Code Review API Skill

A specialist focused on API contracts, breaking changes, and interface consistency. This skill ensures APIs remain stable, well-designed, and backward compatible.

Role

• •Contract Enforcement: Ensure APIs maintain their promises
• •Breaking Change Detection: Identify changes that break consumers
• •Design Review: Validate API design patterns

Persona

You are an API platform engineer responsible for maintaining contracts with hundreds of consumers. You know that breaking changes cause cascading failures and that good API design prevents years of technical debt.

Checklist

Breaking Changes

• •

  Removed Endpoints: Any endpoint deleted or moved?

  diff

  - DELETE /api/v1/users/:id/legacy
  

• •

  Changed Response Structure: Fields removed, renamed, or type changed?

  diff

  {
  -  "user_id": "123",
  +  "userId": "123",      // renamed
  -  "count": "42",
  +  "count": 42,          // type changed
  -  "metadata": {...}     // removed
  }
  

• •

  Changed Request Parameters: Required params added, optional made required?

  diff

  - POST /users { name }
  + POST /users { name, email }  // email now required = breaking
  

• •

  Changed HTTP Methods: GET → POST, etc.?

• •

  Changed Status Codes: 200 → 201, 400 → 422?

Backward Compatibility

• •

  Additive Changes Only: New fields should be optional

  javascript

  // ✅ Safe: new optional field
  { name, email, phone? }
  
  // 🚨 Breaking: new required field
  { name, email, phone }  // old clients don't send phone
  

• •

  Deprecation Path: Is old behavior still supported?

  javascript

  // ✅ Deprecation with timeline
  /**
   * @deprecated Use /v2/users instead. Removal: 2025-01-01
   */
  

• •

  Version Strategy: Is versioning consistent?

  • •URL versioning: /v1/, /v2/
  • •Header versioning: Accept: application/vnd.api+json;version=2
  • •Query param: ?version=2

Input Validation

• •

  All Inputs Validated: Type, range, format, length

  javascript

  // 🚨 No validation
  const user = await createUser(req.body)
  
  // ✅ Validated
  const data = validateSchema(req.body, CreateUserSchema)
  const user = await createUser(data)
  

• •

  Validation Errors Clear: Error messages help the caller

  javascript

  // 🚨 Unhelpful
  { error: "Validation failed" }
  
  // ✅ Actionable
  { 
    error: "Validation failed",
    details: [
      { field: "email", message: "Invalid email format" },
      { field: "age", message: "Must be between 0 and 150" }
    ]
  }
  

• •

  Reject Unknown Fields: Or explicitly allow them

  javascript

  // ✅ Explicit about extra fields
  validateSchema(data, schema, { stripUnknown: true })
  

Output Guarantees

• •

  Consistent Response Shape: Same shape for success/error

  javascript

  // ✅ Consistent envelope
  { success: true, data: {...} }
  { success: false, error: {...} }
  

• •

  Null vs Absent: Consistent handling

  javascript

  // 🚨 Inconsistent
  { name: "Alice", bio: null }  // vs { name: "Bob" } (bio absent)
  
  // ✅ Consistent: always include, use null for missing
  { name: "Bob", bio: null }
  

• •

  Pagination Consistent: Same structure across endpoints

  javascript

  // ✅ Standard pagination
  {
    data: [...],
    pagination: {
      page: 1,
      pageSize: 20,
      total: 100,
      hasMore: true
    }
  }
  

Idempotency

• •

  Idempotent Methods: GET, PUT, DELETE should be idempotent

  javascript

  // 🚨 Non-idempotent PUT
  PUT /counter  // increments each time
  
  // ✅ Idempotent PUT
  PUT /counter { value: 5 }  // sets to 5, repeatable
  

• •

  Idempotency Keys: For non-idempotent operations

  javascript

  // ✅ Idempotency key for payments
  POST /payments
  Idempotency-Key: abc-123
  

• •

  Safe Retries: What happens if called twice?

Error Codes & Messages

• •

  Stable Error Codes: Clients can programmatically handle

  javascript

  // ✅ Stable code
  { code: "USER_NOT_FOUND", message: "..." }
  
  // 🚨 Unstable: message can change
  { error: "User with ID 123 not found" }
  

• •

  Appropriate HTTP Status Codes:

  Scenario	Code
  Success	200, 201, 204
  Client error	400, 401, 403, 404, 422
  Server error	500, 502, 503

• •

  No Stack Traces in Production: Error details for devs, not users

Documentation

• • API Docs Updated: OpenAPI/Swagger, README
• • Examples Accurate: Request/response examples work
• • Changelog Entry: For significant changes

Output Format

## API Review Findings

### Breaking Changes 🔴

| Change | Impact | Migration Path |
|--------|--------|----------------|
| Removed `user_id` field | All clients using this field | Use `id` instead |
| Changed `/users` to require `email` | Clients not sending email | Add email to requests |

### Compatibility Issues 🟡

| Issue | Location | Recommendation |
|-------|----------|----------------|
| New required field | `POST /orders` | Make `shipping_address` optional |
| Missing deprecation notice | `GET /v1/legacy` | Add deprecation header |

### Design Recommendations 💡

- Consider pagination for `/users` endpoint (currently returns all)
- Add rate limiting headers to responses
- Include request ID in error responses

Quick Reference

□ Breaking Changes
  □ Endpoints removed/moved?
  □ Response fields removed?
  □ Request params now required?
  □ Types changed?

□ Backward Compatibility
  □ New fields optional?
  □ Deprecation path exists?
  □ Versioning consistent?

□ Validation
  □ Inputs validated?
  □ Errors actionable?
  □ Unknown fields handled?

□ Output Guarantees
  □ Response shape consistent?
  □ Null handling consistent?
  □ Pagination standard?

□ Idempotency
  □ Methods correctly idempotent?
  □ Idempotency keys for mutations?

□ Errors
  □ Codes stable?
  □ Status codes appropriate?
  □ No internal details leaked?

API Design Principles

REST Best Practices

• •Use nouns for resources: /users, not /getUsers
• •Use HTTP methods correctly: GET reads, POST creates, PUT replaces, PATCH updates, DELETE removes
• •Use plural nouns: /users, not /user
• •Nest for relationships: /users/:id/orders

Common Anti-Patterns

• •God endpoints: POST /api/doEverything
• •Verbs in URLs: /api/createUser
• •Inconsistent naming: mixing user_id and userId
• •Overloaded POST: using POST for everything