API Design

api-design

Grounded corpus (Tier-1 consultation): pagination, versioning, error shape (RFC 9457), idempotency, async ops, rate limiting, bulk, naming, expansion, webhooks — query ./scripts-run <skills-root>/corpus-grounding/scripts/ground search --manifest <skills-root>/api-design/data/manifest.json "<concern>" and propose the grounded pattern (+ hardening + anti-patterns + RFC link) before designing from memory. Corpus: data/api-patterns.csv.

When to use

Use this skill when designing new API endpoints, restructuring existing APIs, or deciding about versioning and deprecation.

Do NOT use when:

• Implementing an already-designed endpoint (use api-endpoint skill)
• Writing tests for APIs (use api-testing skill)

Procedure: Design an API

1. Gather context — read agents/settings/contexts/api-versioning.md, agents/reference/docs/api-resources.md, agents/reference/docs/query-filter.md, agents/reference/docs/controller.md, and guideline php/api-design.md.
2. Identify the resource — determine the domain entity, its attributes, and relationships. Check existing models and resources for field naming patterns.
3. Define endpoints — list each endpoint with HTTP method, URL path, request body, query parameters, and response structure. Follow existing route file patterns.
4. Decide versioning — determine whether this extends the current version or requires a new version (see decision table below).
5. Design error responses — define 4xx/5xx responses matching the project's existing error format.
6. Validate against existing patterns — compare your design with 2-3 similar existing endpoints. Flag any inconsistencies.
7. Run adversarial review — use adversarial-review skill to check for breaking changes, consistency issues, and missing error cases.

Versioning decisions

URL-based versioning

Routes versioned via URL prefix: /api/v1/..., /api/v2/...

routes/api/v1/projects.php  → /api/v1/projects   (Laravel)
app/api/v1/projects/route.ts → /api/v1/projects   (Next.js)
routes/api/v2/projects.php  → /api/v2/projects

Automatic fallback

If a route doesn't exist in the requested version, the system falls back to the next older version. Configured in the framework's app config (config/app.php in Laravel):

'api_versioning' => [
    'versions' => 'v2,v1',  // newest first
],

When to create a new version

If an existing client would break without code changes → new version required.

Deprecation workflow

1. Mark as deprecated — add headers: Deprecation: true, Sunset: YYYY-MM-DD, Link: <successor>
2. Document — add to API changelog with sunset date
3. Monitor usage — track clients still using deprecated endpoints
4. Remove — after sunset date, remove route + controller + docs

Minimum 3 months between deprecation and removal.

Design review

Before presenting an API design, run the adversarial-review skill. Focus on: Breaking changes? Consistency? Error responses?

Output format

1. Endpoint specification — method, path, request/response structure
2. Versioning decision with rationale
3. Error response format following existing project patterns

Gotcha

• Consistency beats "better" design — check existing patterns first.
• Always include pagination on list endpoints.
• Max nesting depth: 2 levels (/users/{id}/orders/{id}).
• Don't version internal APIs only your own frontend consumes.
• Deprecation without migration path is useless — always provide the replacement.
• Don't duplicate controllers for new versions — use fallback logic.

Do NOT

• Do NOT introduce a new response format in an established API — match existing patterns.
• Do NOT create v2 endpoints without a deprecation plan for v1.
• Do NOT skip pagination on list endpoints.

Auto-trigger keywords

• API design
• REST API
• endpoint design
• resource structure
• response format
• API versioning
• deprecation
• breaking changes