HTTP 501 Not Implemented

What HTTP 501 means

HTTP 501 Not Implemented means the server does not recognize or support the functionality required to fulfil the request. The spec reserves it mainly for unrecognized HTTP methods — the server cannot handle the verb for any resource, as opposed to 405 where the verb is known but disallowed for this URL.

In practice 501 also shows up from proxies and legacy servers hitting requests they were never built for (WebDAV verbs, PATCH on old stacks), or as a placeholder from half-configured backends.

Common causes of 501 errors

• An HTTP method the server does not implement at all (e.g. PATCH or WebDAV verbs on an old server).
• A proxy or gateway receiving a request method it cannot forward.
• A stub endpoint deployed before its implementation was finished.
• Malformed method names from buggy clients (typos like GETT).

How to fix it as a developer

• Check the exact method being sent — typos and wrong-cased custom verbs are surprisingly common.
• If the method is legitimate (e.g. PATCH), upgrade or configure the server/framework to support it, or tunnel via POST with an override header where the stack allows.
• Distinguish from 405: if only one route rejects the verb, fix the route (405 territory); if the whole server does, it is a platform capability gap.
• Do not return 501 for unfinished features in production APIs — prefer 404 or a feature flag, since 501 is cacheable by default.

Example response

HTTP/1.1 501 Not Implemented
Content-Type: application/json

{"error":"not_implemented","message":"PATCH is not supported by this server"}

FAQ