5xx Server error
501 Not Implemented
The server doesn’t support the functionality required to fulfil the request.
Usually the HTTP method isn’t recognised by the server at all (unlike 405, where the method exists but isn’t allowed for this resource).
Common causes and fixes
- Check for unusual methods or proxies that don’t support them.
Example response
HTTP/1.1 501 Not Implemented
Date: Thu, 22 Oct 2026 08:00:00 GMT Check a URL’s status with curl -I https://example.com, which prints the status line and headers.