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api-design

Skill8 starsChanged 2 months agoMITView on GitHub

Use when designing or reviewing an API surface — REST, GraphQL, or gRPC. Contract-first methodology: the contract artifact (OpenAPI 3.1, SDL, proto3) is written and reviewed BEFORE implementation, lives in the repo, and Phase 6 verifies implementation matches it.

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SKILL.md

API Design — the contract IS the spec

Contract-first: write the machine-readable contract, review it like code, implement to it. Never code-first with docs generated after.

Style selection (decision table — pick ONE, record an ADR)

Use caseStyle
Public/partner API, resource-shaped, broad client baseREST + OpenAPI 3.1
Product frontend with many views over the same graph, client-driven field selectionGraphQL + SDL
Internal service-to-service, low latency, strong typing across languagesgRPC + proto3

Ambiguous → one question to the human with the trade-off, never two styles at once without justification.

REST rules (OpenAPI 3.1)

  • Contract file in-repo: api/openapi.yaml (or the repo's existing convention). Reviewed in the same PR discipline as code.
  • Resources are plural nouns; no verbs in paths (POST /orders, not /createOrder). Consistent casing (pick kebab or snake for paths, camel for JSON; record it).
  • Methods carry semantics: GET safe, PUT/DELETE idempotent, POST for creation/actions. Idempotency keys for payment-shaped POSTs.
  • Errors: RFC 9457 application/problem+json — every operation lists its error responses in the contract. No bare 500s as design.
  • Pagination: cursor-based by default; offset only with a written justification (deep-page cost). Filtering/sorting as documented query params.
  • Versioning: choose URL prefix (/v1) or header once, record the ADR, never mix.
  • AuthN/authZ in the contract: securitySchemes + per-operation scopes/roles. An operation without a declared auth requirement is a finding, not an oversight.

GraphQL rules (SDL)

  • Schema file in-repo. Nullability is design: non-null by intent, not by default.
  • Typed errors (union results or errors interface) over throwing strings.
  • Pagination: Relay-style connections. N+1: name the dataloader plan at design time.
  • Depth/complexity limits stated in the contract docs (abuse surface).

gRPC rules (proto3)

  • Proto files versioned in-repo; packages versioned (v1).
  • Field numbers are forever: reserve removed numbers, never reuse.
  • Deadlines/timeouts and idempotency noted per RPC; streaming only with a stated reason.

Verification hook (Phase 6)

The implementation must match the contract: routes/fields/status codes vs the contract file (drift check — route list diff, schema validation of real responses where the test suite allows). Contract drift found in verify = the run is not done.

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