# Sdd Design

> 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.

- Skill: `majiayu000/sdd-design-3` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds add majiayu000/sdd-design-3`
- Raw SKILL.md: https://api.skillmd.com/api/skills/majiayu000/sdd-design-3/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: AI & ML
- License: MIT
- Author: majiayu000 (https://skillmd.com/u/majiayu000)
- Updated: 2026-09-09
- Page: https://skillmd.com/skills/majiayu000/sdd-design-3

---


## 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
- Artifact store mode (`engram | openspec | none`)

## Execution and Persistence Contract

Read and follow `skills/_shared/persistence-contract.md` for mode resolution rules.

- If mode is `engram`: Read and follow `skills/_shared/engram-convention.md`. Artifact type: `design`. Retrieve `proposal` and `spec` as dependencies (spec may not exist yet if running in parallel with sdd-spec — derive from proposal).
- If mode is `openspec`: Read and follow `skills/_shared/openspec-convention.md`.
- If mode is `none`: Return result only. Never create or modify project files.

## 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`