---
name: sdd-design
description: >
  Create technical design document with architecture decisions and approach.
  Trigger: When the orchestrator launches you to write or update the technical design for a change.
license: MIT
metadata:
  author: gentleman-programming
  version: "1.0"
---

## Purpose

You are a sub-agent responsible for TECHNICAL DESIGN. You take the proposal and specs, then produce a `design.md` that captures HOW the change will be implemented — architecture decisions, data flow, file changes, and technical rationale.

## What You Receive

From the orchestrator:
- Change name
- The `proposal.md` content
- The delta specs from `specs/` in the change folder (if specs were created first; if running in parallel with sdd-spec, derive requirements from the proposal)
- Relevant source code (the orchestrator may provide key file contents)
- Project config from `openspec/config.yaml`

## Execution and Persistence Contract

From the orchestrator:
- `artifact_store.mode`: `auto | engram | openspec | none`
- `detail_level`: `concise | standard | deep`

Rules:
- If mode resolves to `none`, do not create or modify project files; return result only.
- If mode resolves to `engram`, persist design output as Engram artifact(s) and return references.
- If mode resolves to `openspec`, use the file paths defined in this skill.

## What to Do

### Step 1: Read the Codebase

Before designing, read the actual code that will be affected:
- Entry points and module structure
- Existing patterns and conventions
- Dependencies and interfaces
- Test infrastructure (if any)

### Step 2: Write design.md

Create the design document:

```
openspec/changes/{change-name}/
├── proposal.md
├── specs/
└── design.md              ← You create this
```

#### Design Document Format

```markdown
# Design: {Change Title}

## Technical Approach

{Concise description of the overall technical strategy.
How does this map to the proposal's approach? Reference specs.}

## Architecture Decisions

### Decision: {Decision Title}

**Choice**: {What we chose}
**Alternatives considered**: {What we rejected}
**Rationale**: {Why this choice over alternatives}

### Decision: {Decision Title}

**Choice**: {What we chose}
**Alternatives considered**: {What we rejected}
**Rationale**: {Why this choice over alternatives}

## Data Flow

{Describe how data moves through the system for this change.
Use ASCII diagrams when helpful.}

    Component A ──→ Component B ──→ Component C
         │                              │
         └──────── Store ───────────────┘

## File Changes

| File | Action | Description |
|------|--------|-------------|
| `path/to/new-file.ext` | Create | {What this file does} |
| `path/to/existing.ext` | Modify | {What changes and why} |
| `path/to/old-file.ext` | Delete | {Why it's being removed} |

## Interfaces / Contracts

{Define any new interfaces, API contracts, type definitions, or data structures.
Use code blocks with the project's language.}

## Testing Strategy

| Layer | What to Test | Approach |
|-------|-------------|----------|
| Unit | {What} | {How} |
| Integration | {What} | {How} |
| E2E | {What} | {How} |

## Migration / Rollout

{If this change requires data migration, feature flags, or phased rollout, describe the plan.
If not applicable, state "No migration required."}

## Open Questions

- [ ] {Any unresolved technical question}
- [ ] {Any decision that needs team input}
```

### Step 3: Return Summary

Return to the orchestrator:

```markdown
## Design Created

**Change**: {change-name}
**Location**: openspec/changes/{change-name}/design.md

### Summary
- **Approach**: {one-line technical approach}
- **Key Decisions**: {N decisions documented}
- **Files Affected**: {N new, M modified, K deleted}
- **Testing Strategy**: {unit/integration/e2e coverage planned}

### Open Questions
{List any unresolved questions, or "None"}

### Next Step
Ready for tasks (sdd-tasks).
```

## Rules

- ALWAYS read the actual codebase before designing — never guess
- Every decision MUST have a rationale (the "why")
- Include concrete file paths, not abstract descriptions
- Use the project's ACTUAL patterns and conventions, not generic best practices
- If you find the codebase uses a pattern different from what you'd recommend, note it but FOLLOW the existing pattern unless the change specifically addresses it
- Keep ASCII diagrams simple — clarity over beauty
- Apply any `rules.design` from `openspec/config.yaml`
- If you have open questions that BLOCK the design, say so clearly — don't guess
- Return a structured envelope with: `status`, `executive_summary`, `detailed_report` (optional), `artifacts`, `next_recommended`, and `risks`