---

name: phase-4-api description: | Skill for designing and implementing backend APIs. Includes Zero Script QA methodology for validating APIs without test scripts.

Use proactively when user needs to design or implement backend APIs.

Triggers: API design, REST API, backend, endpoint, API 설계, API設計, API设计

Do NOT use for: frontend-only projects, static websites, or Starter level projects. hooks: Stop: - hooks: - type: command command: "node ${CLAUDE_PLUGIN_ROOT}/scripts/phase4-api-stop.js" agent: qa-monitor allowed-tools:

• Read
• Write
• Edit
• Glob
• Grep
• Bash user-invocable: false

---

Phase 4: API Design/Implementation + Zero Script QA

Backend API implementation and script-free QA

Purpose

Implement backend APIs that can store and retrieve data. Validate with structured logs instead of test scripts.

What to Do in This Phase

1. API Design: Define endpoints, requests/responses
2. API Implementation: Write actual backend code
3. Zero Script QA: Log-based validation

Deliverables

Copydocs/02-design/
└── api-spec.md             # API specification

src/api/                    # API implementation
├── routes/
├── controllers/
└── services/

docs/03-analysis/
└── api-qa.md               # QA results

PDCA Application

• Plan: Define required API list
• Design: Design endpoints, requests/responses
• Do: Implement APIs
• Check: Validate with Zero Script QA
• Act: Fix bugs and proceed to Phase 5

Level-wise Application

What is Zero Script QA?

CopyInstead of writing test scripts, validate with structured debug logs

[API] POST /api/users
[INPUT] { "email": "test@test.com", "name": "Test" }
[PROCESS] Email duplicate check → Passed
[PROCESS] Password hash → Complete
[PROCESS] DB save → Success
[OUTPUT] { "id": 1, "email": "test@test.com" }
[RESULT] ✅ Success

Advantages:
- Save test code writing time
- See actual behavior with your eyes
- Easy debugging

RESTful API Principles

What is REST?

REpresentational State Transfer - an architecture style for designing web services.

6 Core Principles

Uniform Interface Details

The core of RESTful APIs is a uniform interface.

1. Resource-Based URLs

Copy✅ Good (nouns, plural)
GET    /users          # User list
GET    /users/123      # Specific user
POST   /users          # Create user
PUT    /users/123      # Update user
DELETE /users/123      # Delete user

❌ Bad (using verbs)
GET    /getUsers
POST   /createUser
POST   /deleteUser/123

2. HTTP Method Meanings

Idempotent: Same result even if requested multiple times Safe: Doesn't change server state

3. HTTP Status Codes

Copy2xx Success
├── 200 OK              # Success (read, update)
├── 201 Created         # Creation success
└── 204 No Content      # Success but no response body (delete)

4xx Client Error
├── 400 Bad Request     # Invalid request (validation failure)
├── 401 Unauthorized    # Authentication required
├── 403 Forbidden       # No permission
├── 404 Not Found       # Resource not found
└── 409 Conflict        # Conflict (duplicate, etc.)

5xx Server Error
├── 500 Internal Error  # Internal server error
└── 503 Service Unavailable  # Service unavailable

4. Consistent Response Format

Copy// Success response
{
  "data": {
    "id": 123,
    "email": "user@example.com",
    "name": "John Doe"
  },
  "meta": {
    "timestamp": "2026-01-08T10:00:00Z"
  }
}

// Error response
{
  "error": {
    "code": "VALIDATION_ERROR",
    "message": "Email format is invalid.",
    "details": [
      { "field": "email", "message": "Please enter a valid email" }
    ]
  }
}

// List response (pagination)
{
  "data": [...],
  "pagination": {
    "page": 1,
    "limit": 20,
    "total": 100,
    "totalPages": 5
  }
}

URL Design Rules

Copy1. Use lowercase
   ✅ /users/123/orders
   ❌ /Users/123/Orders

2. Use hyphens (-), avoid underscores (_)
   ✅ /user-profiles
   ❌ /user_profiles

3. Express hierarchical relationships
   ✅ /users/123/orders/456

4. Filtering via query parameters
   ✅ /users?status=active&sort=created_at
   ❌ /users/active/sort/created_at

5. Version management
   ✅ /api/v1/users
   ✅ Header: Accept: application/vnd.api+json;version=1

API Documentation Tools

---

API Design Checklist

• RESTful Principles Compliance
  • Resource-based URLs (nouns, plural)
  • Appropriate HTTP methods
  • Correct status codes
• Unified error response format
• Authentication/authorization method defined
• Pagination method defined
• Versioning method (optional)

Templates

• templates/pipeline/phase-4-api.template.md
• templates/pipeline/zero-script-qa.template.md

Next Phase

Phase 5: Design System → APIs are ready, now build UI components