HTTP 501 — Not Implemented
The server either does not recognize the request method, or it lacks the ability to fulfill the request
When to use it
Use status 501 when this is the correct HTTP response semantic: The server either does not recognize the request method, or it lacks the ability to fulfill the request
Request and response example
GET /service HTTP/1.1
Host: api.example.com
HTTP/1.1 501 Not Implemented
Content-Type: application/json
{"error":"Service failed to process the request"}