api-endpoint-builder skill

API Endpoint Builder

Build complete, production-ready REST API endpoints with proper validation, error handling, authentication, and documentation.

When to Use This Skill

• User asks to "create an API endpoint" or "build a REST API"
• Building new backend features
• Adding endpoints to existing APIs
• User mentions "API", "endpoint", "route", or "REST"
• Creating CRUD operations

What You'll Build

For each endpoint, you create:

• Route handler with proper HTTP method
• Input validation (request body, params, query)
• Authentication/authorization checks
• Business logic
• Error handling
• Response formatting
• API documentation
• Tests (if requested)

Endpoint Structure

1. Route Definition

// Express example
router.post('/api/users', authenticate, validateUser, createUser);

// Fastify example
fastify.post('/api/users', {
  preHandler: [authenticate],
  schema: userSchema
}, createUser);

2. Input Validation

Always validate before processing:

const validateUser = (req, res, next) => {
  const { email, name, password } = req.body;
  
  if (!email || !email.includes('@')) {
    return res.status(400).json({ error: 'Valid email required' });
  }
  
  if (!name || name.length < 2) {
    return res.status(400).json({ error: 'Name must be at least 2 characters' });
  }
  
  if (!password || password.length < 8) {
    return res.status(400).json({ error: 'Password must be at least 8 characters' });
  }
  
  next();
};

3. Handler Implementation

const createUser = async (req, res) => {
  try {
    const { email, name, password } = req.body;
    
    // Check if user exists
    const existing = await db.users.findOne({ email });
    if (existing) {
      return res.status(409).json({ error: 'User already exists' });
    }
    
    // Hash password
    const hashedPassword = await bcrypt.hash(password, 10);
    
    // Create user
    const user = await db.users.create({
      email,
      name,
      password: hashedPassword,
      createdAt: new Date()
    });
    
    // Don't return password
    const { password: _, ...userWithoutPassword } = user;
    
    res.status(201).json({
      success: true,
      data: userWithoutPassword
    });
    
  } catch (error) {
    console.error('Create user error:', error);
    res.status(500).json({ error: 'Internal server error' });
  }
};

Best Practices

HTTP Status Codes

• 200 - Success (GET, PUT, PATCH)
• 201 - Created (POST)
• 204 - No Content (DELETE)
• 400 - Bad Request (validation failed)
• 401 - Unauthorized (not authenticated)
• 403 - Forbidden (not authorized)
• 404 - Not Found
• 409 - Conflict (duplicate)
• 500 - Internal Server Error

Response Format

Consistent structure:

// Success
{
  "success": true,
  "data": { ... }
}

// Error
{
  "error": "Error message",
  "details": { ... } // optional
}

// List with pagination
{
  "success": true,
  "data": [...],
  "pagination": {
    "page": 1,
    "limit": 20,
    "total": 100
  }
}

Security Checklist

• [ ] Authentication required for protected routes
• [ ] Authorization checks (user owns resource)
• [ ] Input validation on all fields
• [ ] SQL injection prevention (use parameterized queries)
• [ ] Rate limiting on public endpoints
• [ ] No sensitive data in responses (passwords, tokens)
• [ ] CORS configured properly
• [ ] Request size limits set

Error Handling

// Centralized error handler
app.use((err, req, res, next) => {
  console.error(err.stack);
  
  // Don't leak error details in production
  const message = process.env.NODE_ENV === 'production' 
    ? 'Internal server error' 
    : err.message;
  
  res.status(err.status || 500).json({ error: message });
});

Common Patterns

CRUD Operations

// Create
POST /api/resources
Body: { name, description }

// Read (list)
GET /api/resources?page=1&limit=20

// Read (single)
GET /api/resources/:id

// Update
PUT /api/resources/:id
Body: { name, description }

// Delete
DELETE /api/resources/:id

Pagination

const getResources = async (req, res) => {
  const page = parseInt(req.query.page) || 1;
  const limit = parseInt(req.query.limit) || 20;
  const skip = (page - 1) * limit;
  
  const [resources, total] = await Promise.all([
    db.resources.find().skip(skip).limit(limit),
    db.resources.countDocuments()
  ]);
  
  res.json({
    success: true,
    data: resources,
    pagination: {
      page,
      limit,
      total,
      pages: Math.ceil(total / limit)
    }
  });
};

Filtering & Sorting

const getResources = async (req, res) => {
  const { status, sort = '-createdAt' } = req.query;
  
  const filter = {};
  if (status) filter.status = status;
  
  const resources = await db.resources
    .find(filter)
    .sort(sort)
    .limit(20);
  
  res.json({ success: true, data: resources });
};

Documentation Template

/**
 * @route POST /api/users
 * @desc Create a new user
 * @access Public
 * 
 * @body {string} email - User email (required)
 * @body {string} name - User name (required)
 * @body {string} password - Password, min 8 chars (required)
 * 
 * @returns {201} User created successfully
 * @returns {400} Validation error
 * @returns {409} User already exists
 * @returns {500} Server error
 * 
 * @example
 * POST /api/users
 * {
 *   "email": "user@example.com",
 *   "name": "John Doe",
 *   "password": "securepass123"
 * }
 */

Testing Example

describe('POST /api/users', () => {
  it('should create a new user', async () => {
    const response = await request(app)
      .post('/api/users')
      .send({
        email: 'test@example.com',
        name: 'Test User',
        password: 'password123'
      });
    
    expect(response.status).toBe(201);
    expect(response.body.success).toBe(true);
    expect(response.body.data.email).toBe('test@example.com');
    expect(response.body.data.password).toBeUndefined();
  });
  
  it('should reject invalid email', async () => {
    const response = await request(app)
      .post('/api/users')
      .send({
        email: 'invalid',
        name: 'Test User',
        password: 'password123'
      });
    
    expect(response.status).toBe(400);
    expect(response.body.error).toContain('email');
  });
});

Key Principles

• Validate all inputs before processing
• Use proper HTTP status codes
• Handle errors gracefully
• Never expose sensitive data
• Keep responses consistent
• Add authentication where needed
• Document your endpoints
• Write tests for critical paths

Related Skills

• @security-auditor - Security review
• @test-driven-development - Testing
• @database-design - Data modeling

Limitations

• Use this skill only when the task clearly matches the scope described above.
• Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
• Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.