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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

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: []

Template 2: Code-First Generation (Python/FastAPI)

# FastAPI with automatic OpenAPI generation
from fastapi import FastAPI, HTTPException, Query, Path, Depends
from pydantic import BaseModel, Field, EmailStr
from typing import Optional, List
from datetime import datetime
from uuid import UUID
from enum import Enum

app = FastAPI(
    title="User Management API",
    description="API for managing users and profiles",
    version="2.0.0",
    openapi_tags=[
        {"name": "Users", "description": "User operations"},
        {"name": "Profiles", "description": "Profile operations"},
    ],
    servers=[
        {"url": "https://api.example.com/v2", "description": "Production"},
        {"url": "http://localhost:8000", "description": "Development"},
    ],
)

# Enums
class UserStatus(str, Enum):
    active = "active"
    inactive = "inactive"
    suspended = "suspended"
    pending = "pending"

class UserRole(str, Enum):
    user = "user"
    moderator = "moderator"
    admin = "admin"

# Models
class UserBase(BaseModel):
    email: EmailStr = Field(..., description="User email address")
    name: str = Field(..., min_length=1, max_length=100, description="Display name")

class UserCreate(UserBase):
    role: UserRole = Field(default=UserRole.user)
    metadata: Optional[dict] = Field(default=None, description="Custom metadata")

    model_config = {
        "json_schema_extra": {
            "examples": [
                {
                    "email": "user@example.com",
                    "name": "John Doe",
                    "role": "user"
                }
            ]
        }
    }

class UserUpdate(BaseModel):
    name: Optional[str] = Field(None, min_length=1, max_length=100)
    status: Optional[UserStatus] = None
    role: Optional[UserRole] = None
    metadata: Optional[dict] = None

class User(UserBase):
    id: UUID = Field(..., description="Unique identifier")
    status: UserStatus
    role: UserRole
    avatar: Optional[str] = Field(None, description="Avatar URL")
    metadata: Optional[dict] = None
    created_at: datetime = Field(..., alias="createdAt")
    updated_at: Optional[datetime] = Field(None, alias="updatedAt")

    model_config = {"populate_by_name": True}

class Pagination(BaseModel):
    page: int = Field(..., ge=1)
    limit: int = Field(..., ge=1, le=100)
    total: int = Field(..., ge=0)
    total_pages: int = Field(..., ge=0, alias="totalPages")
    has_next: bool = Field(..., alias="hasNext")
    has_prev: bool = Field(..., alias="hasPrev")

class UserListResponse(BaseModel):
    data: List[User]
    pagination: Pagination

class ErrorDetail(BaseModel):
    field: str
    message: str

class ErrorResponse(BaseModel):
    code: str = Field(..., description="Error code")
    message: str = Field(..., description="Error message")
    details: Optional[List[ErrorDetail]] = None
    request_id: Optional[str] = Field(None, alias="requestId")

# Endpoints
@app.get(
    "/users",
    response_model=UserListResponse,
    tags=["Users"],
    summary="List all users",
    description="Returns a paginated list of users with optional filtering.",
    responses={
        400: {"model": ErrorResponse, "description": "Invalid request"},
        401: {"model": ErrorResponse, "description": "Unauthorized"},
    },
)
async def list_users(
    page: int = Query(1, ge=1, description="Page number"),
    limit: int = Query(20, ge=1, le=100, description="Items per page"),
    status: Optional[UserStatus] = Query(None, description="Filter by status"),
    search: Optional[str] = Query(None, min_length=2, max_length=100),
):
    """
    List users with pagination and filtering.

    - **page**: Page number (1-based)
    - **limit**: Number of items per page (max 100)
    - **status**: Filter by user status
    - **search**: Search by name or email
    """
    # Implementation
    pass

@app.post(
    "/users",
    response_model=User,
    status_code=201,
    tags=["Users"],
    summary="Create a new user",
    responses={
        400: {"model": ErrorResponse},
        409: {"model": ErrorResponse, "description": "Email already exists"},
    },
)
async def create_user(user: UserCreate):
    """Create a new user and send welcome email."""
    pass

@app.get(
    "/users/{user_id}",
    response_model=User,
    tags=["Users"],
    summary="Get user by ID",
    responses={404: {"model": ErrorResponse}},
)
async def get_user(
    user_id: UUID = Path(..., description="User ID"),
):
    """Retrieve a specific user by their ID."""
    pass

@app.patch(
    "/users/{user_id}",
    response_model=User,
    tags=["Users"],
    summary="Update user",
    responses={
        400: {"model": ErrorResponse},
        404: {"model": ErrorResponse},
    },
)
async def update_user(
    user_id: UUID = Path(..., description="User ID"),
    user: UserUpdate = ...,
):
    """Update user attributes."""
    pass

@app.delete(
    "/users/{user_id}",
    status_code=204,
    tags=["Users", "Admin"],
    summary="Delete user",
    responses={404: {"model": ErrorResponse}},
)
async def delete_user(
    user_id: UUID = Path(..., description="User ID"),
):
    """Permanently delete a user."""
    pass

# Export OpenAPI spec
if __name__ == "__main__":
    import json
    print(json.dumps(app.openapi(), indent=2))

Template 3: Code-First (TypeScript/Express with tsoa)

// tsoa generates OpenAPI from TypeScript decorators

import {
  Controller,
  Get,
  Post,
  Patch,
  Delete,
  Route,
  Path,
  Query,
  Body,
  Response,
  SuccessResponse,
  Tags,
  Security,
  Example,
} from "tsoa";

// Models
interface User {
  /** Unique identifier */
  id: string;
  /** User email address */
  email: string;
  /** Display name */
  name: string;
  status: UserStatus;
  role: UserRole;
  /** Avatar URL */
  avatar?: string;
  /** Custom metadata */
  metadata?: Record<string, unknown>;
  createdAt: Date;
  updatedAt?: Date;
}

enum UserStatus {
  Active = "active",
  Inactive = "inactive",
  Suspended = "suspended",
  Pending = "pending",
}

enum UserRole {
  User = "user",
  Moderator = "moderator",
  Admin = "admin",
}

interface CreateUserRequest {
  email: string;
  name: string;
  role?: UserRole;
  metadata?: Record<string, unknown>;
}

interface UpdateUserRequest {
  name?: string;
  status?: UserStatus;
  role?: UserRole;
  metadata?: Record<string, unknown>;
}

interface Pagination {
  page: number;
  limit: number;
  total: number;
  totalPages: number;
  hasNext: boolean;
  hasPrev: boolean;
}

interface UserListResponse {
  data: User[];
  pagination: Pagination;
}

interface ErrorResponse {
  code: string;
  message: string;
  details?: { field: string; message: string }[];
  requestId?: string;
}

@Route("users")
@Tags("Users")
export class UsersController extends Controller {
  /**
   * List all users with pagination and filtering
   * @param page Page number (1-based)
   * @param limit Items per page (max 100)
   * @param status Filter by user status
   * @param search Search by name or email
   */
  @Get()
  @Security("bearerAuth")
  @Response<ErrorResponse>(400, "Invalid request")
  @Response<ErrorResponse>(401, "Unauthorized")
  @Example<UserListResponse>({
    data: [
      {
        id: "550e8400-e29b-41d4-a716-446655440000",
        email: "john@example.com",
        name: "John Doe",
        status: UserStatus.Active,
        role: UserRole.User,
        createdAt: new Date("2024-01-15T10:30:00Z"),
      },
    ],
    pagination: {
      page: 1,
      limit: 20,
      total: 1,
      totalPages: 1,
      hasNext: false,
      hasPrev: false,
    },
  })
  public async listUsers(
    @Query() page: number = 1,
    @Query() limit: number = 20,
    @Query() status?: UserStatus,
    @Query() search?: string,
  ): Promise<UserListResponse> {
    // Implementation
    throw new Error("Not implemented");
  }

  /**
   * Create a new user
   */
  @Post()
  @Security("bearerAuth")
  @SuccessResponse(201, "Created")
  @Response<ErrorResponse>(400, "Invalid request")
  @Response<ErrorResponse>(409, "Email already exists")
  public async createUser(@Body() body: CreateUserRequest): Promise<User> {
    this.setStatus(201);
    throw new Error("Not implemented");
  }

  /**
   * Get user by ID
   * @param userId User ID
   */
  @Get("{userId}")
  @Security("bearerAuth")
  @Response<ErrorResponse>(404, "User not found")
  public async getUser(@Path() userId: string): Promise<User> {
    throw new Error("Not implemented");
  }

  /**
   * Update user attributes
   * @param userId User ID
   */
  @Patch("{userId}")
  @Security("bearerAuth")
  @Response<ErrorResponse>(400, "Invalid request")
  @Response<ErrorResponse>(404, "User not found")
  public async updateUser(
    @Path() userId: string,
    @Body() body: UpdateUserRequest,
  ): Promise<User> {
    throw new Error("Not implemented");
  }

  /**
   * Delete user
   * @param userId User ID
   */
  @Delete("{userId}")
  @Tags("Users", "Admin")
  @Security("bearerAuth")
  @SuccessResponse(204, "Deleted")
  @Response<ErrorResponse>(404, "User not found")
  public async deleteUser(@Path() userId: string): Promise<void> {
    this.setStatus(204);
  }
}

Template 4: Validation & Linting

# Install validation tools
npm install -g @stoplight/spectral-cli
npm install -g @redocly/cli

# Spectral ruleset (.spectral.yaml)
cat > .spectral.yaml << 'EOF'
extends: ["spectral:oas", "spectral:asyncapi"]

rules:
  # Enforce operation IDs
  operation-operationId: error

  # Require descriptions
  operation-description: warn
  info-description: error

  # Naming conventions
  operation-operationId-valid-in-url: true

  # Security
  operation-security-defined: error

  # Response codes
  operation-success-response: error

  # Custom rules
  path-params-snake-case:
    description: Path parameters should be snake_case
    severity: warn
    given: "$.paths[*].parameters[?(@.in == 'path')].name"
    then:
      function: pattern
      functionOptions:
        match: "^[a-z][a-z0-9_]*$"

  schema-properties-camelCase:
    description: Schema properties should be camelCase
    severity: warn
    given: "$.components.schemas[*].properties[*]~"
    then:
      function: casing
      functionOptions:
        type: camel
EOF

# Run Spectral
spectral lint openapi.yaml

# Redocly config (redocly.yaml)
cat > redocly.yaml << 'EOF'
extends:
  - recommended

rules:
  no-invalid-media-type-examples: error
  no-invalid-schema-examples: error
  operation-4xx-response: warn
  request-mime-type:
    severity: error
    allowedValues:
      - application/json
  response-mime-type:
    severity: error
    allowedValues:
      - application/json
      - application/problem+json

theme:
  openapi:
    generateCodeSamples:
      languages:
        - lang: curl
        - lang: python
        - lang: javascript
EOF

# Run Redocly
redocly lint openapi.yaml
redocly bundle openapi.yaml -o bundled.yaml
redocly preview-docs openapi.yaml

SDK Generation

# OpenAPI Generator
npm install -g @openapitools/openapi-generator-cli

# Generate TypeScript client
openapi-generator-cli generate \
  -i openapi.yaml \
  -g typescript-fetch \
  -o ./generated/typescript-client \
  --additional-properties=supportsES6=true,npmName=@myorg/api-client

# Generate Python client
openapi-generator-cli generate \
  -i openapi.yaml \
  -g python \
  -o ./generated/python-client \
  --additional-properties=packageName=api_client

# Generate Go client
openapi-generator-cli generate \
  -i openapi.yaml \
  -g go \
  -o ./generated/go-client

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

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