Designing APIs Skill | Agent Skills

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

Agent Skills: Designing APIs

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