2xx Success
  200 OK - General success
  201 Created - Resource created (return Location header)
  204 No Content - Success with no body (DELETE)

4xx Client Error
  400 Bad Request - Invalid input
  401 Unauthorized - Authentication required
  403 Forbidden - Authenticated but not permitted
  404 Not Found - Resource doesn't exist
  409 Conflict - State conflict (duplicate)
  422 Unprocessable Entity - Validation failed
  429 Too Many Requests - Rate limited

5xx Server Error
  500 Internal Server Error - Unexpected failure
  502 Bad Gateway - Upstream failure
  503 Service Unavailable - Temporary unavailability

// Success (single resource)
{
  "data": { "id": "123", "name": "Example" },
  "meta": { "requestId": "abc-123" }
}

// Success (collection)
{
  "data": [{ "id": "1" }, { "id": "2" }],
  "meta": { "total": 100, "page": 1, "perPage": 20 }
}

// Error
{
  "error": {
    "code": "VALIDATION_ERROR",
    "message": "Invalid email format",
    "details": [
      { "field": "email", "message": "Must be valid email" }
    ]
  }
}

# Offset-based (simple, not scalable)
GET /users?page=2&per_page=20

# Cursor-based (scalable, recommended)
GET /users?cursor=eyJpZCI6MTAwfQ&limit=20

# Filtering
GET /users?status=active&role=admin

# Sorting
GET /users?sort=created_at:desc,name:asc

# Field selection
GET /users?fields=id,name,email

# URL versioning (recommended)
GET /v1/users

# Header versioning
Accept: application/vnd.api+json; version=1

# Good: Specific query
query GetUserOrders($userId: ID!, $limit: Int = 10) {
  user(id: $userId) {
    orders(first: $limit) {
      edges {
        node { id, total, status }
      }
      pageInfo { hasNextPage, endCursor }
    }
  }
}

# Good: Input types and payloads
mutation CreateUser($input: CreateUserInput!) {
  createUser(input: $input) {
    user { id, name }
    errors { field, message }
  }
}

# Response headers
X-RateLimit-Limit: 1000
X-RateLimit-Remaining: 999
X-RateLimit-Reset: 1609459200
Retry-After: 3600