OpenAPI Spec Generation

Comprehensive patterns for creating, maintaining, and validating OpenAPI 3.1 specifications for RESTful APIs.

When to Use This Skill

• Creating API documentation from scratch
• Generating OpenAPI specs from existing code
• Designing API contracts (design-first approach)
• Validating API implementations against specs
• Generating client SDKs from specs
• Setting up API documentation portals

Core Concepts

1. OpenAPI 3.1 Structure

openapi: 3.1.0
info:
  title: API Title
  version: 1.0.0
servers:
  - url: https://api.example.com/v1
paths:
  /resources:
    get: ...
components:
  schemas: ...
  securitySchemes: ...

2. Design Approaches

Approach	Description	Best For
Design-First	Write spec before code	New APIs, contracts
Code-First	Generate spec from code	Existing APIs
Hybrid	Annotate code, generate spec	Evolving APIs

Templates

Template 1: Complete API Specification

openapi: 3.1.0
info:
  title: User Management API
  description: |
    API for managing users and their profiles.

    ## Authentication
    All endpoints require Bearer token authentication.

    ## Rate Limiting
    - 1000 requests per minute for standard tier
    - 10000 requests per minute for enterprise tier
  version: 2.0.0
  contact:
    name: API Support
    email: api-support@example.com
    url: https://docs.example.com
  license:
    name: MIT
    url: https://opensource.org/licenses/MIT

servers:
  - url: https://api.example.com/v2
    description: Production
  - url: https://staging-api.example.com/v2
    description: Staging
  - url: http://localhost:3000/v2
    description: Local development

tags:
  - name: Users
    description: User management operations
  - name: Profiles
    description: User profile operations
  - name: Admin
    description: Administrative operations

paths:
  /users:
    get:
      operationId: listUsers
      summary: List all users
      description: Returns a paginated list of users with optional filtering.
      tags:
        - Users
      parameters:
        - $ref: "#/components/parameters/PageParam"
        - $ref: "#/components/parameters/LimitParam"
        - name: status
          in: query
          description: Filter by user status
          schema:
            $ref: "#/components/schemas/UserStatus"
        - name: search
          in: query
          description: Search by name or email
          schema:
            type: string
            minLength: 2
            maxLength: 100
      responses:
        "200":
          description: Successful response
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/UserListResponse"
              examples:
                default:
                  $ref: "#/components/examples/UserListExample"
        "400":
          $ref: "#/components/responses/BadRequest"
        "401":
          $ref: "#/components/responses/Unauthorized"
        "429":
          $ref: "#/components/responses/RateLimited"
      security:
        - bearerAuth: []

    post:
      operationId: createUser
      summary: Create a new user
      description: Creates a new user account and sends welcome email.
      tags:
        - Users
      requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: "#/components/schemas/CreateUserRequest"
            examples:
              standard:
                summary: Standard user
                value:
                  email: user@example.com
                  name: John Doe
                  role: user
              admin:
                summary: Admin user
                value:
                  email: admin@example.com
                  name: Admin User
                  role: admin
      responses:
        "201":
          description: User created successfully
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/User"
          headers:
            Location:
              description: URL of created user
              schema:
                type: string
                format: uri
        "400":
          $ref: "#/components/responses/BadRequest"
        "409":
          description: Email already exists
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/Error"
      security:
        - bearerAuth: []

  /users/{userId}:
    parameters:
      - $ref: "#/components/parameters/UserIdParam"

    get:
      operationId: getUser
      summary: Get user by ID
      tags:
        - Users
      responses:
        "200":
          description: Successful response
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/User"
        "404":
          $ref: "#/components/responses/NotFound"
      security:
        - bearerAuth: []

    patch:
      operationId: updateUser
      summary: Update user
      tags:
        - Users
      requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: "#/components/schemas/UpdateUserRequest"
      responses:
        "200":
          description: User updated
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/User"
        "400":
          $ref: "#/components/responses/BadRequest"
        "404":
          $ref: "#/components/responses/NotFound"
      security:
        - bearerAuth: []

    delete:
      operationId: deleteUser
      summary: Delete user
      tags:
        - Users
        - Admin
      responses:
        "204":
          description: User deleted
        "404":
          $ref: "#/components/responses/NotFound"
      security:
        - bearerAuth: []
        - apiKey: []

components:
  schemas:
    User:
      type: object
      required:
        - id
        - email
        - name
        - status
        - createdAt
      properties:
        id:
          type: string
          format: uuid
          readOnly: true
          description: Unique user identifier
        email:
          type: string
          format: email
          description: User email address
        name:
          type: string
          minLength: 1
          maxLength: 100
          description: User display name
        status:
          $ref: "#/components/schemas/UserStatus"
        role:
          type: string
          enum: [user, moderator, admin]
          default: user
        avatar:
          type: string
          format: uri
          nullable: true
        metadata:
          type: object
          additionalProperties: true
          description: Custom metadata
        createdAt:
          type: string
          format: date-time
          readOnly: true
        updatedAt:
          type: string
          format: date-time
          readOnly: true

    UserStatus:
      type: string
      enum: [active, inactive, suspended, pending]
      description: User account status

    CreateUserRequest:
      type: object
      required:
        - email
        - name
      properties:
        email:
          type: string
          format: email
        name:
          type: string
          minLength: 1
          maxLength: 100
        role:
          type: string
          enum: [user, moderator, admin]
          default: user
        metadata:
          type: object
          additionalProperties: true

    UpdateUserRequest:
      type: object
      minProperties: 1
      properties:
        name:
          type: string
          minLength: 1
          maxLength: 100
        status:
          $ref: "#/components/schemas/UserStatus"
        role:
          type: string
          enum: [user, moderator, admin]
        metadata:
          type: object
          additionalProperties: true

    UserListResponse:
      type: object
      required:
        - data
        - pagination
      properties:
        data:
          type: array
          items:
            $ref: "#/components/schemas/User"
        pagination:
          $ref: "#/components/schemas/Pagination"

    Pagination:
      type: object
      required:
        - page
        - limit
        - total
        - totalPages
      properties:
        page:
          type: integer
          minimum: 1
        limit:
          type: integer
          minimum: 1
          maximum: 100
        total:
          type: integer
          minimum: 0
        totalPages:
          type: integer
          minimum: 0
        hasNext:
          type: boolean
        hasPrev:
          type: boolean

    Error:
      type: object
      required:
        - code
        - message
      properties:
        code:
          type: string
          description: Error code for programmatic handling
        message:
          type: string
          description: Human-readable error message
        details:
          type: array
          items:
            type: object
            properties:
              field:
                type: string
              message:
                type: string
        requestId:
          type: string
          description: Request ID for support

  parameters:
    UserIdParam:
      name: userId
      in: path
      required: true
      description: User ID
      schema:
        type: string
        format: uuid

    PageParam:
      name: page
      in: query
      description: Page number (1-based)
      schema:
        type: integer
        minimum: 1
        default: 1

    LimitParam:
      name: limit
      in: query
      description: Items per page
      schema:
        type: integer
        minimum: 1
        maximum: 100
        default: 20

  responses:
    BadRequest:
      description: Invalid request
      content:
        application/json:
          schema:
            $ref: "#/components/schemas/Error"
          example:
            code: VALIDATION_ERROR
            message: Invalid request parameters
            details:
              - field: email
                message: Must be a valid email address

    Unauthorized:
      description: Authentication required
      content:
        application/json:
          schema:
            $ref: "#/components/schemas/Error"
          example:
            code: UNAUTHORIZED
            message: Authentication required

    NotFound:
      description: Resource not found
      content:
        application/json:
          schema:
            $ref: "#/components/schemas/Error"
          example:
            code: NOT_FOUND
            message: User not found

    RateLimited:
      description: Too many requests
      content:
        application/json:
          schema:
            $ref: "#/components/schemas/Error"
      headers:
        Retry-After:
          description: Seconds until rate limit resets
          schema:
            type: integer
        X-RateLimit-Limit:
          description: Request limit per window
          schema:
            type: integer
        X-RateLimit-Remaining:
          description: Remaining requests in window
          schema:
            type: integer

  examples:
    UserListExample:
      value:
        data:
          - id: "550e8400-e29b-41d4-a716-446655440000"
            email: "john@example.com"
            name: "John Doe"
            status: "active"
            role: "user"
            createdAt: "2024-01-15T10:30:00Z"
        pagination:
          page: 1
          limit: 20
          total: 1
          totalPages: 1
          hasNext: false
          hasPrev: false

  securitySchemes:
    bearerAuth:
      type: http
      scheme: bearer
      bearerFormat: JWT
      description: JWT token from /auth/login

    apiKey:
      type: apiKey
      in: header
      name: X-API-Key
      description: API key for service-to-service calls

security:
  - bearerAuth: []

For advanced code-first generation patterns and tooling, see references/code-first-and-tooling.md:

• Template 2: Python/FastAPI — Pydantic models with Field validation, enum types, full CRUD endpoints with response_model and status_code, exporting the spec as JSON
• Template 3: TypeScript/tsoa — Decorator-based controllers (@Route, @Get, @Security, @Example, @Response) that generate OpenAPI from TypeScript types
• Template 4: Validation & Linting — Spectral ruleset (.spectral.yaml) with custom rules for operationId, security, naming conventions; Redocly config with MIME type enforcement and code sample generation
• SDK Generation — openapi-generator-cli for TypeScript (fetch), Python, and Go clients

Best Practices

Do's

• Use $ref - Reuse schemas, parameters, responses
• Add examples - Real-world values help consumers
• Document errors - All possible error codes
• Version your API - In URL or header
• Use semantic versioning - For spec changes

Don'ts

• Don't use generic descriptions - Be specific
• Don't skip security - Define all schemes
• Don't forget nullable - Be explicit about null
• Don't mix styles - Consistent naming throughout
• Don't hardcode URLs - Use server variables