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Designs REST and GraphQL APIs including endpoints, error handling, versioning, and documentation. Use when creating new APIs, designing endpoints, reviewing API contracts, or when asked about REST, GraphQL, or API patterns.

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SKILL.md

name designing-apis
description Designs REST and GraphQL APIs including endpoints, error handling, versioning, and documentation. Use when creating new APIs, designing endpoints, reviewing API contracts, or when asked about REST, GraphQL, or API patterns.

Designing APIs

API Design Workflow

Copy this checklist and track progress:

API Design Progress:
- [ ] Step 1: Define resources and relationships
- [ ] Step 2: Design endpoint structure
- [ ] Step 3: Define request/response formats
- [ ] Step 4: Plan error handling
- [ ] Step 5: Add authentication/authorization
- [ ] Step 6: Document with OpenAPI spec
- [ ] Step 7: Validate design against checklist

REST API Design

URL Structure

# Resource-based URLs (nouns, not verbs)
GET    /users              # List users
GET    /users/:id          # Get user
POST   /users              # Create user
PUT    /users/:id          # Replace user
PATCH  /users/:id          # Update user
DELETE /users/:id          # Delete user

# Nested resources
GET    /users/:id/orders   # User's orders
POST   /users/:id/orders   # Create order for user

# Query parameters for filtering/pagination
GET    /users?role=admin&status=active
GET    /users?page=2&limit=20&sort=-createdAt

HTTP Status Codes

Code Meaning Use Case
200 OK Successful GET, PUT, PATCH
201 Created Successful POST
204 No Content Successful DELETE
400 Bad Request Invalid input
401 Unauthorized Missing/invalid auth
403 Forbidden Valid auth, no permission
404 Not Found Resource doesn't exist
409 Conflict Duplicate, state conflict
422 Unprocessable Validation failed
429 Too Many Requests Rate limited
500 Internal Error Server error

Response Formats

Success Response:

{
  "data": {
    "id": "123",
    "type": "user",
    "attributes": {
      "name": "John Doe",
      "email": "john@example.com"
    }
  },
  "meta": {
    "requestId": "abc-123"
  }
}

List Response with Pagination:

{
  "data": [...],
  "meta": {
    "total": 100,
    "page": 1,
    "limit": 20,
    "totalPages": 5
  },
  "links": {
    "self": "/users?page=1",
    "next": "/users?page=2",
    "last": "/users?page=5"
  }
}

Error Response:

{
  "error": {
    "code": "VALIDATION_ERROR",
    "message": "Invalid input data",
    "details": [
      {
        "field": "email",
        "message": "Must be a valid email address"
      }
    ]
  },
  "meta": {
    "requestId": "abc-123"
  }
}

API Versioning

URL Versioning (Recommended):

/api/v1/users
/api/v2/users

Header Versioning:

Accept: application/vnd.api+json; version=1

Authentication Patterns

JWT Bearer Token:

Authorization: Bearer eyJhbGciOiJIUzI1NiIs...

API Key:

X-API-Key: your-api-key

Rate Limiting Headers

X-RateLimit-Limit: 100
X-RateLimit-Remaining: 95
X-RateLimit-Reset: 1609459200
Retry-After: 60

GraphQL Patterns

Schema Design:

type Query {
  user(id: ID!): User
  users(filter: UserFilter, pagination: Pagination): UserConnection!
}

type Mutation {
  createUser(input: CreateUserInput!): UserPayload!
  updateUser(id: ID!, input: UpdateUserInput!): UserPayload!
}

type User {
  id: ID!
  name: String!
  email: String!
  orders(first: Int, after: String): OrderConnection!
}

input CreateUserInput {
  name: String!
  email: String!
}

type UserPayload {
  user: User
  errors: [Error!]
}

OpenAPI Specification Template

See OPENAPI-TEMPLATE.md for the full OpenAPI 3.0 specification template.

API Design Validation

After completing the design, validate against this checklist:

Validation Checklist:
- [ ] All endpoints use nouns, not verbs
- [ ] HTTP methods match operations correctly
- [ ] Consistent response format across endpoints
- [ ] Error responses include actionable details
- [ ] Pagination implemented for list endpoints
- [ ] Authentication defined for protected endpoints
- [ ] Rate limiting headers documented
- [ ] OpenAPI spec is complete and valid

If validation fails, return to the relevant design step and address the issues.

Security Checklist

  • HTTPS only
  • Authentication on all endpoints
  • Authorization checks
  • Input validation
  • Rate limiting
  • Request size limits
  • CORS properly configured
  • No sensitive data in URLs
  • Audit logging