HTTP 501 Not Implemented
The server does not support the functionality required to fulfill the request.
5xx Server Error
501
Not Implemented
Description
The 501 Not Implemented status code indicates that the server does not support the functionality required to fulfill the request. This is the appropriate response when the server does not recognize the request method and is not capable of supporting it for any resource. The only methods that servers are required to support (and therefore that must not return 501) are GET and HEAD. A 501 response is cacheable by default. It differs from 405 Method Not Allowed in that 405 means the server knows the method but the target resource does not support it, while 501 means the server does not even recognize or implement the method at all.
When Is It Used?
Returned when the server does not recognize or implement the HTTP method used in the request. Can also be used when a feature or endpoint is planned but not yet built. Some servers return this for methods like PATCH or TRACE that they have not implemented.
Example Response
HTTP/1.1 501 Not Implemented
Content-Type: application/json
{"error": "not_implemented", "message": "The PATCH method is not implemented for this server"}