API Design Skill for Cursor

API design

Your one job

You design the contract an API exposes. You do not write the handler. The deliverable is a set of decisions a backend skill can implement directly: resource shapes, URLs, methods, the status-code map, one error envelope, pagination params, versioning rules — ideally captured as an OpenAPI 3.1 document.

When the user names a framework (FastAPI, NestJS, Go, Node), they own the build; you are pulled in for contract questions. Settle the contract first, then hand off (see Handoff). Keep every decision framework-neutral: nothing here should mention an ORM, a router, or a DI container.

REST vs GraphQL vs hybrid

Pick on traffic shape, not fashion. Decide once, write it down.

| Situation | Choose | Why |

|---|---|---|

| CRUD-ish resources, public API, HTTP caching matters | REST | URLs map to resources; CDN/proxy caching works on GET + ETag out of the box |

| Many client shapes, deep nested graphs, mobile over-fetch is real | GraphQL | one round-trip, client picks fields; no N endpoints per screen |

| Stable resource API + one rich read surface for a client app | Hybrid | REST for the system of record, a GraphQL read layer on top |

Operational gotcha that decides monitoring: GraphQL returns HTTP 200 even when a field errored — failures live in an errors] array next to partial data. Your dashboards cannot alert on 5xx; you must alert on the errors[] payload. REST signals failure with the HTTP status itself. If your ops team lives on status-code SLOs, that is a point for REST. Schema/nullability design, mutation and error-union conventions, and this error model in full: [references/graphql-design.md.

Resource & URL modeling

Resources are nouns; HTTP methods are the verbs. Never put a verb in a path.

Rules, each with its reason:

• Plural collections, consistent everywhere — /projects, /projects/{id}. Pick plural and never mix singular in; inconsistent naming is the most-cited design smell.
• Nest sub-resources one level — /projects/{id}/tasks. Deeper than one level (/projects/{p}/tasks/{t}/comments/{c}) gets unreadable; link to the flat resource instead (/comments/{c}).
• Methods carry intent — GET read, POST create, PUT full replace, PATCH partial update, DELETE remove. A path never says what it does.
• Filter/sort/select via query string, not new paths — ?status=open&sort=-created_at&fields=id,title. One GET /projects handles all of it; don't mint /projects/open and /projects/byDate.

| Bad | Good | Why |

|---|---|---|

| POST /createProject | POST /projects | the method is the verb |

| GET /getUserOrders/{id} | GET /users/{id}/orders | noun hierarchy, no verb |

| GET /project and GET /tasks | GET /projects and GET /tasks | one plural convention |

| GET /projects/active | GET /projects?status=active | filter is a query param |

| POST /projects/{id}/delete | DELETE /projects/{id} | method, not path segment |

Full query grammar (filter operators, sparse fieldsets, sort syntax), content negotiation, rate-limit headers and a HATEOAS note: references/rest-conventions.md.

Status codes that matter

You need a small map, used consistently. Don't overload 200.

200 OK            # read / update succeeded, body returned
201 Created       # resource created — include Location: /projects/{id}
202 Accepted      # async accepted, not done — return a status URL
204 No Content    # success, nothing to return (e.g. DELETE)
400 Bad Request   # malformed syntax / unparseable
401 Unauthorized  # not authenticated — who are you?
403 Forbidden     # authenticated but not allowed — I know you, no
404 Not Found     # resource absent (or hidden from this caller)
409 Conflict      # state collision — duplicate, version mismatch
422 Unprocessable # syntactically fine, semantically invalid (validation)
429 Too Many Req  # rate limited — include Retry-After
5xx               # your fault, never the client's; never leak the stack

Two distinctions agents get wrong:

• 401 vs 403 — 401 means *unauthenticated* (no/invalid credentials); 403 means *authenticated but unauthorized*. Returning 401 on a permission failure leaks that re-auth might help when it won't.
• 409 vs 422 — 409 is a *state* conflict (the request fights the current server state: dup key, stale version). 422 is a *content* problem (the body parses but fails business rules). Full status-code table with when-each in references/rest-conventions.md.

Error envelope: RFC 9457

One error shape across every endpoint. Adopt RFC 9457 Problem Details (the current standard; it obsoletes RFC 7807). Media type application/problem+json. Standard members: type, title, status, detail, instance, plus your own extension members.

{
  "type": "https://api.acme.com/problems/validation-error",
  "title": "Your request parameters didn't validate.",
  "status": 422,
  "detail": "due_date must be in the future.",
  "instance": "/projects/8a3/tasks",
  "errors": [
    { "field": "due_date", "message": "must be in the future" }
  ],
  "correlation_id": "req_01H..."
}

Rules:

• type is a stable, machine-readable URI — clients branch on it, not on detail. Never change a type string once published.
• title is human, generic per type; detail is human, specific to this occurrence. detail is for people, not parsers.
• Always carry a correlation/request id (extension member) so a support ticket maps to a log line.
• Never leak internals — no stack traces, SQL, internal hostnames, or raw DB ids in detail. That is both an information leak and a coupling leak.

Pagination

Default to cursor (keyset) pagination. Use offset only for small, bounded sets.

| Approach | Use when | Why |

|---|---|---|

| Cursor / keyset | large or changing datasets, feeds, anything hot | opaque token over an indexed ordered column → constant-time, stable across inserts |

| Offset / limit | small bounded admin lists, fixed reference tables | simple, but the DB scans-and-discards skipped rows (degrades with depth) and skips or duplicates rows when data shifts between page loads |

Decision line: if the list can grow unbounded or rows can be inserted between page fetches, use cursor.

REST cursor envelope — same keys on every list endpoint:

{
  "data": [ { "id": "...", "title": "..." } ],
  "next_cursor": "eyJpZCI6MTI4N30",
  "has_more": true
}

The next page is GET /projects?cursor=eyJpZCI6MTI4N30&limit=50. The cursor is opaque — clients must not parse or construct it.

GraphQL has its own de-facto standard: Relay Connections — edges { node, cursor }, pageInfo { hasNextPage, endCursor }, args first / after. Use it; don't invent a bespoke GraphQL pagination shape. See references/graphql-design.md.

Versioning & evolution

Prefer additive, non-breaking evolution over a new version. A new version forks your client base and your maintenance. Most changes don't need one.

• Non-breaking (no version bump): adding an optional field, adding a new endpoint, adding a new optional query param, adding a new enum value clients are told to tolerate.
• Breaking (needs a version): removing/renaming a field, changing a type, making an optional field required, changing status-code semantics, changing the error type for a case.

When you must version, use a URL path version (/v1/...) for public APIs — it is visible, cacheable, trivially testable in a browser, and the most common convention clients expect. Header/media-type versioning (Accept: application/vnd.acme.v2+json) keeps URLs clean but is harder to test and cache; query-param versioning (?version=2) pollutes every URL. Default to path.

Deprecate gracefully with the Deprecation and Sunset response headers so clients get programmatic warning before removal:

Deprecation: true
Sunset: Sat, 31 Oct 2026 23:59:59 GMT
Link: <https://api.acme.com/v2/projects>; rel="successor-version"

Full breaking-vs-non-breaking matrix, the three versioning mechanisms and the deprecation/sunset workflow: references/versioning-and-evolution.md.

Idempotency & concurrency

• Make POST/PATCH retry-safe with an Idempotency-Key header. The client sends a unique key; the server replays the original response on a retry instead of double-creating. This is an IETF httpapi draft (not yet an RFC) but is the proven pattern across Stripe, PayPal, and others — adopt it for any create/charge/payment-like operation where a network retry could duplicate work.

POST /payments
Idempotency-Key: 9b1f7c2e-... 

• Use ETag + If-Match for optimistic concurrency on PUT/PATCH. The server returns an ETag (a version fingerprint) on read; the client sends it back in If-Match on write. If it no longer matches, the server returns 412 Precondition Failed — no lost update. Use If-None-Match for conditional GET caching.

Anti-patterns

| Anti-pattern | Why it bites | Do instead |

|---|---|---|

| Verbs in paths (/getUsers, /createProject) | duplicates HTTP semantics, breaks caching/tooling | noun + HTTP method |

| Mixed plural/singular collections | clients can't predict URLs | one plural convention everywhere |

| 200 on error (REST) | breaks status-code monitoring and client error handling | real 4xx/5xx + RFC 9457 body |

| Different error shape per endpoint | every client writes per-endpoint parsing | one application/problem+json shape |

| Leaking stack traces / SQL / DB ids in errors | info leak + couples clients to internals | generic title, safe detail, correlation id |

| Offset pagination on a hot/large feed | slow at depth; skips/dups rows on insert | cursor/keyset pagination |

| Unbounded list endpoint (no limit) | one client can pull the whole table | enforce a default + max limit |

| New version for every change | forks clients, multiplies maintenance | additive non-breaking evolution |

| Breaking a field in place on a live version | silently breaks existing clients | new field/version + deprecation headers |

| 401 for a permission failure | misleads client into re-authing | 403 when authenticated-but-forbidden |

| 200 for a created resource | hides the create, no Location | 201 + Location header |

| Ignoring GraphQL partial errors[] | failures invisible to monitoring | alert on errors[], not just HTTP 5xx |

Handoff

The contract is the artifact. Emit it as an OpenAPI 3.1 document — the checkable deliverable a framework skill generates code from. How to shape it, and what scripts/verify.sh checks: references/openapi-contract.md.

Hand off to the builder:

• Python/async → ../fastapi/SKILL.md
• NestJS / Node DI → ../nestjs/SKILL.md
• Go net/http → ../go/SKILL.md
• Node generally → ../nodejs/SKILL.md

Adjacent concerns you do not own:

• Auth hardening, OWASP, CORS/CSP threat-modeling → ../secure-coding/SKILL.md (you only place 401/403/scopes in the contract)
• Receiving inbound webhooks (HMAC verify, replay, dedupe) → ../webhooks/SKILL.md
• Building an outbound client to a third-party API → ../api-connector-builder/SKILL.md
• Reviewing handler code (not the design) → ../code-review/SKILL.md