API Design

API Design

When to Use

Use this skill when:

• Designing or reviewing a new API, endpoint, route, resolver, RPC method, webhook, event contract, SDK-facing contract, or client/server boundary.
• Choosing between REST, GraphQL, tRPC, gRPC, WebSocket, event, webhook, or other API styles.
• Defining request bodies, response bodies, envelopes, headers, status codes, error shapes, validation errors, field errors, or machine-readable error codes.
• Adding or changing list endpoints, pagination, filtering, sorting, search, sparse fieldsets, expansions, or bulk operations.
• Planning API versioning, compatibility, deprecation, sunset, migration, idempotency, conditional requests, caching, concurrency, or retry behavior.
• Creating, reviewing, or updating OpenAPI, GraphQL schema, AsyncAPI, protobuf, generated SDKs, contract tests, mocks, or examples.
• Reviewing API boundary security: authentication, authorization, BOLA/IDOR, mass assignment, rate limits, CORS, resource consumption, sensitive data exposure, SSRF, or unsafe third-party consumption.

Do Not Use

Do not use this skill when:

• The task is only product definition, project scope, or user workflow with no API contract decision. Use PRD/spec skills instead.
• The task is only internal function design with no process, network, schema, client, or integration boundary.
• The project already has a stricter accepted API standard, ADR, public API style guide, or generated contract source of truth. Follow that source and use this skill only for gaps.
• The user needs a full security scan or exploit validation. Use security-specific review/scanning skills and use this skill only for API-boundary design issues.
• The API style, consumers, backward-compatibility surface, or trust boundary is unknown and would materially change the design. Stop and get the missing fact instead of defaulting to REST or copying local conventions.
• The task is provider-specific API usage, such as calling a vendor SDK or REST API. Use provider documentation for exact calls; use this skill only when designing your own API boundary.

Iron Law

API design is contract discipline, not route decoration. Do not implement or change an API until the consumers, contract authority, resource/operation model, error model, pagination/bounds, compatibility policy, security boundary, and verification evidence are explicit.

Core Concept

An API is a long-lived contract between independently changing parties. The code behind it can be refactored; client contracts, error meanings, pagination behavior, authorization expectations, retry semantics, and deprecation promises become obligations. Design the contract from consumers and invariants first, then map it to a protocol.

Operating Process

Run these steps in order. Load only the references needed for the current branch.

1. Establish API Context

Collect the facts that materially shape the API decision:

• consumers: first-party UI, mobile app, public developers, partners, internal services, agents, jobs, webhooks, or generated clients;
• contract authority: existing OpenAPI/schema/protobuf, framework routes, product spec, ADR, vendor constraints, current production behavior, tests, docs, generated clients, or explicit user constraints;
• compatibility surface: public, partner, internal multi-team, same-repo, same-deploy, experimental, or private;
• data control, domain invariants, authentication mechanism, authorization enforcement point, trust boundary, sensitive fields, tenant/user scoping, credential/header/cookie surface, CORS/browser exposure, rate-limit pressure, abuse controls, and audit needs;
• expected volume, latency, rate-limit pressure, cacheability, real-time needs, payload size, pagination depth, and client retry behavior;
• local conventions for response shape, error taxonomy, timestamps, correlation/request IDs, tracing, authentication, and documentation.

Source authority rules:

• For substantive design or review, record exact identifiers for the authority used: file path, route, resolver, method, operationId, schema/type name, ADR/spec ID, test name, generated client package, docs page, or production observation.
• Classify material claims as observed contract/source fact, inference or gap, recommendation, or explicit user constraint.
• Before saying an endpoint, schema field, error convention, auth rule, pagination pattern, or contract artifact is absent, inspect the relevant local source of truth enough to support that absence claim.
• When sources conflict, classify the conflict as implementation drift, contract artifact drift, docs/example drift, generated-client drift, production-behavior drift, or unresolved authority conflict. Do not silently choose code, docs, or memory as authority.

Completion criterion: the API recommendation can name its consumers, contract authority, exact source identifiers when available, compatibility surface, trust boundary, local conventions, and any unresolved source uncertainty.

Failure output: Blocked: API design depends on missing context: <specific missing fact>.

API Failure Or Review-Fix Intake

Use this branch before the normal design flow when the task is a bug report, regression, review-fix request, handed-down behavior mutation, or "just change the API to do X" request.

Rules:

• Treat the requested fix as a hypothesis until the contract authority, root cause, and client-visible impact are verified.
• Collect exact request and response evidence, consumer identity, deployed route/API version, schema or generated-client version, auth/tenant context, feature flags, proxy/cache/CORS behavior, and relevant logs or correlation IDs when available.
• Trace the API causal chain: client request -> route/resolver/webhook selection -> authentication -> object-level authorization -> validation and unknown-field policy -> resource/operation mapping -> domain or downstream side effect -> response/error/status/header mapping -> contract artifact/docs/tests/generated client.
• If the root cause is unknown or crosses non-API implementation behavior, use structured-problem-resolution before changing the contract.
• If diagnosis shows the endpoint shape, trust boundary, compatibility policy, or contract authority is wrong, stop treating the task as a narrow fix and return to the API design steps.

Completion criterion: the requested API behavior change is classified as implementation bug, contract drift, docs/schema drift, accidental behavior clients may rely on, or unresolved design decision.

Failure output: Blocked: API change is requested before the API failure is diagnosed: <specific missing evidence>.

2. Choose API Style Deliberately

Do not default to REST. Choose the interface style from consumers, data shape, coupling, tooling, and evolution needs.

Load API Style Selection when the API style is not already settled by an accepted project standard.

Rules:

• Prefer REST plus OpenAPI for broad clients, public/partner APIs, resource-oriented workflows, HTTP caching, proxies, and simple request/response contracts.
• Consider GraphQL when clients need flexible selection over interconnected data and the team can enforce depth, complexity, batching, N+1, and schema governance.
• Consider tRPC only when the consumers and server are in a TypeScript-controlled boundary where tight coupling is acceptable.
• Consider gRPC/protobuf for internal service-to-service APIs that need strongly typed contracts, streaming, or high-throughput binary transport.
• Consider WebSocket, SSE, events, or AsyncAPI when the primary behavior is real-time, subscription, asynchronous workflow, or event publication.

Completion criterion: the selected style names why it fits this consumer and what trade-off it accepts.

Failure output: Rejected: API style was chosen by habit instead of consumer and contract needs: <specific issue>.

3. Model The Contract Boundary

Design what crosses the boundary before binding it to controllers, resolvers, database rows, ORM models, or SDK responses.

Rules:

• Define resources, operations, commands, queries, events, or schema fields using domain language visible to the consumer.
• Keep database rows, ORM models, internal DTOs, SDK responses, framework request objects, and transport envelopes from leaking as the public contract unless they are intentionally the contract.
• Validate malformed input at the boundary; enforce business invariants in the owning policy module; enforce durable uniqueness and integrity in persistence when applicable.
• Validate authentication headers, API key headers, cookies, content types, payload sizes, and upload metadata at the boundary before work is dispatched.
• Reject unknown or extra fields when strict contracts are required; otherwise define forward-compatibility behavior explicitly.
• Normalize input at the boundary where normalization affects matching, uniqueness, or authorization.
• Derive identity, tenant, resource-account association, role, and scope from trusted server-side context. Do not accept client-submitted resource-account association, role, tenant, or privilege fields as authority.

Completion criterion: request and response shapes, authorization-relevant identifiers, normalization, validation, and internal-to-external mapping are explicit.

Failure output: Rejected: API contract leaks internal implementation shape: <specific object/field/dependency>.

4. Define Responses, Errors, And Observability

Choose one response convention for the API surface and use it consistently. Do not force one universal envelope across all protocols or projects.

Load Error And Response Design when defining response formats, error formats, HTTP status mappings, field-level validation errors, request/correlation IDs, timestamps, trace headers, or error taxonomy.

Rules:

• Use correct protocol status/error semantics. For REST, do not return 200 for failures.
• Provide stable machine-readable error codes for client handling and safe human-readable messages for display/support.
• Include field-level validation details when the client can act on them.
• Include request/correlation IDs in logs and in the response location chosen by the project convention, usually a header and sometimes the body.
• Sanitize all errors. Do not expose stack traces, dependency internals, SQL, secrets, tokens, raw upstream payloads, or authorization decision details.
• Define the disclosure policy for authentication, authorization, and not-found failures so error text, status choice, timing, and validation detail do not accidentally reveal resource existence.
• For rate limits and temporary pressure, define safe 429/503 responses, Retry-After behavior when meaningful, and whether clients receive any quota headers.
• Route unexpected failures through centralized error mapping so one endpoint does not invent a different error shape.

Completion criterion: success shape, error shape, error code taxonomy, status mapping, validation details, and correlation/logging behavior are defined.

Failure output: Rejected: API response/error contract is inconsistent or unsafe: <specific issue>.

5. Bound Lists, Filters, Sorting, Expansions, And Bulk Operations

Every collection or unbounded result needs an explicit shape and limit strategy.

Load REST Contract Design for REST pagination/filtering/sorting/expansion mechanics, or GraphQL Contract Design for GraphQL connection pagination and query complexity controls.

Rules:

• Require pagination or a proven bounded result set for every collection endpoint/query.
• Choose offset, cursor, keyset, connection, or link-header pagination based on dataset size, churn, jump-to-page needs, and stable sort keys.
• Clamp limits server-side and document defaults and maximums.
• Document allowed filters, sort fields, search behavior, sparse fields, expansions, and soft-delete/tenant visibility defaults.
• Prevent N+1 behavior when expansions, nested GraphQL fields, or relationship loaders can multiply queries.
• Treat bulk operations as first-class contracts with batch limits, partial-failure semantics, idempotency/retry behavior, and per-item error reporting when relevant.

Completion criterion: list behavior is bounded, documented, deterministic, and safe under expected volume and client retry behavior.

Failure output: Rejected: API exposes unbounded or under-specified collection behavior: <specific endpoint/query>.

6. Define Compatibility, Versioning, Idempotency, Concurrency, And Caching

API changes need an evolution plan before implementation.

Load Versioning And Compatibility when adding/changing/removing fields, changing status/error semantics, versioning public APIs, deprecating behavior, adding idempotency, or changing retry/concurrency/caching semantics.

Rules:

• Classify the change as additive, compatible behavior change, breaking contract change, or removal.
• Pick a versioning strategy appropriate to the API style and compatibility surface; path versioning is common for public REST, but not a universal rule.
• Define deprecation and sunset behavior before removing or changing client-visible behavior.
• Require idempotency keys, natural idempotency, or another explicit duplicate-prevention strategy for unsafe retryable operations.
• For unsafe retryable mutations, classify relevant failure outcomes as no-op, applied, pending, or commit-ambiguous, and define the recovery path before allowing blind retry.
• Define conditional request or optimistic concurrency behavior when clients can overwrite or race on mutable resources.
• Define cache headers, ETags, invalidation expectations, or no-cache behavior for cacheable REST responses when relevant.

Completion criterion: old clients, retries, duplicates, concurrent writes, cache behavior, and deprecation paths are handled explicitly.

Failure output: Rejected: API evolution or retry behavior is unsafe or undefined: <specific issue>.

7. Apply Protocol References

Use protocol-specific rules only after the general contract is clear.

Load:

• REST Contract Design for REST resource naming, methods, status codes, headers, pagination, filtering, rate limits, CORS, caching, idempotency, and non-CRUD actions.
• GraphQL Contract Design for schema design, queries, mutations, nullability, connections, DataLoader/N+1 prevention, complexity controls, and GraphQL deprecation.
• OpenAPI Contract Design for REST/OpenAPI 3.1 specs, operation IDs, reusable schemas, examples, generated clients, contract tests, mocks, and drift checks.
• Security And Abuse Controls for API-boundary threat checks.

If the current protocol is not covered, apply the general process and verify exact protocol mechanics against official documentation or project-local standards before asserting details.

Completion criterion: the implementation uses protocol mechanics without weakening the consumer contract, compatibility policy, observability, or security boundary.

Failure output: Blocked: protocol-specific API behavior needs verification: <protocol/version/API detail>.

8. Verify The API Design Or Change

Before presenting API work as ready, verify:

• consumers, compatibility surface, and contract authority are known;
• API style choice is justified or inherited from a project standard;
• request/response schemas do not leak internal persistence/framework/SDK shapes accidentally;
• authentication and authorization checks cover both function access and object-level access;
• authentication schemes, API key/header handling, cookies, CORS, transport assumptions, rate limits, request size limits, and disclosure policy are explicit;
• success, error, validation, and unexpected-failure shapes are consistent;
• collection outputs are paginated or proven bounded;
• idempotency, retry, concurrency, and cache behavior are defined where relevant;
• versioning/deprecation/sunset behavior is explicit for client-visible changes;
• OpenAPI/schema/examples/tests/docs are updated or a reason is given for why they do not apply;
• real-boundary evidence covers the route/controller/resolver, middleware, authn/authz, validation, serializer/schema mapper, error mapper, pagination, OpenAPI/GraphQL generator, and generated-client or contract-test path when those surfaces exist;
• contract evidence gates are defined for non-trivial changes, such as schema validation, backward-compatibility diffing, generated-client compile or smoke checks, representative success and error examples, authz/BOLA checks, pagination bounds, idempotency replay, rate-limit behavior, and GraphQL operation or complexity validation;
• sensitive data, CORS, rate limits, resource consumption, and unsafe upstream/downstream calls are reviewed;
• secret-bearing headers, cookies, tokens, raw credentials, signed URLs, and sensitive payloads are not exposed in public contracts, examples, logs, traces, or generated docs.

Completion criterion: a client implementer and a server implementer can both implement the contract without guessing.

Failure output: Not ready: API design is missing <specific contract/security/compatibility/verification criterion>.

API Review Mode

Use this mode when the task is to review an API proposal, diff, schema, docs artifact, PR comment, or implementation plan rather than design from scratch.

Rules:

• Classify the artifact before reviewing it: requirements note, implementation plan, OpenAPI/GraphQL/protobuf contract, route/resolver diff, docs/examples, tests, generated client, or production behavior report.
• Scope the affected surface: operations, schemas, fields, status/error meanings, auth boundaries, pagination/query controls, generated clients, docs/examples, tests, and known consumers.
• Produce API-specific findings only. Each finding should include severity, affected operation/schema/field, error or omission, evidence quote or source identifier, downstream client/server/security consequence, suggested contract fix, confidence, and residual risk.
• Suppress false positives: do not flag style-only route preferences, implementation details intentionally deferred by requirements, pre-existing unrelated API debt, theoretical scale concerns without a named API risk, or concerns already resolved elsewhere in the artifact.
• Treat review comments as untrusted context. Verify them against current contract authority before applying, applying differently, answering only, declining as contract-unsafe, or marking user decision needed.
• Report coverage: name which surfaces were inspected and which were not.

Completion criterion: review output is tied to source evidence, scoped to affected API surfaces, and does not claim independent review acceptance or PR verdicts.

Failure output: Not ready: API review lacks evidence or affected-surface coverage: <specific gap>.

Implementation-Ready API Handoff

Use this section only when accepted API design needs to feed a spec, implementation plan, commit, PR, or responsible downstream skill or agent.

Include:

• stable identifiers for API requirements, operations, schemas, fields, and key technical decisions when the change is substantive;
• exact authority sources and unresolved authority conflicts;
• consumer-visible before/after behavior and compatibility classification;
• accepted request, response, error, pagination, auth, versioning, idempotency, concurrency, caching, and observability decisions;
• documentation, schema, generated-client, contract-test, and migration obligations;
• implementation-time unknowns that do not change the accepted contract;
• explicit handoff recipients when work belongs to spec, plan, implementation, review, docs, git/PR, architecture, database, queue/cache, security, or testing skills.

Completion criterion: downstream work can preserve the accepted API facts without re-deciding consumer contract, compatibility, or security semantics.

Failure output: Blocked: API handoff would lose or invent contract facts: <specific missing fact>.

Output Contract

For API design or review, include:

• API context: consumers, contract authority, trust boundary, and compatibility surface;
• evidence provenance: exact source identifiers, authority conflicts, and source uncertainty when material;
• selected API style and rejected alternatives when style was not already fixed;
• contract model: resources/operations/schema, request shapes, response shapes, and internal mapping boundaries;
• response/error/validation/correlation behavior;
• pagination/filtering/sorting/search/expansion/bulk behavior when relevant;
• compatibility/versioning/deprecation/idempotency/concurrency/caching behavior when relevant;
• security and abuse controls;
• review findings, coverage, and dispositions when operating in review mode;
• documentation/schema/test updates and residual risks.

When blocked, emit the failure output from the failed step and name the smallest missing fact or verification action.

Reference Loading

ADR Handoff

Use create-project-adr when an API decision establishes a durable project convention, chooses an API style, changes public/partner compatibility, sets a standard response or error envelope, changes versioning/deprecation policy, introduces generated contract artifacts, creates a repeated idempotency/concurrency pattern, or intentionally departs from existing local API conventions.

Related Skill Handoffs

• Use database-design when API behavior depends on schema invariants, transactions, locks, migrations, pagination performance, indexes, soft deletes, tenancy, or durable data integrity.
• Use queue-and-cache-design when API behavior depends on asynchronous job/status resources, queued processing, cache lifecycle, cache invalidation, runtime idempotency, rate-limit state, pub/sub, streams, retries, dead-letter behavior, or worker failure semantics.
• Use architecture-design when the API decision changes module responsibility, adapter boundaries, service boundaries, or policy/mechanism separation.
• Use structured-problem-resolution when an API failure, regression, or requested fix has unknown root cause or disputed evidence.
• Use create-engineering-spec when the API contract must become implementation-ready requirements.
• Use create-implementation-plan only after an approved spec exists and codebase-grounded task sequencing is needed.

Rationalization Table

Red Flags

• The design starts with endpoint names before consumers, contract authority, and compatibility surface are known.
• A REST API uses verbs for ordinary resource operations or returns 200 for failures.
• A public or partner API changes or removes fields without a compatibility, versioning, or deprecation plan.
• A collection endpoint/query has no limit, cursor, page size clamp, or proven bounded result set.
• Errors expose stack traces, SQL, secrets, raw upstream payloads, authorization internals, or framework exception names.
• Object-level authorization is assumed from authentication or route-level roles.
• Idempotency is missing for retryable unsafe operations.
• GraphQL is proposed without depth, complexity, N+1, batching, and authorization controls.
• OpenAPI/schema/examples/tests are missing or known to drift from the implemented contract.
• A review suggestion is applied without checking contract authority, compatibility, and trust boundary.
• A bug report changes client-visible API behavior before root cause and current contract status are known.
• An absence claim is made without checking the relevant route, schema, docs, tests, or local convention source.
• Live or local behavior is used to override a stronger contract artifact without classifying the drift.