API Documentation Generator Agent Skill

API Documentation Generator

Overview

Automatically generate clear, comprehensive API documentation from your codebase. This skill helps you create professional documentation that includes endpoint descriptions, request/response examples, authentication details, error handling, and usage guidelines.

Perfect for REST APIs, GraphQL APIs, and WebSocket APIs.

When to Use This Skill

• Use when you need to document a new API
• Use when updating existing API documentation
• Use when your API lacks clear documentation
• Use when onboarding new developers to your API
• Use when preparing API documentation for external users
• Use when creating OpenAPI/Swagger specifications

How It Works

Step 1: Analyze the API Structure

First, I'll examine your API codebase to understand:

• Available endpoints and routes
• HTTP methods (GET, POST, PUT, DELETE, etc.)
• Request parameters and body structure
• Response formats and status codes
• Authentication and authorization requirements
• Error handling patterns

Step 2: Generate Endpoint Documentation

For each endpoint, I'll create documentation including:

Endpoint Details:

• HTTP method and URL path
• Brief description of what it does
• Authentication requirements
• Rate limiting information (if applicable)

Request Specification:

• Path parameters
• Query parameters
• Request headers
• Request body schema (with types and validation rules)

Response Specification:

• Success response (status code + body structure)
• Error responses (all possible error codes)
• Response headers

Code Examples:

• cURL command
• JavaScript/TypeScript (fetch/axios)
• Python (requests)
• Other languages as needed

Step 3: Add Usage Guidelines

I'll include:

• Getting started guide
• Authentication setup
• Common use cases
• Best practices
• Rate limiting details
• Pagination patterns
• Filtering and sorting options

Step 4: Document Error Handling

Clear error documentation including:

• All possible error codes
• Error message formats
• Troubleshooting guide
• Common error scenarios and solutions

Step 5: Create Interactive Examples

Where possible, I'll provide:

• Postman collection
• OpenAPI/Swagger specification
• Interactive code examples
• Sample responses

Examples

Example 1: REST API Endpoint Documentation

## Create User

Creates a new user account.

**Endpoint:** `POST /api/v1/users`

**Authentication:** Required (Bearer token)

**Request Body:**
\`\`\`json
{
  "email": "[email protected]",      // Required: Valid email address
  "password": "SecurePass123!",     // Required: Min 8 chars, 1 uppercase, 1 number
  "name": "John Doe",               // Required: 2-50 characters
  "role": "user"                    // Optional: "user" or "admin" (default: "user")
}
\`\`\`

**Success Response (201 Created):**
\`\`\`json
{
  "id": "usr_1234567890",
  "email": "[email protected]",
  "name": "John Doe",
  "role": "user",
  "createdAt": "2026-01-20T10:30:00Z",
  "emailVerified": false
}
\`\`\`

**Error Responses:**

- `400 Bad Request` - Invalid input data
  \`\`\`json
  {
    "error": "VALIDATION_ERROR",
    "message": "Invalid email format",
    "field": "email"
  }
  \`\`\`

- `409 Conflict` - Email already exists
  \`\`\`json
  {
    "error": "EMAIL_EXISTS",
    "message": "An account with this email already exists"
  }
  \`\`\`

- `401 Unauthorized` - Missing or invalid authentication token

**Example Request (cURL):**
\`\`\`bash
curl -X POST https://api.example.com/api/v1/users \
  -H "Authorization: Bearer YOUR_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "email": "[email protected]",
    "password": "SecurePass123!",
    "name": "John Doe"
  }'
\`\`\`

**Example Request (JavaScript):**
\`\`\`javascript
const response = await fetch('https://api.example.com/api/v1/users', {
  method: 'POST',
  headers: {
    'Authorization': `Bearer ${token}`,
    'Content-Type': 'application/json'
  },
  body: JSON.stringify({
    email: '[email protected]',
    password: 'SecurePass123!',
    name: 'John Doe'
  })
});

const user = await response.json();
console.log(user);
\`\`\`

**Example Request (Python):**
\`\`\`python
import requests

response = requests.post(
    'https://api.example.com/api/v1/users',
    headers={
        'Authorization': f'Bearer {token}',
        'Content-Type': 'application/json'
    },
    json={
        'email': '[email protected]',
        'password': 'SecurePass123!',
        'name': 'John Doe'
    }
)

user = response.json()
print(user)
\`\`\`

Example 2: GraphQL API Documentation

## User Query

Fetch user information by ID.

**Query:**
\`\`\`graphql
query GetUser($id: ID!) {
  user(id: $id) {
    id
    email
    name
    role
    createdAt
    posts {
      id
      title
      publishedAt
    }
  }
}
\`\`\`

**Variables:**
\`\`\`json
{
  "id": "usr_1234567890"
}
\`\`\`

**Response:**
\`\`\`json
{
  "data": {
    "user": {
      "id": "usr_1234567890",
      "email": "[email protected]",
      "name": "John Doe",
      "role": "user",
      "createdAt": "2026-01-20T10:30:00Z",
      "posts": [
        {
          "id": "post_123",
          "title": "My First Post",
          "publishedAt": "2026-01-21T14:00:00Z"
        }
      ]
    }
  }
}
\`\`\`

**Errors:**
\`\`\`json
{
  "errors": [
    {
      "message": "User not found",
      "extensions": {
        "code": "USER_NOT_FOUND",
        "userId": "usr_1234567890"
      }
    }
  ]
}
\`\`\`

Example 3: Authentication Documentation

## Authentication

All API requests require authentication using Bearer tokens.

### Getting a Token

**Endpoint:** `POST /api/v1/auth/login`

**Request:**
\`\`\`json
{
  "email": "[email protected]",
  "password": "your-password"
}
\`\`\`

**Response:**
\`\`\`json
{
  "token": "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...",
  "expiresIn": 3600,
  "refreshToken": "refresh_token_here"
}
\`\`\`

### Using the Token

Include the token in the Authorization header:

\`\`\`
Authorization: Bearer YOUR_TOKEN
\`\`\`

### Token Expiration

Tokens expire after 1 hour. Use the refresh token to get a new access token:

**Endpoint:** `POST /api/v1/auth/refresh`

**Request:**
\`\`\`json
{
  "refreshToken": "refresh_token_here"
}
\`\`\`

Best Practices

✅ Do This

• Be Consistent - Use the same format for all endpoints
• Include Examples - Provide working code examples in multiple languages
• Document Errors - List all possible error codes and their meanings
• Show Real Data - Use realistic example data, not "foo" and "bar"
• Explain Parameters - Describe what each parameter does and its constraints
• Version Your API - Include version numbers in URLs (/api/v1/)
• Add Timestamps - Show when documentation was last updated
• Link Related Endpoints - Help users discover related functionality
• Include Rate Limits - Document any rate limiting policies
• Provide Postman Collection - Make it easy to test your API

❌ Don't Do This

• Don't Skip Error Cases - Users need to know what can go wrong
• Don't Use Vague Descriptions - "Gets data" is not helpful
• Don't Forget Authentication - Always document auth requirements
• Don't Ignore Edge Cases - Document pagination, filtering, sorting
• Don't Leave Examples Broken - Test all code examples
• Don't Use Outdated Info - Keep documentation in sync with code
• Don't Overcomplicate - Keep it simple and scannable
• Don't Forget Response Headers - Document important headers

Documentation Structure

Recommended Sections

• Introduction
• What the API does
• Base URL
• API version
• Support contact

• Authentication
• How to authenticate
• Token management
• Security best practices

• Quick Start
• Simple example to get started
• Common use case walkthrough

• Endpoints
• Organized by resource
• Full details for each endpoint

• Data Models
• Schema definitions
• Field descriptions
• Validation rules

• Error Handling
• Error code reference
• Error response format
• Troubleshooting guide

• Rate Limiting
• Limits and quotas
• Headers to check
• Handling rate limit errors

• Changelog
• API version history
• Breaking changes
• Deprecation notices

• SDKs and Tools
• Official client libraries
• Postman collection
• OpenAPI specification

Common Pitfalls

Problem: Documentation Gets Out of Sync

Symptoms: Examples don't work, parameters are wrong, endpoints return different data

Solution:

• Generate docs from code comments/annotations
• Use tools like Swagger/OpenAPI
• Add API tests that validate documentation
• Review docs with every API change

Problem: Missing Error Documentation

Symptoms: Users don't know how to handle errors, support tickets increase

Solution:

• Document every possible error code
• Provide clear error messages
• Include troubleshooting steps
• Show example error responses

Problem: Examples Don't Work

Symptoms: Users can't get started, frustration increases

Solution:

• Test every code example
• Use real, working endpoints
• Include complete examples (not fragments)
• Provide a sandbox environment

Problem: Unclear Parameter Requirements

Symptoms: Users send invalid requests, validation errors

Solution:

• Mark required vs optional clearly
• Document data types and formats
• Show validation rules
• Provide example values

Tools and Formats

OpenAPI/Swagger

Generate interactive documentation:

openapi: 3.0.0
info:
  title: My API
  version: 1.0.0
paths:
  /users:
    post:
      summary: Create a new user
      requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/CreateUserRequest'

Postman Collection

Export collection for easy testing:

{
  "info": {
    "name": "My API",
    "schema": "https://schema.getpostman.com/json/collection/v2.1.0/collection.json"
  },
  "item": [
    {
      "name": "Create User",
      "request": {
        "method": "POST",
        "url": "{{baseUrl}}/api/v1/users"
      }
    }
  ]
}

Related Skills

• @doc-coauthoring - For collaborative documentation writing
• @copywriting - For clear, user-friendly descriptions
• @test-driven-development - For ensuring API behavior matches docs
• @systematic-debugging - For troubleshooting API issues

Additional Resources

• OpenAPI Specification
• REST API Best Practices
• GraphQL Documentation
• API Design Patterns
• Postman Documentation

Pro Tip: Keep your API documentation as close to your code as possible. Use tools that generate docs from code comments to ensure they stay in sync!

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.