501 Not Implemented

HTTP 501 Not Implemented tells the client that the server does not support the functionality needed to fulfill the request, most often because it does not recognize the HTTP method or request feature at all. Per RFC 9110, this is distinct from a request that is valid but temporarily unavailable: a 501 means the server, as currently built, is fundamentally unable to handle this kind of request, regardless of retries. It is one of the least common status codes in practice because modern web servers and frameworks support the small set of standard methods, such as GET, POST, PUT, and DELETE, that almost all traffic uses.

In practice 501 shows up when a client sends an HTTP method the server has never implemented, such as PATCH on an old API that only added it years after launch, or a custom or WebDAV method like PROPFIND hitting a server with no WebDAV module. It can also appear when a reverse proxy or CDN edge worker receives a method it deliberately blocks, or when an API gateway routes to a backend that has not yet shipped support for a newly documented endpoint. Unlike 405 Method Not Allowed, which implies the resource exists but rejects this specific method, 501 says the server itself lacks the capability.

Because 501 is rare, seeing it unexpectedly is usually a strong signal of a client bug, such as a typo in the HTTP verb or a library defaulting to an unsupported method, or a server that is older or more minimal than the client assumes, such as a static file server receiving an API-style request. Load balancers and CDNs sometimes generate a synthetic 501 themselves when they cannot map a request to any known handler, in which case the fix lives in routing rules rather than application code.