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

A method the server does not implement

HTTP request

PATCH /api/v2/order/40213 HTTP/1.1
Host: legacy.example.com

HTTP response

HTTP/1.1 501 Not Implemented
Content-Type: text/plain