API Documentation

When to Use This Skill

Use this skill when you need to:

• Create API documentation for a REST API with endpoint references
• Write authentication guides, request/response examples, and error handling docs
• Build a getting started guide for API consumers
• Structure a complete API reference for developer onboarding

DO NOT use this skill for internal code documentation, SDK documentation, or webhook setup guides. This is for external-facing REST API documentation.

Core Principle

API DOCUMENTATION IS WRITTEN FOR DEVELOPERS WHO WANT TO INTEGRATE, NOT READ — EVERY PAGE MUST ANSWER "HOW DO I DO THIS?" WITH A COPY-PASTE EXAMPLE.

Phase 1: Brief

Required Inputs

GATE: Confirm the brief before structuring the documentation.

Phase 2: Structure

Documentation Architecture

1. Overview — What the API does, who it is for, base URL
2. Authentication — How to get and use API keys
3. Quick Start — First API call in under 5 minutes
4. Endpoints Reference — Full endpoint documentation
5. Error Handling — Error codes, messages, and resolution
6. Rate Limits — Limits, headers, and handling 429 responses
7. Changelog — Version history and breaking changes

Endpoint Documentation Template

For each endpoint, document:

• HTTP method and path
• Description (one sentence)
• Authentication required (yes/no)
• Request parameters (path, query, body)
• Request example (cURL)
• Response example (JSON)
• Error responses
• Rate limit notes

GATE: Confirm structure and endpoint list before writing.

Phase 3: Write

Authentication Section

## Authentication

All API requests require an API key passed in the header:

\`\`\`
Authorization: Bearer YOUR_API_KEY
\`\`\`

**Getting your API key:**
1. Log in to your dashboard
2. Navigate to Settings → API
3. Click "Generate API Key"
4. Copy and store securely — it will not be shown again

**Security:** Never expose your API key in client-side code, public repositories, or URLs.

Quick Start Section

Write a complete first-request walkthrough:

1. Get your API key (link to auth section)
2. Make your first request (cURL example)
3. Understand the response
4. Next steps (link to full reference)

Endpoint Reference Format

## [METHOD] /endpoint/path

[One-sentence description of what this endpoint does.]

### Parameters

| Name | Type | Required | Description |
|------|------|----------|-------------|
| id | string | Yes | The resource ID |
| limit | integer | No | Max results (default: 20, max: 100) |

### Request Example

\`\`\`bash
curl -X GET "https://api.example.com/v1/resource" \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json"
\`\`\`

### Response Example

\`\`\`json
{
  "data": [...],
  "meta": {
    "total": 42,
    "page": 1,
    "per_page": 20
  }
}
\`\`\`

### Error Responses

| Code | Message | Resolution |
|------|---------|------------|
| 401 | Unauthorized | Check your API key |
| 404 | Not found | Verify the resource ID exists |
| 429 | Rate limited | Wait and retry after the Retry-After header value |

Writing Rules

• Every endpoint has a cURL example that can be copied and run immediately
• Response examples use realistic (but fake) data
• Error responses include resolution steps, not just codes
• Parameters table specifies type, required/optional, and default values
• Use consistent formatting across all endpoints

Phase 4: Polish

1. Error Reference Table

## Error Codes

| HTTP Code | Error Type | Description | Resolution |
|-----------|-----------|-------------|------------|
| 400 | Bad Request | Invalid parameters | Check request body against schema |
| 401 | Unauthorized | Missing or invalid API key | Verify your API key |
| 403 | Forbidden | Insufficient permissions | Check your plan's API access level |
| 404 | Not Found | Resource does not exist | Verify the resource ID |
| 429 | Too Many Requests | Rate limit exceeded | Retry after Retry-After header value |
| 500 | Internal Server Error | Server-side issue | Retry; contact support if persistent |

2. SDK and Library Suggestions

List official or community SDKs if available. If none exist, provide example code in 2-3 popular languages (Python, JavaScript, cURL).

3. Quality Checklist

## API Documentation Checklist

- [ ] Base URL is clearly stated at the top
- [ ] Authentication section includes step-by-step key setup
- [ ] Quick start gets a developer to their first response in under 5 minutes
- [ ] Every endpoint has method, path, description, parameters, and examples
- [ ] cURL examples are copy-paste ready
- [ ] Response examples use realistic fake data
- [ ] Error codes include resolution steps
- [ ] Rate limit policy is documented with handling guidance
- [ ] Changelog tracks breaking changes with dates
- [ ] No placeholder text or TODO items remain

Example

API: Invoice management API for a billing SaaS

Quick start excerpt:

## Quick Start

Make your first API call in 3 steps:

### 1. Get your API key
Go to Settings → API in your dashboard and generate a key.

### 2. List your invoices
curl -X GET "https://api.invoicebot.com/v1/invoices" \
  -H "Authorization: Bearer sk_test_abc123"

### 3. Check the response
{
  "data": [
    {
      "id": "inv_001",
      "client": "Acme Corp",
      "amount": 2500.00,
      "status": "paid"
    }
  ]
}

You are ready. Explore the full endpoint reference below.

Anti-Patterns

• No examples — documentation without copy-paste examples is unusable. Every endpoint needs a request and response sample.
• Outdated examples — examples that return errors destroy developer trust. Test every example before publishing.
• Missing error documentation — developers spend more time handling errors than happy paths. Document every error code.
• Jargon-heavy descriptions — "Retrieves a paginated collection of resource entities" should be "Lists all invoices."
• No quick start — forcing developers to read the entire reference before making their first call is a conversion killer.

Recovery

• API is still changing: Mark the docs as "beta" and version the API. Document known limitations upfront.
• Too many endpoints to document at once: Start with the 5 most-used endpoints. Add the rest incrementally.
• No SDK available: Provide examples in cURL, Python (requests), and JavaScript (fetch) for the top 3 endpoints.
• Authentication is complex (OAuth): Create a dedicated authentication guide with a flow diagram and step-by-step walkthrough.