Agent Skills: API Design Patterns

API Design Patterns

Critical rules

• Handlers only translate HTTP ↔ domain: validate at the edge, call fn(args, deps) / Result, map to response. No business logic in handlers.
• One error envelope everywhere; ORPCError(code) must match ErrorResponse.code.
• Shared domain-error → status mapping; /health and /ready; X-Request-ID in middleware once.
• Mutations require Idempotency-Key; 429 includes Retry-After; drain with 503 on shutdown.
• One file per route with co-located tests; compose routers via factories.
• Before implementing routes or ops concerns, read references/patterns.md.

Workflow

1. Define Zod input/output schemas at the framework boundary.
2. Create a route factory createX({ deps }) that delegates to domain functions.
3. Map domain errors through the shared status table and consistent envelope.
4. Add health/ready, CORS, request ID middleware, idempotency, rate limits, shutdown — see references/patterns.md.
5. Compose routers at module boundaries (createPostsRouter → createApiRouter).
6. Write co-located tests with call() — see references/testing.md.

Resources

• references/patterns.md — factories, errors, health, CORS, idempotency, organization. Read when designing endpoints.
• references/testing.md — route unit tests with mocked deps. Read when writing route tests.

Validation

• [ ] Routes are factories with injected deps; handlers stay thin
• [ ] Errors use one envelope; codes match
• [ ] Shared status mapping; /health and /ready exist
• [ ] Mutations honor Idempotency-Key; 429 has Retry-After; shutdown → 503
• [ ] X-Request-ID set in middleware
• [ ] Co-located success and error tests per route

Constraints

• Internal calls and background jobs without HTTP are out of scope. Retry policy belongs in resilience.
• Related: fn-args-deps, validation-boundary, result-types, resilience, observability.