish(status: Exclude<LongTaskStatus, \"active\">, reason: string): LongTaskState {\n+    this.state.status = status;\n+    this.state.reason = reason;\n+    this.#session.append({\n+      type: \"goal/status-changed\",\n+      goalId: this.state.goalId,\n+      status,\n+      reason,\n+    });\n+    return structuredClone(this.state);\n+  }\n+}\n",
      "diffStats": {
        "filesChanged": 1,
        "additions": 115,
        "deletions": 0,
        "files": [
          {
            "path": "src/long-task.ts",
            "additions": 115,
            "deletions": 0
          }
        ]
      },
      "requests": [
        {
          "step": 1,
          "request": {
            "system": "Use a tool when the answer requires external facts. Answer directly after receiving the result.",
            "tools": [],
            "messages": [
              {
                "role": "user",
                "content": "Goal: prepare three release notes.\nThis Round: collect existing information."
              }
            ],
            "dynamicContext": "Round 1 / 2"
          },
          "parts": [
            {
              "id": "system",
              "kind": "system",
              "stability": "stable",
              "label": "System prompt",
              "value": "Use a tool when the answer requires external facts. Answer directly after receiving the result.",
              "approximateTokens": 5
            },
            {
              "id": "tools",
              "kind": "tools",
              "stability": "stable",
              "label": "Tool schemas",
              "value": [],
              "approximateTokens": 1
            },
            {
              "id": "message-0",
              "kind": "message",
              "stability": "append-only",
              "label": "user",
              "value": {
                "role": "user",
                "content": "Goal: prepare three release notes.\nThis Round: collect existing information."
              },
              "approximateTokens": 13
            },
            {
              "id": "dynamic",
              "kind": "dynamic",
              "stability": "step-variable",
              "label": "Step context",
              "value": "Round 1 / 2",
              "approximateTokens": 3
            }
          ],
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          "prefix": {
            "sharedParts": 0,
            "sharedApproximateTokens": 0,
            "previousParts": 0,
            "currentParts": 4,
            "firstInvalidation": "First request has no predecessor.",
            "note": "Teaching estimate only; no provider cache was queried."
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        },
        {
          "step": 2,
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              {
                "role": "user",
                "content": "Goal: prepare three release notes.\nThis Round: collect existing information."
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              {
                "role": "assistant",
                "content": "Two items are ready; one is still missing.",
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              },
              {
                "role": "user",
                "content": "Goal: prepare three release notes.\nThis Round: complete and deliver."
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          },
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            },
            {
              "id": "tools",
              "kind": "tools",
              "stability": "stable",
              "label": "Tool schemas",
              "value": [],
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            },
            {
              "id": "message-0",
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              "stability": "append-only",
              "label": "user",
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                "content": "Goal: prepare three release notes.\nThis Round: collect existing information."
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            {
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                "role": "user",
                "content": "Goal: prepare three release notes.\nThis Round: complete and deliver."
              },
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            },
            {
              "id": "dynamic",
              "kind": "dynamic",
              "stability": "step-variable",
              "label": "Step context",
              "value": "Round 2 / 2",
              "approximateTokens": 3
            }
          ],
          "totalApproximateTokens": 50,
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            "sharedApproximateTokens": 19,
            "previousParts": 4,
            "currentParts": 6,
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            "note": "Longest identical canonical prefix; approximate tokens, not provider cache usage."
          }
        }
      ],
      "events": [
        {
          "id": 1,
          "type": "goal/created",
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          "objective": "prepare three release notes",
          "maxRounds": 2
        },
        {
          "id": 2,
          "type": "goal/round-started",
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        },
        {
          "id": 3,
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          "label": "complete and deliver"
        },
        {
          "id": 4,
          "type": "goal/status-changed",
          "goalId": "goal-1",
          "status": "completed",
          "reason": "All three items have been added to the deliverable."
        }
      ],
      "trace": [
        {
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        },
        {
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          "detail": "completed: all three items have been added to the deliverable."
        }
      ],
      "graphs": []
    }
  ]
}