501 Not Implemented
The server does not support the functionality required to fulfil the request, usually the method itself.
Status
HTTP/1.1 501 Not Implemented
Details
- Category: 5xx
- Status: Standard. Registered with IANA and defined by a current standards-track RFC.
- Specification: RFC 9110 §15.6.2
- Cacheable: Cacheable by default
- Response body: Allowed
- Retry: Do not retry
501 means the server does not recognize the request method at all, or cannot support it for any resource. It is a statement about the server's capabilities rather than about one URL, which is what separates it from 405: 405 says this resource does not accept that method, 501 says this server does not implement it anywhere.
Only GET and HEAD are required of every server. Anything else, including PATCH and the WebDAV methods, may legitimately produce 501.
Headers
- Retry-After: Optional. Include only if support is expected to arrive at a known time.
Common causes
- An unrecognized or non-standard HTTP method.
- A proxy asked to handle a method or protocol feature it does not implement.
- An API stub that returns 501 for endpoints planned but not built.
- A server refusing an unsupported Transfer-Encoding or content coding.
How to fix it
As the client
- Use a supported method. GET and HEAD are the safe baseline.
- Check whether the API offers a method-override convention for clients or intermediaries that cannot send PATCH or DELETE.
As the server
- Use 405 with an Allow header when the method exists elsewhere on the server but not on this resource.
- Document unimplemented endpoints rather than leaving callers to discover them by trying.
Examples
PATCH /api/v2/order/40213 HTTP/1.1 Host: legacy.example.com HTTP/1.1 501 Not Implemented Content-Type: text/plain PATCH is not supported by this server