API Design

Design APIs that are consistent, predictable, and easy to consume. A well-designed API is intuitive to use without reading documentation — but has great documentation anyway.

Scope Boundary

This skill designs the contract (endpoints, request/response shapes, error formats, pagination). For higher-level decisions (REST vs GraphQL, API gateway, authentication strategy), use the architecture-design skill first, then return here to design the specifics.

The Deliverable

An API design is not finished until it carries all six of these. A list of endpoints is a third of an answer:

1. Resources and endpoints — URLs, methods, and how relationships are expressed
2. Request/response schemas — field names, types, and the response envelope
3. The error format — one shape for every error, plus the status-code mapping
4. The pagination contract — parameters and response shape for every list endpoint
5. Auth per endpoint — scheme, who may call it, and which endpoints are public
6. The written spec — markdown or OpenAPI, using templates/api-spec.md

Deliver all six in the same response. Where a choice is genuinely open, pick the default this skill recommends, say you picked it, and keep going — never return a design as a list of questions.

Workflow

Step 1: Identify Resources and Operations

Start from the domain model, not the UI:

• What are the resources? (nouns: users, orders, products, comments)
• What operations exist? (CRUD + domain-specific: archive, publish, approve)
• What are the relationships? (user has orders, order contains items)
• Who consumes this API? (web app, mobile app, third-party, internal service)

Map each resource to its operations before choosing URLs or methods.

Step 2: Design Endpoints

Apply RESTful naming conventions — see references/rest-conventions.md:

GET    /api/v1/users          → List users
POST   /api/v1/users          → Create user
GET    /api/v1/users/:id      → Get user
PATCH  /api/v1/users/:id      → Update user (partial)
DELETE /api/v1/users/:id      → Delete user

# Nested resources (when the child only makes sense in parent context)
GET    /api/v1/users/:id/orders    → List user's orders
POST   /api/v1/users/:id/orders    → Create order for user

# Actions that don't map to CRUD
POST   /api/v1/orders/:id/cancel   → Cancel an order
POST   /api/v1/users/:id/verify    → Verify a user's email

Present the endpoint list, then design the schemas in the same response — the list is a checkpoint, not a stopping point. Invite correction on the naming while delivering the rest.

Step 3: Define Request/Response Schemas

For each endpoint, define:

Request: Query parameters (for filtering/pagination), path parameters, request body with field types and validation rules.

Response: Status code, response body shape, included relationships.

Use consistent patterns across all endpoints — see references/rest-conventions.md for standard shapes.

Decisions to make now, with a stated default rather than a question — flag them for correction and move on:

• ID format: default ULID (sortable, no coordination); integer or UUID if the stack forces it
• Date format: ISO 8601 always (2025-03-05T14:30:00Z) — not a decision
• Null vs absent: default omit absent fields, return null only for a known-empty value
• Envelope or not: default { data, pagination } on collections, flat on single resources

Step 4: Standardize Error Responses

Every API error should use the same shape. Recommend this format:

{
  "error": {
    "code": "VALIDATION_ERROR",
    "message": "Human-readable description for developers",
    "details": [
      {
        "field": "email",
        "message": "Must be a valid email address"
      }
    ]
  }
}

Map HTTP status codes consistently:

• 400 — Client sent invalid data (validation errors)
• 401 — Authentication required or invalid
• 403 — Authenticated but not authorized for this action
• 404 — Resource not found
• 409 — Conflict (duplicate, state violation)
• 422 — Valid syntax but semantically invalid (business rule violation)
• 429 — Rate limited
• 500 — Server error (never expose internals)

Step 5: Design Pagination, Filtering, Sorting

For any list endpoint that could return many results:

Pagination (recommend cursor-based for most cases):

GET /api/v1/orders?cursor=abc123&limit=20

Response:
{
  "data": [...],
  "pagination": {
    "next_cursor": "def456",
    "has_more": true
  }
}

Filtering: Use query parameters with clear naming:

GET /api/v1/orders?status=pending&created_after=2025-01-01

Sorting: Use a sort parameter with - prefix for descending:

GET /api/v1/orders?sort=-created_at,total

Step 6: Define Authentication and Authorization

For each endpoint, specify:

• Authentication: Required? What scheme? (Bearer token, API key, session)
• Authorization: What roles/permissions can access this? Document per-endpoint.
• Public endpoints: Explicitly mark which endpoints don't require auth.

Step 7: Produce the Specification

Output the API design as one of:

• Markdown document — for internal APIs and quick designs
• OpenAPI 3.x spec — for formal APIs, enables codegen and tooling

Use the template at templates/api-spec.md for markdown output.

Principles Applied

• KISS: Prefer flat resource URLs over deeply nested ones. /orders?user_id=123 is simpler than /users/123/orders/456/items/789.
• DRY: Standardize error format, pagination, and envelope structure once. Don't reinvent per-endpoint.
• YAGNI: Don't add filtering or sorting until a list endpoint actually needs them. Pagination is not in this category — any collection that can grow ships with the pagination contract from day one, because adding it later is a breaking change for every consumer.
• Functional Independence: Each endpoint should do one thing. Avoid "Swiss army knife" endpoints that change behavior based on query parameters.