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)
• Versioning model named: URL path (/v1/), header, or content-type negotiation
• Deprecation timeline written into the spec or linked doc
• Error model: RFC 9457 Problem Details with stable type URI and code field
• 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: RateLimit + RateLimit-Policy headers (current IETF httpapi draft, still not an RFC as of July 2026) or documented legacy X-RateLimit-* triad; 429 includes Retry-After
• Idempotency: POST/PATCH operations document idempotency key or mark non-idempotent explicitly
• 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.

Route Elsewhere

• Backend implementation: [software-backend](../software-backend/SKILL.md)
• AppSec review and auth hardening: [software-security-appsec](../software-security-appsec/SKILL.md)
• System-wide architecture: [software-architecture-design](../software-architecture-design/SKILL.md)
• Rollout and migration sequencing: [dev-workflow-planning](../dev-workflow-planning/SKILL.md)

Defaults

• Define the contract before implementation or code generation.
• Use RFC 9457 Problem Details with stable machine-readable error codes.
• Make versioning, deprecation, idempotency, pagination, and rate limits explicit in the spec.
• Prefer OpenAPI 3.2.0 or AsyncAPI as the canonical source for HTTP or event-driven interfaces.
• Treat agent APIs as domain contracts with clear side effects, not thin wrappers around random endpoints.
• MCP is the standard tool-exposure layer for agent consumers; OAuth 2.1 (servers as formal OAuth 2.1 resource servers, RFC 9728 Protected Resource Metadata, RFC 8707 Resource Indicators) is the direction locked into the 2026-07-28 MCP spec — final publication date, still forthcoming as of this writing; verify current status before depending on it.

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), version explicitly (URL path or dated header à la Stripe) and support N-1 for a published window — undo cost after ship is high because 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 (2026-07-11 style, not v3) beat integer majors once you have more than a handful of releases: they let you pin per-account instead of forcing a global cutover, and the date itself communicates recency without a changelog lookup.
• 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."

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](../software-database-design/references/migration-strategies.md#hyrums-law-and-the-adds-vs-drops-rule).) 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 (GraphQL responses are POST-only and cache-hostile by default), 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.
• gRPC is wrong for anything a browser calls directly, for public third-party integrators (protobuf tooling and HTTP/2 trailers are still a barrier outside internal or mobile-native ecosystems), and for APIs whose primary value is discoverability/self-documentation over raw performance — reach for Connect-RPC or REST+OpenAPI instead.
• 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](references/restful-design-patterns.md) — HTTP method semantics, URL structure, idempotency
• [references/pagination-filtering.md](references/pagination-filtering.md) — cursor vs offset, filtering contracts
• [references/error-handling-patterns.md](references/error-handling-patterns.md) — RFC 9457, error taxonomy
• [references/authentication-patterns.md](references/authentication-patterns.md) — OAuth 2.1, JWT, API keys, mTLS
• [references/rate-limiting-patterns.md](references/rate-limiting-patterns.md) — headers, 429 handling, burst strategies

Style-specific:

• [references/api-design-best-practices.md](references/api-design-best-practices.md) — cross-style DX and governance
• [references/versioning-strategies.md](references/versioning-strategies.md) — breaking change detection, deprecation
• [references/api-security-checklist.md](references/api-security-checklist.md) — OWASP API Top 10 checklist
• [references/graphql-patterns.md](references/graphql-patterns.md) — schema design, federation v2, Cosmo Router
• [references/grpc-patterns.md](references/grpc-patterns.md) — protobuf design, streaming, deadlines
• [references/trpc-patterns.md](references/trpc-patterns.md) — TypeScript end-to-end type sharing
• [references/openapi-guide.md](references/openapi-guide.md) — OpenAPI 3.2.0, Spectral, Redocly
• [references/openapi-32-arazzo-101.md](references/openapi-32-arazzo-101.md) — OpenAPI 3.2 new features (streaming types, QUERY method, OAuth flows) and Arazzo 1.1.0 multi-step workflow chains; load when specifying multi-call API sequences or adopting 3.2 features
• [references/asyncapi-patterns.md](references/asyncapi-patterns.md) — event-driven contracts, pub/sub
• [references/webhook-patterns.md](references/webhook-patterns.md) — HMAC signing, replay protection, delivery guarantees
• [references/real-time-api-patterns.md](references/real-time-api-patterns.md) — SSE, WebSocket, long-polling tradeoffs
• [references/api-testing-patterns.md](references/api-testing-patterns.md) — contract tests, property-based testing, Schemathesis
• [references/llm-agent-api-contracts.md](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/openapi-template.yaml)
• [assets/fastapi/fastapi-complete-api.md](assets/fastapi/fastapi-complete-api.md)
• [assets/express-nodejs/express-complete-api.md](assets/express-nodejs/express-complete-api.md)
• [assets/django-rest/django-rest-complete-api.md](assets/django-rest/django-rest-complete-api.md)
• [assets/spring-boot/spring-boot-complete-api.md](assets/spring-boot/spring-boot-complete-api.md)
• [assets/cross-platform/api-patterns-universal.md](assets/cross-platform/api-patterns-universal.md)
• [assets/cross-platform/template-api-governance.md](assets/cross-platform/template-api-governance.md)
• [assets/cross-platform/template-api-design-review-checklist.md](assets/cross-platform/template-api-design-review-checklist.md)
• [assets/cross-platform/template-api-error-model.md](assets/cross-platform/template-api-error-model.md)
• [data/sources.json](data/sources.json)

Related skills:

• [software-backend](../software-backend/SKILL.md), [software-security-appsec](../software-security-appsec/SKILL.md), [data-sql-optimization](../data-sql-optimization/SKILL.md), [qa-testing-strategy](../qa-testing-strategy/SKILL.md), [qa-observability](../qa-observability/SKILL.md), [docs-codebase](../docs-codebase/SKILL.md), [docs-ai-prd](../docs-ai-prd/SKILL.md), [dev-workflow-planning](../dev-workflow-planning/SKILL.md), [software-architecture-design](../software-architecture-design/SKILL.md)

Fact-Checking

• Verify current standards, library behavior, and tooling claims against primary sources before presenting them as fact.
• Prefer primary specs, official docs, and official tool documentation over summaries.
• If live verification is unavailable, mark version-sensitive claims as unverified.

Learnings Loop

Before applying this skill on a non-trivial task, read learnings.consolidated.md in this directory (and learnings.md if present).

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.