501 Not Implemented
5xx Server Error

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.

RFC 7231 6.6.2


This is also an excellent status code for cases where things may be implemented in the future. Just note that this status code can be cached.


A 501 response is cacheable by default.