server error RFC 9110
HTTP 501 Not Implemented
Server doesn't support the functionality required to fulfill the request.
What it means
The server does not recognize the request method or does not support fulfilling it. Rare in practice — most 'unsupported method' errors are 405. 501 is for cases where the method itself is fundamentally unimplemented (e.g., custom methods, HTTP extensions the server doesn't speak).
When servers send it
- •Non-standard HTTP method the server doesn't implement
- •HTTP extension the server doesn't support (rare in modern web)
What the client does
Cannot retry — the feature just doesn't exist on this server.
Common causes
- •Client using a method the server never supports
Similar codes
405 Method Not Allowed client error
The URL exists but doesn't support the HTTP method the client used.
500 Internal Server Error server error
Something went wrong on the server side — usually an unhandled exception.
502 Bad Gateway server error
A gateway or proxy received an invalid response from the upstream server.
503 Service Unavailable server error
Server is temporarily unable to handle the request — overloaded or down for maintenance.
504 Gateway Timeout server error
A gateway or proxy timed out waiting for a response from the upstream server.
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