API Design Reviewer

Tier: POWERFUL
Category: Engineering / Architecture
Maintainer: Claude Skills Team

Overview

The API Design Reviewer skill provides comprehensive analysis and review of API designs, focusing on REST conventions, best practices, and industry standards. This skill helps engineering teams build consistent, maintainable, and well-designed APIs through automated linting, breaking change detection, and design scorecards.

Quick Start — run the tools first

# 1. Lint an OpenAPI/Swagger spec for convention violations
python3 scripts/api_linter.py openapi.json --format json -o lint.json

# 2. Detect breaking changes between two spec versions (gate: exits non-zero with --exit-on-breaking)
python3 scripts/breaking_change_detector.py openapi-v1.json openapi-v2.json --format json --exit-on-breaking -o breaking.json

# 3. Score overall design quality (gate: --min-grade fails below threshold)
python3 scripts/api_scorecard.py openapi.json --format json --min-grade B -o scorecard.json

Review flow: run all three, report linter findings + breaking changes + grade to the user, fix, then re-run until the linter is clean, --exit-on-breaking passes (or breaking changes are version-bumped), and the scorecard meets the agreed --min-grade. Never sign off an API review on prose alone — attach the tool outputs.

Core Capabilities

1. API Linting and Convention Analysis

• Resource Naming Conventions: Enforces kebab-case for resources, camelCase for fields
• HTTP Method Usage: Validates proper use of GET, POST, PUT, PATCH, DELETE
• URL Structure: Analyzes endpoint patterns for consistency and RESTful design
• Status Code Compliance: Ensures appropriate HTTP status codes are used
• Error Response Formats: Validates consistent error response structures
• Documentation Coverage: Checks for missing descriptions and documentation gaps

2. Breaking Change Detection

• Endpoint Removal: Detects removed or deprecated endpoints
• Response Shape Changes: Identifies modifications to response structures
• Field Removal: Tracks removed or renamed fields in API responses
• Type Changes: Catches field type modifications that could break clients
• Required Field Additions: Flags new required fields that could break existing integrations
• Status Code Changes: Detects changes to expected status codes

3. API Design Scoring and Assessment

• Consistency Analysis (30%): Evaluates naming conventions, response patterns, and structural consistency
• Documentation Quality (20%): Assesses completeness and clarity of API documentation
• Security Implementation (20%): Reviews authentication, authorization, and security headers
• Usability Design (15%): Analyzes ease of use, discoverability, and developer experience
• Performance Patterns (15%): Evaluates caching, pagination, and efficiency patterns

REST Design Principles

Resource Naming Conventions

✅ Good Examples:
- /api/v1/users
- /api/v1/user-profiles
- /api/v1/orders/123/line-items

❌ Bad Examples:
- /api/v1/getUsers
- /api/v1/user_profiles
- /api/v1/orders/123/lineItems

HTTP Method Usage

• GET: Retrieve resources (safe, idempotent)
• POST: Create new resources (not idempotent)
• PUT: Replace entire resources (idempotent)
• PATCH: Partial resource updates (not necessarily idempotent)
• DELETE: Remove resources (idempotent)

URL Structure Best Practices

Collection Resources: /api/v1/users
Individual Resources: /api/v1/users/123
Nested Resources: /api/v1/users/123/orders
Actions: /api/v1/users/123/activate (POST)
Filtering: /api/v1/users?status=active&role=admin

Versioning Strategies

1. URL Versioning (Recommended)

/api/v1/users
/api/v2/users

Pros: Clear, explicit, easy to route
Cons: URL proliferation, caching complexity

2. Header Versioning

GET /api/users
Accept: application/vnd.api+json;version=1

Pros: Clean URLs, content negotiation
Cons: Less visible, harder to test manually

3. Media Type Versioning

GET /api/users
Accept: application/vnd.myapi.v1+json

Pros: RESTful, supports multiple representations
Cons: Complex, harder to implement

4. Query Parameter Versioning

/api/users?version=1

Pros: Simple to implement
Cons: Not RESTful, can be ignored

Pagination Patterns

Offset-Based Pagination

{
  "data": [...],
  "pagination": {
    "offset": 20,
    "limit": 10,
    "total": 150,
    "hasMore": true
  }
}

Cursor-Based Pagination

{
  "data": [...],
  "pagination": {
    "nextCursor": "eyJpZCI6MTIzfQ==",
    "hasMore": true
  }
}

Page-Based Pagination

{
  "data": [...],
  "pagination": {
    "page": 3,
    "pageSize": 10,
    "totalPages": 15,
    "totalItems": 150
  }
}

Error Response Formats

Standard Error Structure

{
  "error": {
    "code": "VALIDATION_ERROR",
    "message": "The request contains invalid parameters",
    "details": [
      {
        "field": "email",
        "code": "INVALID_FORMAT",
        "message": "Email address is not valid"
      }
    ],
    "requestId": "req-123456",
    "timestamp": "2026-02-16T13:00:00Z"
  }
}

HTTP Status Code Usage

• 400 Bad Request: Invalid request syntax or parameters
• 401 Unauthorized: Authentication required
• 403 Forbidden: Access denied (authenticated but not authorized)
• 404 Not Found: Resource not found
• 409 Conflict: Resource conflict (duplicate, version mismatch)
• 422 Unprocessable Entity: Valid syntax but semantic errors
• 429 Too Many Requests: Rate limit exceeded
• 500 Internal Server Error: Unexpected server error

Authentication and Authorization Patterns

Bearer Token Authentication

Authorization: Bearer <token>

API Key Authentication

X-API-Key: <api-key>
Authorization: Api-Key <api-key>

OAuth 2.0 Flow

Authorization: Bearer <oauth-access-token>

Role-Based Access Control (RBAC)

{
  "user": {
    "id": "123",
    "roles": ["admin", "editor"],
    "permissions": ["read:users", "write:orders"]
  }
}

Rate Limiting Implementation

Headers

X-RateLimit-Limit: 1000
X-RateLimit-Remaining: 999
X-RateLimit-Reset: 1640995200

Response on Limit Exceeded

{
  "error": {
    "code": "RATE_LIMIT_EXCEEDED",
    "message": "Too many requests",
    "retryAfter": 3600
  }
}

HATEOAS (Hypermedia as the Engine of Application State)

Example Implementation

{
  "id": "123",
  "name": "John Doe",
  "email": "john@example.com",
  "_links": {
    "self": { "href": "/api/v1/users/123" },
    "orders": { "href": "/api/v1/users/123/orders" },
    "profile": { "href": "/api/v1/users/123/profile" },
    "deactivate": { 
      "href": "/api/v1/users/123/deactivate",
      "method": "POST"
    }
  }
}

Idempotency

Idempotent Methods

• GET: Always safe and idempotent
• PUT: Should be idempotent (replace entire resource)
• DELETE: Should be idempotent (same result)
• PATCH: May or may not be idempotent

Idempotency Keys

POST /api/v1/payments
Idempotency-Key: 123e4567-e89b-12d3-a456-426614174000

Backward Compatibility Guidelines

Safe Changes (Non-Breaking)

• Adding optional fields to requests
• Adding fields to responses
• Adding new endpoints
• Making required fields optional
• Adding new enum values (with graceful handling)

Breaking Changes (Require Version Bump)

• Removing fields from responses
• Making optional fields required
• Changing field types
• Removing endpoints
• Changing URL structures
• Modifying error response formats

OpenAPI/Swagger Validation

Required Components

• API Information: Title, description, version
• Server Information: Base URLs and descriptions
• Path Definitions: All endpoints with methods
• Parameter Definitions: Query, path, header parameters
• Request/Response Schemas: Complete data models
• Security Definitions: Authentication schemes
• Error Responses: Standard error formats

Best Practices

• Use consistent naming conventions
• Provide detailed descriptions for all components
• Include examples for complex objects
• Define reusable components and schemas
• Validate against OpenAPI specification

Performance Considerations

Caching Strategies

Cache-Control: public, max-age=3600
ETag: "123456789"
Last-Modified: Wed, 21 Oct 2015 07:28:00 GMT

Efficient Data Transfer

• Use appropriate HTTP methods
• Implement field selection (?fields=id,name,email)
• Support compression (gzip)
• Implement efficient pagination
• Use ETags for conditional requests

Resource Optimization

• Avoid N+1 queries
• Implement batch operations
• Use async processing for heavy operations
• Support partial updates (PATCH)

Security Best Practices

Input Validation

• Validate all input parameters
• Sanitize user data
• Use parameterized queries
• Implement request size limits

Authentication Security

• Use HTTPS everywhere
• Implement secure token storage
• Support token expiration and refresh
• Use strong authentication mechanisms

Authorization Controls

• Implement principle of least privilege
• Use resource-based permissions
• Support fine-grained access control
• Audit access patterns

Tools and Scripts

api_linter.py

Analyzes API specifications for compliance with REST conventions and best practices.

Features:

• OpenAPI/Swagger spec validation
• Naming convention checks
• HTTP method usage validation
• Error format consistency
• Documentation completeness analysis

breaking_change_detector.py

Compares API specification versions to identify breaking changes.

Features:

• Endpoint comparison
• Schema change detection
• Field removal/modification tracking
• Migration guide generation
• Impact severity assessment

api_scorecard.py

Provides comprehensive scoring of API design quality.

Features:

• Multi-dimensional scoring
• Detailed improvement recommendations
• Letter grade assessment (A-F)
• Benchmark comparisons
• Progress tracking

Integration Examples

CI/CD Integration

- name: "api-linting"
  run: python scripts/api_linter.py openapi.json

- name: "breaking-change-detection"
  run: python scripts/breaking_change_detector.py openapi-v1.json openapi-v2.json

- name: "api-scorecard"
  run: python scripts/api_scorecard.py openapi.json

Pre-commit Hooks

#!/bin/bash
python engineering/skills/api-design-reviewer/scripts/api_linter.py api/openapi.json
if [ $? -ne 0 ]; then
  echo "API linting failed. Please fix the issues before committing."
  exit 1
fi

Best Practices Summary

1. Consistency First: Maintain consistent naming, response formats, and patterns
2. Documentation: Provide comprehensive, up-to-date API documentation
3. Versioning: Plan for evolution with clear versioning strategies
4. Error Handling: Implement consistent, informative error responses
5. Security: Build security into every layer of the API
6. Performance: Design for scale and efficiency from the start
7. Backward Compatibility: Minimize breaking changes and provide migration paths
8. Testing: Implement comprehensive testing including contract testing
9. Monitoring: Add observability for API usage and performance
10. Developer Experience: Prioritize ease of use and clear documentation

Common Anti-Patterns to Avoid

1. Verb-based URLs: Use nouns for resources, not actions
2. Inconsistent Response Formats: Maintain standard response structures
3. Over-nesting: Avoid deeply nested resource hierarchies
4. Ignoring HTTP Status Codes: Use appropriate status codes for different scenarios
5. Poor Error Messages: Provide actionable, specific error information
6. Missing Pagination: Always paginate list endpoints
7. No Versioning Strategy: Plan for API evolution from day one
8. Exposing Internal Structure: Design APIs for external consumption, not internal convenience
9. Missing Rate Limiting: Protect your API from abuse and overload
10. Inadequate Testing: Test all aspects including error cases and edge conditions

Regular use of the linting, breaking change detection, and scoring tools ensures continuous improvement and helps maintain API quality throughout the development lifecycle.