API Documentation Generator

What this skill does

This skill reads REST API route handlers (Express, FastAPI, Go net/http, Rails, Django, or any framework) and generates complete OpenAPI 3.0 YAML documentation. It extracts paths, HTTP methods, path/query/body parameters, response shapes, and error codes, then writes properly structured YAML with example request and response bodies. The output can be loaded directly into Swagger UI, Redoc, or any OpenAPI-compatible tool.

Use this when you have undocumented API code and need to produce accurate, usable API documentation quickly.

How to use

Claude Code / Cline

Copy this file to .agents/skills/api-docs-generator/SKILL.md in your project root.

Then point the agent at your routes file and ask:

• "Use the API Documentation Generator skill on server/routes.ts."
• "Generate OpenAPI docs for all routes in src/api/ using the API Documentation Generator skill."

The agent will read the route files and any referenced handler functions to extract parameter and response shapes.

Cursor

Add the "Prompt / Instructions" section to your .cursorrules file. Open your routes file in the editor and ask Cursor to generate the OpenAPI YAML.

Codex

Paste your route definitions and handler code into the chat along with the instructions below. Include type definitions or schema files if they exist — they help Codex produce accurate request/response schemas.

The Prompt / Instructions for the Agent

When asked to generate API documentation, follow these steps:

1. Read the route definitions. For each route, extract:

   • HTTP method (GET, POST, PUT, PATCH, DELETE)
   • Path, including path parameters (e.g., /users/:id → /users/{id})
   • Router-level middleware (e.g., authentication guards — these become security entries)
   • The handler function name

2. Read the handler functions. For each handler, extract:

   • Path parameters (e.g., req.params.id)
   • Query parameters (e.g., req.query.page, req.query.limit)
   • Request body shape — read from validation schema (Zod, Joi, Pydantic, etc.) if present, otherwise infer from how req.body fields are used
   • All res.json() / return response shapes — enumerate every response including error paths
   • HTTP status codes returned (200, 201, 400, 401, 404, 500, etc.)

3. Infer data types accurately:

   • If a Zod/Joi/Pydantic/serializer schema exists for the body or response, use it as the source of truth
   • Otherwise infer types from usage: string, number, boolean, array, object
   • Mark fields as required if they are destructured without a default or checked for presence
   • Note nullable: true on fields that may be null

4. Write the OpenAPI 3.0 YAML with this structure:

   • openapi: "3.0.3"
   • info: title, version, description
   • servers: include a localhost dev server at minimum
   • paths: one entry per route
   • components/schemas: reusable schema objects referenced with $ref
   • components/securitySchemes: if auth middleware is detected (Bearer, API key, cookie)

5. For each path entry, include:

   • summary: one sentence describing what the endpoint does
   • description: optional longer explanation
   • parameters: path, query, and header params with name, in, required, schema, description
   • requestBody: with content/application/json/schema and a fully-formed example
   • responses: at minimum the success response and common error responses (400, 401, 404, 500)
   • Each response must have a description and a content/application/json/schema

6. Include realistic example values in request and response bodies. Do not use placeholder values like "string" or 0 — use realistic data (e.g., "john@example.com", 42, "2026-03-15T12:00:00Z").

7. Output the complete YAML in a single code block. Do not truncate.

Example

Input to Agent:

"Use the API Documentation Generator skill on this Express route:

router.post('/users', async (req, res) => {
  const { name, email, role } = req.body;
  if (!name || !email) return res.status(400).json({ error: 'name and email are required' });
  const user = await db.users.create({ name, email, role: role ?? 'viewer' });
  res.status(201).json({ id: user.id, name: user.name, email: user.email, role: user.role });
});

router.get('/users/:id', requireAuth, async (req, res) => {
  const user = await db.users.findById(req.params.id);
  if (!user) return res.status(404).json({ error: 'User not found' });
  res.json({ id: user.id, name: user.name, email: user.email, role: user.role });
});
```"

Output from Agent:

openapi: "3.0.3"
info:
  title: Users API
  version: "1.0.0"
  description: Endpoints for managing users.
servers:
  - url: http://localhost:3000
    description: Local development
components:
  securitySchemes:
    bearerAuth:
      type: http
      scheme: bearer
  schemas:
    User:
      type: object
      properties:
        id:
          type: string
          example: "usr_a1b2c3"
        name:
          type: string
          example: "Jane Smith"
        email:
          type: string
          format: email
          example: "jane@example.com"
        role:
          type: string
          enum: [viewer, admin]
          example: "viewer"
    Error:
      type: object
      properties:
        error:
          type: string
paths:
  /users:
    post:
      summary: Create a new user
      requestBody:
        required: true
        content:
          application/json:
            schema:
              type: object
              required: [name, email]
              properties:
                name:
                  type: string
                  example: "Jane Smith"
                email:
                  type: string
                  format: email
                  example: "jane@example.com"
                role:
                  type: string
                  enum: [viewer, admin]
                  default: viewer
      responses:
        "201":
          description: User created successfully.
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/User'
        "400":
          description: Missing required fields.
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Error'
  /users/{id}:
    get:
      summary: Get a user by ID
      security:
        - bearerAuth: []
      parameters:
        - name: id
          in: path
          required: true
          schema:
            type: string
          example: "usr_a1b2c3"
      responses:
        "200":
          description: User found.
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/User'
        "401":
          description: Not authenticated.
        "404":
          description: User not found.
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Error'