HTTP 501 Not Implemented indicates the server does not recognize or support the HTTP method in the request. While 405 Method Not Allowed means the resource doesn't accept that method, 501 means the server itself doesn't support the method at all. This is rare in modern HTTP servers since all standard methods are implemented. You might encounter 501 with: custom HTTP methods (PATCH on older servers), WebDAV methods (PROPFIND, MKCOL), or when a proxy encounters an unknown method.
Response includes the status code, standard headers (including Content-Type), and a small diagnostic JSON body describing the request and returned status.
Simulator URL (copy in the app after load — not a normal link):
https://httpstatus.com/api/status/501
Example request:
curl -i "https://httpstatus.com/api/status/501"