API Design

Use this skill for contract-first API design across REST, GraphQL, gRPC, tRPC, AsyncAPI, and agent-facing interfaces. It owns contract choice, auth boundaries, versioning, errors, pagination, rate limits, idempotency, and validation; it does not replace backend implementation or security review.

Style Decision Table

Quick Reference

Contract Review Checklist

Run on every API contract before handoff:

• Canonical spec artifact exists (OpenAPI, AsyncAPI, protobuf, GraphQL SDL, or MCP schema)
• Compatibility and versioning model named: evolutionary changes, URL path (/v1/), header, or content-type negotiation
• Deprecation timeline written into the spec or linked doc; Deprecation header uses the RFC 9745 @<unix-seconds> form, Sunset uses RFC 8594
• Error model: for REST/HTTP, use RFC 9457 Problem Details with stable type URI and an application code extension when clients need one; for GraphQL, gRPC, and events, define errors in their native contract
• Auth boundary: which endpoints require which scopes; token type (JWT, opaque); revocation path
• Pagination: cursor-based for high-cardinality; offset only for small, stable sets
• Rate limits: either the IETF httpapi RateLimit / RateLimit-Policy fields or a documented legacy X-RateLimit-* triad; 429 includes Retry-After. The IETF fields have changed shape between drafts: look up the current revision and RFC status on the IETF datatracker and copy the syntax from that revision, and name the revision in the spec
• Idempotency: POST/PATCH operations document idempotency key or mark non-idempotent explicitly; whatever the IETF Idempotency-Key header's status (check datatracker), the spec must define key scope, retention, and the reused-key response
• Long-running jobs: 202 + Location poll URL; state enum with terminal states named
• Webhooks: HMAC signature; replay protection; trace_id on payload
• Breaking-change detection: oasdiff or equivalent configured in CI
• Contract tests: Schemathesis (property-based) or Pact (consumer-driven) wired up

Workflow

1. Choose the API style using the decision table above.
2. Define the canonical contract artifact.
3. Run the contract review checklist.
4. Add contract validation, breaking-change detection, and documentation.
5. Hand off spec, examples, and rollout notes.

Compatibility evidence by change class

Classify each change before calling the contract compatible:

For provider/consumer deployments, derive the safe order from message direction, changed payload side, and actual consumer tolerance. For a new optional request capability, ship provider support before consumers send it. For response enum/event expansion, or whenever a consumer may reject unknown fields or variants, ship and verify tolerant consumers before the provider emits the new value. For removals, ship consumers first, confirm old-field traffic is gone, then remove the provider behavior. Prove the relevant consumer behavior rather than inferring safety from schema additivity; a green schema diff alone is insufficient evidence for semantic compatibility or deploy order.

Route Elsewhere

• Backend implementation: software-backend
• AppSec review and auth hardening: software-security-appsec
• System-wide architecture: software-architecture-design
• Rollout and migration sequencing: dev-workflow-planning

Defaults

• Define the contract before implementation or code generation.
• Use RFC 9457 Problem Details for REST/HTTP errors that need a response body; define native error shapes for other API styles.
• Make versioning, deprecation, idempotency, pagination, and rate limits explicit in the spec.
• Prefer OpenAPI 3.x or AsyncAPI as the canonical source for HTTP or event-driven interfaces; pin the newest minor that every validator, linter, and generator in your pipeline supports.
• Treat agent APIs as domain contracts with clear side effects, not thin wrappers around random endpoints.
• Use MCP as the tool-exposure layer when an agent is the primary consumer. Its authorization model builds on OAuth resource-server standards (RFC 9728 Protected Resource Metadata, RFC 8707 Resource Indicators), but session, handshake, and auth requirements change between spec revisions: look up the current revision at modelcontextprotocol.io before designing around sessions or the initialize handshake. See references/llm-agent-api-contracts.md.

Versioning Strategy Table

Expert Judgment

Versioning strategy — pick based on who controls the client, not team preference.

• If you do not control every client (public API, third-party integrators, mobile apps you cannot force-update), publish a version policy and a support window based on observed consumer use and migration lead time; you cannot silently migrate callers.
• If you control every client (internal service mesh, monorepo with generated clients), prefer evolutionary compatibility (additive fields, deprecate-then-remove) over versioning — a new version number is a coordination tax you don't need to pay.
• Dated versions can be useful when each account or request pins behavior independently. If requests omit a version, resolve to a documented pinned default rather than silently upgrading callers, and echo the resolved version in a response header.
• Never let "evolutionary" become an excuse to skip a compatibility gate — GraphQL and gRPC still need CI-enforced schema diffing (GraphQL Inspector, buf breaking); "no versioning" is not "no discipline."

Rule: rules/api/contracts.md loads this invariant when Claude edits a matching file.

Breaking-change detection instincts — what schema-diff tools structurally cannot catch:

The instances below are all applications of Hyrum's Law: with enough consumers of an API, every observable behavior — not just the documented schema — becomes a de facto contract, whether or not you ever promised it. (Same law, applied to schema deploy-sequencing rather than API surface, in software-database-design/references/migration-strategies.md.) That is why a clean schema diff is not proof of compatibility:

• Semantic changes with no shape change: tightening an existing enum's allowed values, narrowing a previously-permissive validation rule, or changing what a field means while keeping its type — oasdiff/buf breaking will report zero diff.
• Behavioral defaults: changing a default sort order, default page size, or default timeout is a breaking change for callers who rely on the default, even though the schema is untouched.
• Cross-field coupling: a field that used to be optional-but-ignored becoming optional-but-enforced (e.g., a previously-cosmetic region field now affecting routing).
• Rate-limit and quota tightening: not a contract break in the schema sense, but it breaks production traffic identically to a removed field — treat quota changes with the same deprecation-notice discipline as field removal.
• Error-code reclassification: moving a case from 404 to 410, or from a generic code to a more specific one, breaks clients that pattern-match on the old code even though the Problem Details shape is unchanged.
• Treat automated diffing (oasdiff, GraphQL Inspector, buf breaking, Pact) as a floor, not a ceiling — pair it with a changelog review by someone who understands what callers actually depend on.

Hyrum's Law framing adapted from addyosmani/agent-skills (MIT), commit 7676817, 2026-08-09.

When GraphQL, gRPC, or event-driven contracts are the wrong choice:

• GraphQL is wrong for simple CRUD with one client shape, for teams that need HTTP-cache semantics (the GraphQL-over-HTTP spec requires POST support and makes GET optional, so many servers accept only POST; HTTP caching then needs GET plus persisted queries), and for single-team ownership where the query-flexibility payoff is never realized — you inherit N+1 risk, query-complexity DoS surface, and federation tooling cost for no client benefit.
• Native gRPC is a poor default for browser clients, which need gRPC-Web or a compatible transport layer. For public third-party integrators, account for protobuf tooling and transport support; choose REST+OpenAPI when broad HTTP interoperability and discoverability matter more.
• AsyncAPI/event contracts are wrong when the caller needs an immediate, correlated answer to a specific request — forcing request/response workflows through pub/sub adds correlation-ID bookkeeping and timeout ambiguity that plain synchronous HTTP avoids.
• The tell that a style choice was fashion, not fit: nobody on the team can name the specific latency budget, client platform constraint, or multi-team ownership problem the chosen style solves.

Known Traps

• Picking GraphQL, gRPC, or AsyncAPI for architectural fashion instead of actual client, latency, or interoperability constraints.
• Designing happy-path resources without an explicit idempotency and retry story for duplicated or partial-failure requests.
• Letting auth stay implicit until implementation, producing inconsistent enforcement across endpoints.
• Reusing pagination models that leak internal storage semantics into the public contract.
• Treating webhook or event delivery as reliable push without signature validation, replay protection, or consumer backpressure.
• Generating a spec from code after implementation and calling it contract-first.
• Wrapping arbitrary internal endpoints as agent APIs without stable side-effect, auth, and validation rules.

Navigation

Core patterns:

• references/restful-design-patterns.md — HTTP method semantics, URL structure, idempotency
• references/pagination-filtering.md — cursor vs offset, filtering contracts
• references/error-handling-patterns.md — RFC 9457, error taxonomy
• references/authentication-patterns.md — OAuth 2.1, JWT, API keys, mTLS
• references/rate-limiting-patterns.md — headers, 429 handling, burst strategies

Style-specific:

• references/api-design-best-practices.md — cross-style DX and governance
• references/versioning-strategies.md — breaking change detection, deprecation
• references/api-security-checklist.md — OWASP API Top 10 checklist
• references/graphql-patterns.md — schema design, federation, when to adopt a supergraph
• references/grpc-patterns.md — protobuf design, streaming, deadlines
• references/trpc-patterns.md — TypeScript end-to-end type sharing
• references/openapi-guide.md — OpenAPI 3.x authoring, linting, bundling
• references/openapi-32-arazzo-101.md — what OpenAPI 3.2 added over 3.1 (streaming types, QUERY method, OAuth flows) and Arazzo multi-step workflow chains; load when specifying multi-call API sequences or adopting 3.2 features
• references/asyncapi-patterns.md — event-driven contracts, pub/sub
• references/webhook-patterns.md — HMAC signing, replay protection, delivery guarantees
• references/real-time-api-patterns.md — SSE, WebSocket, long-polling tradeoffs
• references/api-testing-patterns.md — contract tests, property-based testing, Schemathesis
• references/llm-agent-api-contracts.md — MCP integration, AX design, agent-first patterns, LLM-consumer rules (summary-stat endpoints, language-first search, payload pruning as accuracy control)

Assets and templates:

• assets/openapi-template.yaml
• assets/spectral-ruleset.yaml — custom Spectral lint ruleset for OpenAPI
• assets/oasdiff-ci.yml — CI workflow for oasdiff breaking-change detection
• Full-stack framework scaffolds (FastAPI, Express, Django REST, Spring Boot) are implementation, not contract design — moved to software-backend: assets/python/template-python-fastapi-sqlalchemy.md, assets/nodejs-express/template-nodejs-express.md, assets/python-django/template-python-django-rest.md, assets/java/template-java-spring-boot.md
• assets/cross-platform/api-patterns-universal.md
• assets/cross-platform/template-api-governance.md
• assets/cross-platform/template-api-design-review-checklist.md
• assets/cross-platform/template-api-error-model.md
• data/sources.json

Related skills:

• software-backend, software-security-appsec, data-sql-optimization, qa-testing-strategy, qa-observability, docs-codebase, docs-ai-prd, dev-workflow-planning, software-architecture-design

Learnings Loop

When prior decisions or pitfalls are relevant, consult learnings.consolidated.md if present; use learnings.md only for needed history or as the available fallback. Otherwise skip both.

After applying it, if you encountered a pattern worth remembering, a mistake worth preventing, or a domain fact that surprised you, append one dated bullet to learnings.md via agents-skills-feedback-loop/scripts/append_learning.py. Do not modify SKILL.md itself.