HTTP Status 428 - Precondition Required

PUT /api/documents/42 HTTP/1.1
Host: api.example.com
Content-Type: application/json

{"title": "Updated Document", "content": "New content"}

HTTP/1.1 428 Precondition Required
Content-Type: application/json

{
  "error": "precondition_required",
  "message": "This endpoint requires If-Match header for updates",
  "details": "Perform GET request first to obtain current ETag",
  "example": "If-Match: \"abc123def456\"",
  "docs": "https://api.example.com/docs/optimistic-locking"
}

DELETE /api/users/alice HTTP/1.1
Host: api.example.com
Authorization: Bearer token123

HTTP/1.1 428 Precondition Required
Content-Type: application/json
ETag: "user-v42"

{
  "error": "conditional_request_required",
  "message": "User deletion requires If-Match header",
  "current_etag": "user-v42",
  "reason": "Prevents accidental deletion of modified resource",
  "retry": "Include 'If-Match: \"user-v42\"' header"
}

PATCH /api/settings HTTP/1.1
Host: api.example.com
Content-Type: application/json

{"theme": "dark", "notifications": true}

HTTP/1.1 428 Precondition Required
Content-Type: application/json

{
  "status": 428,
  "error": "precondition_required",
  "message": "Settings updates require If-Unmodified-Since or If-Match header",
  "details": "This prevents overwriting changes made by other sessions",
  "last_modified": "2025-10-01T14:30:00Z",
  "current_etag": "settings-abc-123"
}