ForgeLM/.agents/rules/api-design-principles.md

trigger, description

trigger	description
model_decision	When implementing REST/HTTP APIs (endpoints, handlers, middleware, or response formatting)

API Design Principles

RESTful API Standards

Resource-Based URLs:

• Use plural nouns for resources: /api/{version}/users, /api/{version}/orders
• Hierarchical relationships: /api/{version}/users/:userId/orders
• Avoid verbs in URLs: /api/{version}/getUser ❌ → /api/{version}/users/:id ✅

HTTP Methods:

• GET: Read/retrieve resource (safe, idempotent, cacheable)
• POST: Create new resource (not idempotent)
• PUT: Replace entire resource (idempotent)
• PATCH: Partial update (idempotent)
• DELETE: Remove resource (idempotent)

Versioning:

• URL path versioning: /api/{version}/users e.g./api/v1/users (explicit, clear)

Pagination:

• Limit results per page (default 20, max 100)
• Cursor-based: ?cursor=abc123 (better for real-time data)
• Offset-based: ?page=2&limit=20 (simpler, less accurate for changing data)

Filtering and Sorting:

• Filtering: ?status=active&role=admin
• Sorting: ?sort=created_at:desc,name:asc
• Searching: ?q=search+term

HTTP Status Codes and Error Categories

Success Codes:

• 200 OK: Success (GET, PUT, PATCH)
• 201 Created: Resource created successfully (POST)
• 204 No Content: Success with no response body (DELETE)

Client Error Codes (4xx) - User Can Fix:

Validation Errors (400 Bad Request):

• User input doesn't meet requirements
• Examples: Invalid email, password too short, required field missing
• Response: Detailed field-level errors
• User action: Correct input and retry

Authentication Errors (401 Unauthorized):

• Identity verification failed
• Examples: Invalid credentials, expired token, missing token
• User action: Provide valid credentials

Authorization Errors (403 Forbidden):

• Permission denied (user identified but lacks permission)
• User action: Contact admin for permission

Not Found Errors (404 Not Found):

• Resource doesn't exist or user lacks permission to know it exists
• User action: None (doesn't exist)

Business Rule Violations (409 Conflict / 422 Unprocessable Entity):

• Domain rule violations
• Examples: Insufficient balance, duplicate email, order already shipped
• Response: Business rule explanation
• User action: Depends on business context

Rate Limiting (429 Too Many Requests):

• Too many requests in time window
• User action: Wait and retry

Server Error Codes (5xx) - System Issue:

Infrastructure Errors (500/502/503):

• Database down, network timeout, external service failure
• Response: Generic message with correlation ID
• User action: None (system issue, retry later)

API Success Response Format

{
  "data": { /* resource or array of resources */ },
  "meta": {
    "total": 100,
    "page": 1,
    "perPage": 20
  },
  "links": {
    "self": "/api/v1/users?page=1",
    "next": "/api/v1/users?page=2",
    "prev": null
  }
}

API Error Response Format

All API errors must follow a consistent envelope structure, matching the success format where possible or using a standard error envelope.

{
  "status": "error",                      // Transport: Always "error" or "fail"
  "code": 400,                            // Transport: Redundant HTTP Status
  "error": {                              // Domain: The actual business problem
    "code": "VALIDATION_ERROR",           // Machine-readable business code (UPPER_SNAKE)
    "message": "Invalid email format",    // Human-readable message
    "details": {                          // Optional: Structured context
      "field": "email",
      "reason": "Must be a valid address"
    },
  "correlationId": "req-1234567890",      // Ops: Traceability
  "doc_url": "https://..."                // Optional: Help link
  }
}

Related Principles

• Error Handling Principles @error-handling-principles.md
• Security Mandate @security-mandate.md
• Security Principles @security-principles.md
• Logging and Observability Mandate @logging-and-observability-mandate.md
• Data Serialization and Interchange Principles @data-serialization-and-interchange-principles.md