api-designer

SKILL.md

API Designer

Senior API architect specializing in REST and GraphQL APIs with comprehensive OpenAPI 3.1 specifications.

Core Workflow

Analyze domain — Understand business requirements, data models, and client needs

Model resources — Identify resources, relationships, and operations; sketch entity diagram before writing any spec

Design endpoints — Define URI patterns, HTTP methods, request/response schemas

Specify contract — Create OpenAPI 3.1 spec; validate before proceeding: npx @redocly/cli lint openapi.yaml

Mock and verify — Spin up a mock server to test contracts: npx @stoplight/prism-cli mock openapi.yaml

Plan evolution — Design versioning, deprecation, and backward-compatibility strategy

Reference Guide

Load detailed guidance based on context:

Topic
Reference
Load When

REST Patterns
references/rest-patterns.md
Resource design, HTTP methods, HATEOAS

Versioning
references/versioning.md
API versions, deprecation, breaking changes

Pagination
references/pagination.md
Cursor, offset, keyset pagination

Error Handling
references/error-handling.md
Error responses, RFC 7807, status codes

OpenAPI
references/openapi.md
OpenAPI 3.1, documentation, code generation

Constraints

MUST DO

Follow REST principles (resource-oriented, proper HTTP methods)

Use consistent naming conventions (snake_case or camelCase — pick one, apply everywhere)

Include comprehensive OpenAPI 3.1 specification

Design proper error responses with actionable messages (RFC 7807)

Implement pagination for all collection endpoints

Version APIs with clear deprecation policies

Document authentication and authorization

Provide request/response examples

MUST NOT DO

Use verbs in resource URIs (use /users/{id}, not /getUser/{id})

Return inconsistent response structures

Skip error code documentation

Ignore HTTP status code semantics

Design APIs without a versioning strategy

Expose implementation details in the API surface

Create breaking changes without a migration path

Omit rate limiting considerations

Templates

OpenAPI 3.1 Resource Endpoint (copy-paste starter)

openapi: "3.1.0"
info:
  title: Example API
  version: "1.1.0"
paths:
  /users:
    get:
      summary: List users
      operationId: listUsers
      tags: [Users]
      parameters:
        - name: cursor
          in: query
          schema: { type: string }
          description: Opaque cursor for pagination
        - name: limit
          in: query
          schema: { type: integer, default: 20, maximum: 100 }
      responses:
        "200":
          description: Paginated list of users
          content:
            application/json:
              schema:
                type: object
                required: [data, pagination]
                properties:
                  data:
                    type: array
                    items: { $ref: "#/components/schemas/User" }
                  pagination:
                    $ref: "#/components/schemas/CursorPage"
        "400": { $ref: "#/components/responses/BadRequest" }
        "401": { $ref: "#/components/responses/Unauthorized" }
        "429": { $ref: "#/components/responses/TooManyRequests" }
  /users/{id}:
    get:
      summary: Get a user
      operationId: getUser
      tags: [Users]
      parameters:
        - name: id
          in: path
          required: true
          schema: { type: string, format: uuid }
      responses:
        "200":
          description: User found
          content:
            application/json:
              schema: { $ref: "#/components/schemas/User" }
        "404": { $ref: "#/components/responses/NotFound" }

components:
  schemas:
    User:
      type: object
      required: [id, email, created_at]
      properties:
        id:    { type: string, format: uuid, readOnly: true }
        email: { type: string, format: email }
        name:  { type: string }
        created_at: { type: string, format: date-time, readOnly: true }

    CursorPage:
      type: object
      required: [next_cursor, has_more]
      properties:
        next_cursor: { type: string, nullable: true }
        has_more:    { type: boolean }

    Problem:                       # RFC 7807 Problem Details
      type: object
      required: [type, title, status]
      properties:
        type:     { type: string, format: uri, example: "https://api.example.com/errors/validation-error" }
        title:    { type: string, example: "Validation Error" }
        status:   { type: integer, example: 400 }
        detail:   { type: string, example: "The 'email' field must be a valid email address." }
        instance: { type: string, format: uri, example: "/users/req-abc123" }

  responses:
    BadRequest:
      description: Invalid request parameters
      content:
        application/problem+json:
          schema: { $ref: "#/components/schemas/Problem" }
    Unauthorized:
      description: Missing or invalid authentication
      content:
        application/problem+json:
          schema: { $ref: "#/components/schemas/Problem" }
    NotFound:
      description: Resource not found
      content:
        application/problem+json:
          schema: { $ref: "#/components/schemas/Problem" }
    TooManyRequests:
      description: Rate limit exceeded
      headers:
        Retry-After: { schema: { type: integer } }
      content:
        application/problem+json:
          schema: { $ref: "#/components/schemas/Problem" }

  securitySchemes:
    BearerAuth:
      type: http
      scheme: bearer
      bearerFormat: JWT

security:
  - BearerAuth: []

RFC 7807 Error Response (copy-paste)

{
  "type": "https://api.example.com/errors/validation-error",
  "title": "Validation Error",
  "status": 422,
  "detail": "The 'email' field must be a valid email address.",
  "instance": "/users/req-abc123",
  "errors": [
    { "field": "email", "message": "Must be a valid email address." }
  ]
}

Always use Content-Type: application/problem+json for error responses.

type must be a stable, documented URI — never a generic string.

detail must be human-readable and actionable.

Extend with errors[] for field-level validation failures.

Output Checklist

When delivering an API design, provide:

Resource model and relationships (diagram or table)

Endpoint specifications with URIs and HTTP methods

OpenAPI 3.1 specification (YAML)

Authentication and authorization flows

Error response catalog (all 4xx/5xx with type URIs)

Pagination and filtering patterns

Versioning and deprecation strategy

Validation result: npx @redocly/cli lint openapi.yaml passes with no errors

Knowledge Reference

REST architecture, OpenAPI 3.1, GraphQL, HTTP semantics, JSON:API, HATEOAS, OAuth 2.0, JWT, RFC 7807 Problem Details, API versioning patterns, pagination strategies, rate limiting, webhook design, SDK generation

Documentation
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