# Architecture

`fullstack-expert` is a static Cordis plugin mounted into an existing Harness composition. It is not a second agent loop.

## Planes

- **Host-native capabilities:** filesystem, shell, jobs, sandbox, approval, session persistence, model routing, and browser support remain owned by Harness rows.
- **Agent/preset contribution:** this plugin adds one scoped methodology section, one dynamic state context, embedded skills, two tools, and an optional monotonic guard.
- **Private runtime seam:** `FullstackExpertRuntime` owns deterministic plan/check state and keeps only scalar projections. It does not serialize live Services, Sessions, Agents, Tool executions, or credentials.

## Interfaces

- `inspectProject(fs, options)` — bounded read-only ProjectModel builder over `ctx.fs`.
- `createPlan(input)` — pure request-to-vertical-slice planner.
- `checkProject(fs, input)` — bounded read-only checks with pass/warn/unknown results.
- `applyEvidence(state, evidence)` — immutable evidence transition; VERIFIED requires a passing verification record.
- `fullstack_plan` / `fullstack_check` — small model-facing interfaces; existing native tools perform edits, commands, browser actions, and approvals.

The plugin observes `session/event` and `tools/result` for compact facts, but the native session log remains the durable source of truth. There is no competing event bus or persistence format.

## Composition

The supplied `cordis.patch.yml` inserts the plugin row after the profile's base bundle. End users can install that layer with `dsh plugin --profile <name> add @deepseek-ai/fullstack-expert`; see the [quickstart](../QUICKSTART.md). It assumes `dsh-tools`, `dsh-skill`, `dsh-system-prompt`, and an optional `dsh-fs` implementation already exist. If the filesystem capability is absent, the plugin still mounts but plan/check tools fail closed with an actionable error.