# API Designer > REST and GraphQL API design expertise covering REST maturity model, resource naming, pagination patterns, filtering and sorting, HATEOAS, versioning strategies, error response formats, rate limiting headers, and OpenAPI specification. Use when the user asks about api designer, api designer best practices, or needs guidance on api designer implementation. Do NOT use when the user needs a different specialized skill or is asking about an unrelated technology domain. - Skill: `ferroxlabs/api-designer` (Agent Skill, multi-file: 2 files) - Install (CLI): `npx skillmds@latest add ferroxlabs/api-designer` - Raw SKILL.md: https://api.skillmd.com/api/skills/ferroxlabs/api-designer/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: AI & ML - License: Apache-2.0 - Author: FerroxLabs (https://skillmd.com/u/ferroxlabs) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/ferroxlabs/api-designer --- # API Designer ## Purpose Design consistent, intuitive, and evolvable HTTP APIs. This skill covers REST API design from resource modeling through error handling, pagination, versioning, and documentation with OpenAPI. GraphQL design considerations are included for comparison and hybrid architectures. ## REST Maturity Model (Richardson) ``` Level 0: The Swamp of POX - Single endpoint, single HTTP method (POST) - RPC-style: POST /api { "action": "getUser", "id": 123 } - Avoid this. Level 1: Resources - Multiple endpoints, one per resource - POST /users, POST /users/123/delete - Still misusing HTTP methods. Level 2: HTTP Verbs (TARGET for most APIs) - Resources + correct HTTP methods - GET /users, POST /users, PUT /users/123, DELETE /users/123 - Correct status codes (200, 201, 204, 400, 404, etc.) Level 3: HATEOAS (Hypermedia) - Level 2 + hypermedia controls in responses - Clients discover actions from response links - Full REST (rarely implemented in practice) ``` ## Resource Naming Conventions ### Rules ``` 1. Use NOUNS, not verbs GOOD: GET /users BAD: GET /getUsers 2. Use PLURAL nouns GOOD: /users, /orders, /products BAD: /user, /order, /product 3. Use kebab-case for multi-word resources GOOD: /order-items, /user-profiles BAD: /orderItems, /user_profiles 4. Nest resources to show relationships (max 2 levels) GOOD: /users/123/orders BAD: /users/123/orders/456/items/789/reviews (too deep) 5. Use query parameters for filtering, not path segments GOOD: GET /orders?status=pending&userId=123 BAD: GET /orders/pending/user/123 6. Actions on resources (when necessary) POST /orders/123/cancel (state transition) POST /users/123/verify (trigger action) ``` ### Standard HTTP Methods ``` GET /resources -> List resources (200) GET /resources/:id -> Get single resource (200 or 404) POST /resources -> Create resource (201 with Location header) PUT /resources/:id -> Full replace (200 or 204) PATCH /resources/:id -> Partial update (200 or 204) DELETE /resources/:id -> Delete resource (204 or 200) HEAD /resources/:id -> Same as GET but no body (check existence) OPTIONS /resources -> Available methods (CORS preflight) ``` ### Standard Status Codes ``` 2xx SUCCESS: 200 OK -> GET, PUT, PATCH with response body 201 Created -> POST (include Location header) 204 No Content -> DELETE, PUT/PATCH with no response body 3xx REDIRECTION: 301 Moved Permanently -> Resource permanently at new URL 304 Not Modified -> Conditional GET (ETag match) 4xx CLIENT ERROR: 400 Bad Request -> Validation error, malformed request 401 Unauthorized -> Missing or invalid authentication 403 Forbidden -> Authenticated but not authorized 404 Not Found -> Resource does not exist 405 Method Not Allowed -> HTTP method not supported 409 Conflict -> Resource state conflict (duplicate, version) 422 Unprocessable Entity -> Valid syntax but semantic errors 429 Too Many Requests -> Rate limit exceeded 5xx SERVER ERROR: 500 Internal Server Error -> Unexpected server failure 502 Bad Gateway -> Upstream service failure 503 Service Unavailable -> Temporary (include Retry-After header) 504 Gateway Timeout -> Upstream timeout ``` ## Pagination Patterns ### Offset-Based Pagination ``` REQUEST: GET /users?page=2&per_page=20 RESPONSE: { "data": [...], "pagination": { "page": 2, "per_page": 20, "total_items": 157, "total_pages": 8, "has_next": true, "has_prev": true }, "links": { "first": "/users?page=1&per_page=20", "prev": "/users?page=1&per_page=20", "next": "/users?page=3&per_page=20", "last": "/users?page=8&per_page=20" } } PROS: - Simple to implement - Allows jumping to arbitrary page - Total count available CONS: - Inconsistent with inserts/deletes during pagination - Poor performance on large offsets (OFFSET N is O(N)) - Items can be skipped or duplicated between pages ``` ### Cursor-Based Pagination (Recommended for Large Datasets) ``` REQUEST: GET /users?limit=20&after=eyJ0IjoxNjk... RESPONSE: { "data": [...], "pagination": { "has_next": true, "has_prev": true, "next_cursor": "eyJ0IjoxNjk...", "prev_cursor": "eyJ0IjoxNjg..." } } PROS: - Consistent pagination (no skips/dupes) - Efficient for any position in dataset - Works well with real-time data CONS: - Cannot jump to arbitrary page - No total count (requires separate query) - Cursor is opaque (not user-friendly) CURSOR IMPLEMENTATION: Encode the last item's sort key(s) as base64: { "created_at": "2025-01-15T10:30:00Z", "id": "abc123" } -> base64 -> "eyJ0IjoxNjk..." SQL: WHERE (created_at, id) < (:cursor_created_at, :cursor_id) ORDER BY created_at DESC, id DESC LIMIT :limit + 1 -- get one extra to check has_next ``` ### Keyset Pagination ``` REQUEST: GET /users?limit=20&created_before=2025-01-15T10:30:00Z&id_before=abc123 RESPONSE: Same as cursor-based but with explicit keys instead of opaque cursor. PROS: Transparent and debuggable CONS: Exposes sort implementation ``` ## Filtering and Sorting ### Filtering Patterns ``` Simple equality: GET /users?status=active&role=admin Operators (LHS brackets): GET /orders?amount[gte]=100&amount[lte]=500 GET /users?name[contains]=john GET /orders?created_at[after]=2025-01-01 Multiple values (comma-separated OR): GET /users?role=admin,editor Search: GET /users?q=john+doe Nested fields (dot notation): GET /users?address.city=Austin ``` ### Sorting ``` Single sort: GET /users?sort=created_at GET /users?sort=-created_at (descending with - prefix) Multi-sort: GET /users?sort=-created_at,name (primary: created_at desc, secondary: name asc) Alternative format: GET /users?sort_by=created_at&sort_order=desc ``` ### Field Selection (Sparse Fieldsets) ``` GET /users?fields=id,name,email (only return these fields) GET /users?include=orders,profile (include related resources) GET /users?exclude=password_hash (exclude sensitive fields) ``` ## Error Response Format ### Standard Error Envelope ```json { "error": { "code": "VALIDATION_ERROR", "message": "The request body contains invalid data.", "details": [ { "field": "email", "code": "INVALID_FORMAT", "message": "Must be a valid email address." }, { "field": "age", "code": "OUT_OF_RANGE", "message": "Must be between 13 and 120.", "meta": { "min": 13, "max": 120 } } ], "request_id": "req_abc123xyz", "documentation_url": "[reference URL]" } } ``` ### Error Code Taxonomy ``` AUTHENTICATION: AUTH_REQUIRED -> 401: No credentials provided AUTH_INVALID -> 401: Invalid credentials AUTH_EXPIRED -> 401: Token expired AUTHORIZATION: FORBIDDEN -> 403: Insufficient permissions RESOURCE_FORBIDDEN -> 403: No access to this resource VALIDATION: VALIDATION_ERROR -> 400/422: Input validation failed INVALID_FORMAT -> 400: Wrong data format MISSING_FIELD -> 400: Required field missing RESOURCE: NOT_FOUND -> 404: Resource does not exist ALREADY_EXISTS -> 409: Duplicate resource CONFLICT -> 409: State conflict (version mismatch) GONE -> 410: Resource permanently deleted RATE_LIMIT: RATE_LIMIT_EXCEEDED -> 429: Too many requests SERVER: INTERNAL_ERROR -> 500: Unexpected error SERVICE_UNAVAILABLE -> 503: Temporary outage ``` ## Versioning Strategies ### Decision Matrix ``` STRATEGY MECHANISM WHEN TO USE ----------------------------------------------------------------------- URL Path /v1/users, /v2/users Simple, explicit, most common Header Accept: application/vnd. More "RESTful", harder to test api.v2+json Query Param /users?version=2 Easy migration, but ugly RECOMMENDATION: URL path versioning (/v1/) for simplicity and clarity. ``` ### Versioning Rules ``` 1. Only bump major version for BREAKING changes: - Removing a field - Changing field type - Changing resource structure - Changing error format 2. NON-BREAKING changes (no version bump): - Adding new fields (with defaults) - Adding new endpoints - Adding new query parameters - Adding new enum values 3. Support at most 2 versions simultaneously v1 (deprecated, sunset date set) and v2 (current) 4. Include deprecation headers: Sunset: Sat, 01 Mar 2026 00:00:00 GMT Deprecation: true Link: <[reference URL]>; rel="successor-version" ``` ## Rate Limiting Headers ``` Response headers for rate-limited endpoints: X-RateLimit-Limit: 100 # Max requests per window X-RateLimit-Remaining: 67 # Remaining requests in window X-RateLimit-Reset: 1705312800 # Unix timestamp when window resets Retry-After: 30 # Seconds to wait (when 429 returned) Alternative (IETF draft standard): RateLimit-Policy: 100;w=3600 # 100 requests per 3600 seconds RateLimit-Limit: 100 RateLimit-Remaining: 67 RateLimit-Reset: 42 # Seconds until reset (not timestamp) ``` ## HATEOAS ```json { "id": "ord_123", "status": "pending", "total": 99.99, "links": { "self": { "href": "/orders/ord_123", "method": "GET" }, "cancel": { "href": "/orders/ord_123/cancel", "method": "POST" }, "pay": { "href": "/orders/ord_123/pay", "method": "POST" }, "items": { "href": "/orders/ord_123/items", "method": "GET" } } } // After payment, links change: { "id": "ord_123", "status": "paid", "links": { "self": { "href": "/orders/ord_123", "method": "GET" }, "refund": { "href": "/orders/ord_123/refund", "method": "POST" }, "receipt": { "href": "/orders/ord_123/receipt", "method": "GET" } // "cancel" and "pay" no longer available } } ``` ## OpenAPI Specification ```yaml openapi: 3.1.0 info: title: Users API version: 1.0.0 description: User management API contact: email: api@example.com servers: - url: [reference URL] description: Production - url: [reference URL] description: Staging paths: /users: get: operationId: listUsers summary: List all users tags: [Users] parameters: - name: page # ... (condensed) ... securitySchemes: BearerAuth: type: http scheme: bearer bearerFormat: JWT security: - BearerAuth: [] ``` ## API Design Checklist - [ ] Resources are nouns (plural), not verbs - [ ] HTTP methods used correctly (GET reads, POST creates, etc.) - [ ] Status codes are semantic and consistent - [ ] Pagination implemented (cursor-based for large datasets) - [ ] Filtering uses query parameters with consistent operators - [ ] Sorting supports multiple fields with direction prefix - [ ] Error responses follow standard envelope format - [ ] Rate limiting headers included on all responses - [ ] API versioned via URL path (/v1/) - [ ] OpenAPI specification maintained and accurate - [ ] Authentication returns 401, authorization returns 403 - [ ] Created resources return 201 with Location header - [ ] Idempotency keys supported for POST operations - [ ] Request IDs included in error responses for debugging - [ ] Deprecation communicated via Sunset header ## When to Use **Use this skill when:** - Designing or implementing api designer solutions - Reviewing or improving existing api designer approaches - Making architectural or implementation decisions about api designer - Learning api designer patterns and best practices - Troubleshooting api designer-related issues **Do NOT use this skill when:** - The question is about a fundamentally different technology domain - A more specific sibling skill covers the exact topic needed - The user needs a complete hands-on tutorial rather than expert guidance ## Output Format ```markdown # Api Designer Analysis ## Context Assessment [Situation summary and constraints] ## Recommended Approach [Primary recommendation with rationale] ## Implementation Steps 1. [Step with specific details] 2. [Step with specific details] 3. [Step with specific details] ## Trade-offs and Considerations - [Key trade-off 1] - [Key trade-off 2] ## Next Steps - [Immediate action item] - [Follow-up action item] ``` ## Example **Input:** "Help me implement api designer for a medium-scale production application" **Output:** A structured analysis covering current state assessment, recommended api designer approach with specific patterns, implementation roadmap with milestones, and risk mitigation strategies tailored to the application scale and constraints. ## Edge Cases - **Legacy system integration:** When api designer must coexist with legacy approaches, provide a gradual migration path rather than a complete rewrite - **Scale mismatch:** When the solution complexity exceeds the project scale, recommend a simpler approach and note when to revisit - **Team skill gaps:** When the team lacks experience with the recommended approach, include learning resources and simpler alternatives - **Conflicting requirements:** When constraints conflict (e.g., performance vs. maintainability), explicitly state the trade-off and recommend based on stated priorities