Agent Skills: REST API Design

REST API Design

Master REST API design principles to build intuitive, scalable, and developer-friendly APIs that stand the test of time.

When to Use This Skill

• Designing new REST APIs or endpoints
• Refactoring existing APIs for better usability
• Establishing REST design standards for your team
• Reviewing REST API specifications before implementation
• Implementing pagination, filtering, and searching
• Designing error handling and status code strategies
• Planning API versioning and deprecation strategies

Core REST Principles

Resource-Oriented Architecture

REST APIs should be resource-oriented, not action-oriented:

• Resources are nouns (users, orders, products), not verbs
• Use HTTP methods for actions (GET, POST, PUT, PATCH, DELETE)
• URLs represent resource hierarchies
• Consistent naming conventions across endpoints

Good patterns:

GET    /api/users              # List users
POST   /api/users              # Create user
GET    /api/users/{id}         # Get specific user
PUT    /api/users/{id}         # Replace user
PATCH  /api/users/{id}         # Update user fields
DELETE /api/users/{id}         # Delete user

# Nested resources (shallow)
GET    /api/users/{id}/orders  # Get user's orders
POST   /api/users/{id}/orders  # Create order for user

Bad patterns (avoid):

POST /api/createUser
POST /api/getUserById
POST /api/deleteUser
GET  /api/user  (inconsistent singular)

HTTP Methods Semantics

Each HTTP method has specific semantics that must be respected:

• GET: Retrieve resources (idempotent, safe, cacheable)
• POST: Create new resources or trigger actions (not idempotent)
• PUT: Replace entire resource (idempotent)
• PATCH: Partial resource updates (not always idempotent)
• DELETE: Remove resources (idempotent)

Idempotency: GET, PUT, DELETE must be idempotent (same result when called multiple times)

URL Structure Best Practices

Resource Naming:

• Use plural nouns: /api/users not /api/user
• Use lowercase: /api/users not /api/Users
• Use hyphens for multi-word names: /api/user-profiles not /api/userProfiles

Nested Resources (Shallow Preferred):

# Shallow nesting (preferred) - easy to understand
GET /api/users/{id}/orders

# Deep nesting (avoid) - hard to route and query
GET /api/users/{id}/orders/{orderId}/items/{itemId}/reviews

# Better approach for deep hierarchies:
GET /api/order-items/{id}/reviews

HTTP Status Codes

Use status codes correctly to communicate request outcomes:

2xx Success

• 200 OK - Successful GET, PATCH, PUT
• 201 Created - Successful POST (include Location header)
• 204 No Content - Successful DELETE or empty response

4xx Client Errors

• 400 Bad Request - Malformed request syntax
• 401 Unauthorized - Authentication required
• 403 Forbidden - Authenticated but not authorized
• 404 Not Found - Resource doesn't exist
• 409 Conflict - State conflict (duplicate email, etc.)
• 422 Unprocessable Entity - Validation errors

5xx Server Errors

• 500 Internal Server Error - Server error
• 503 Service Unavailable - Temporary downtime

Rate Limiting

• 429 Too Many Requests - Rate limit exceeded

Resource Collection Patterns

Standard CRUD Operations

# List collections
GET    /api/users?page=1&limit=20
→ 200 OK
→ Returns: [user1, user2, ...]

# Get specific resource
GET    /api/users/{id}
→ 200 OK or 404 Not Found
→ Returns: {id, name, email, ...}

# Create new resource
POST   /api/users
Body:  {"name": "John", "email": "john@example.com"}
→ 201 Created
→ Location: /api/users/123
→ Returns: {id: "123", name: "John", ...}

# Update entire resource
PUT    /api/users/{id}
Body:  {complete user object with ALL fields}
→ 200 OK or 404 Not Found

# Partial update
PATCH  /api/users/{id}
Body:  {"name": "Jane"}  (only changed fields)
→ 200 OK or 404 Not Found

# Delete resource
DELETE /api/users/{id}
→ 204 No Content or 404 Not Found

Pagination Strategies

Always paginate large collections. Three main approaches:

1. Offset-Based Pagination

Best for: Small datasets, traditional pagination UI

GET /api/users?page=2&page_size=20

Response:
{
  "items": [...],
  "page": 2,
  "page_size": 20,
  "total": 150,
  "pages": 8,
  "has_next": true,
  "has_prev": true
}

2. Cursor-Based Pagination

Best for: Large datasets, real-time data, consistent results

GET /api/users?limit=20&cursor=eyJpZCI6MTIzfQ

Response:
{
  "items": [...],
  "next_cursor": "eyJpZCI6MTQzfQ",
  "prev_cursor": "eyJpZCI6MTA3fQ",
  "has_more": true
}

3. Link Header Pagination

Most RESTful approach using HTTP Link header:

GET /api/users?page=2

Response Headers:
Link: <https://api.example.com/users?page=3>; rel="next",
      <https://api.example.com/users?page=1>; rel="prev",
      <https://api.example.com/users?page=1>; rel="first",
      <https://api.example.com/users?page=8>; rel="last"

Implementation pattern (FastAPI):

from fastapi import FastAPI, Query
from typing import Optional

@app.get("/api/users")
async def list_users(
    page: int = Query(1, ge=1),
    page_size: int = Query(20, ge=1, le=100)
):
    offset = (page - 1) * page_size
    total = await count_users()
    users = await fetch_users(limit=page_size, offset=offset)

    return {
        "items": users,
        "total": total,
        "page": page,
        "page_size": page_size,
        "pages": (total + page_size - 1) // page_size
    }

Filtering, Sorting, and Searching

Query Parameters

Filtering:

GET /api/users?status=active
GET /api/users?role=admin&status=active

Sorting:

GET /api/users?sort=created_at
GET /api/users?sort=-created_at        # descending
GET /api/users?sort=name,created_at    # multiple fields

Searching:

GET /api/users?search=john
GET /api/users?q=john

Field Selection (Sparse Fieldsets):

GET /api/users?fields=id,name,email

Error Response Format

Standardize error responses for consistency:

{
  "error": {
    "code": "VALIDATION_ERROR",
    "message": "Request validation failed",
    "details": [
      {
        "field": "email",
        "message": "Invalid email format",
        "value": "not-an-email"
      }
    ],
    "timestamp": "2025-10-16T12:00:00Z",
    "path": "/api/users"
  }
}

API Versioning

Plan for breaking changes from day one:

URL Versioning (Recommended)

Clear and easy to route:

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

Header Versioning

Clean URLs but less visible:

GET /api/users
Accept: application/vnd.api+json; version=2

Query Parameter Versioning

Easy to test but easy to forget:

GET /api/users?version=2

Security Patterns

Authentication & Authorization

Bearer Token (JWT):

Authorization: Bearer eyJhbGciOiJIUzI1NiIs...

401 Unauthorized - Missing/invalid token
403 Forbidden - Valid token, insufficient permissions

API Keys:

X-API-Key: your-api-key-here

Rate Limiting

Protect APIs from abuse:

X-RateLimit-Limit: 1000
X-RateLimit-Remaining: 742
X-RateLimit-Reset: 1640000000

Response when limited:
429 Too Many Requests
Retry-After: 3600

CORS Configuration

from fastapi.middleware.cors import CORSMiddleware

app.add_middleware(
    CORSMiddleware,
    allow_origins=["https://example.com"],
    allow_credentials=True,
    allow_methods=["GET", "POST", "PUT", "PATCH", "DELETE"],
    allow_headers=["*"],
)

Advanced Patterns

Idempotency

For non-idempotent operations (POST), use idempotency keys:

POST /api/orders
Idempotency-Key: unique-key-123

If duplicate request: → 200 OK (return cached response)

Bulk Operations

POST /api/users/batch
{
  "items": [
    {"name": "User1", "email": "user1@example.com"},
    {"name": "User2", "email": "user2@example.com"}
  ]
}

Response:
{
  "results": [
    {"id": "1", "status": "created"},
    {"id": null, "status": "failed", "error": "Email already exists"}
  ]
}

HATEOAS (Hypermedia Links)

Include links for related resources:

{
  "id": "123",
  "name": "John",
  "email": "john@example.com",
  "_links": {
    "self": {"href": "/api/users/123"},
    "orders": {"href": "/api/users/123/orders"},
    "update": {"href": "/api/users/123", "method": "PATCH"},
    "delete": {"href": "/api/users/123", "method": "DELETE"}
  }
}

Caching

Cache Headers:

# Client caching
Cache-Control: public, max-age=3600

# No caching
Cache-Control: no-cache, no-store, must-revalidate

# Conditional requests
ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"
If-None-Match: "33a64df551425fcc55e4d42a148795d9f25f89d4"
→ 304 Not Modified

Documentation

OpenAPI/Swagger

Generate interactive API documentation:

from fastapi import FastAPI, Path

app = FastAPI(
    title="My API",
    description="API for managing users",
    version="1.0.0",
    docs_url="/docs",
    redoc_url="/redoc"
)

@app.get(
    "/api/users/{user_id}",
    summary="Get user by ID",
    response_description="User details",
    tags=["Users"]
)
async def get_user(
    user_id: str = Path(..., description="The user ID")
):
    """
    Retrieve user by ID.

    Returns full user profile including:
    - Basic information
    - Contact details
    - Account status
    """
    pass

Best Practices Summary

1. Resource-Oriented: Use nouns for endpoints, verbs for HTTP methods
2. Stateless: Each request contains all necessary information
3. HTTP Semantics: Respect GET/POST/PUT/PATCH/DELETE meanings
4. Status Codes: Use correct codes (2xx, 4xx, 5xx appropriately)
5. Pagination: Always paginate large collections
6. Versioning: Plan for breaking changes from day one
7. Documentation: Use OpenAPI for interactive docs
8. Error Handling: Standardize error response format
9. Security: Implement authentication, authorization, rate limiting
10. Consistency: Maintain consistent naming and structure

Common Pitfalls to Avoid

• Using verbs in endpoints: /api/createUser (wrong)
• Inconsistent naming: Mixing /users and /user
• Ignoring HTTP semantics: POST for idempotent operations
• Missing status codes: Not using 201 for creation
• Poor pagination: Returning all results
• No rate limiting: APIs vulnerable to abuse
• Tight coupling: API structure mirrors database schema
• Undocumented APIs: No OpenAPI/documentation

Cross-Skill References

• graphql-api-design skill - For comparison and GraphQL alternatives
• api-architecture skill - For versioning strategies, security, and monitoring
• api-testing skill - For testing REST endpoints and validation

Reference files for this skill are planned for a future release.