HTTP status · 5xx — server error
501 Not Implemented
The server does not recognize or support the request method.
Key facts
| Class | 5xx — server error |
|---|---|
| Standard | RFC 9110 §15.6.2 |
| Meaning | Method not supported at all |
| Contrast | 405 = method known, not allowed here |
| Cause | Unsupported method or server limitation |
| Fix | Use a supported method or enable it |
501 versus 405
405 says the method is understood but not allowed for this resource, and must include an Allow header listing what is allowed. 501 says the server does not implement the method at all. A server that rejects PATCH everywhere should answer 501; one that allows GET but not DELETE on a specific path should answer 405.
Diagnosing a 501
Check whether a proxy or load balancer is rejecting the method before it reaches the application. Some servers only advertise methods they were compiled or configured to support. For WebDAV-style methods such as PROPFIND, a plain static host will legitimately answer 501.
Frequently asked questions
Is 501 the client’s fault?
It is a server-side limitation. The client can only switch to a supported method or endpoint.
Why does my API return 501 for PATCH?
The framework or server may not implement PATCH for that route. Use PUT or POST, or enable method support.
Does 501 mean the resource does not exist?
No. That is 404. 501 is about the method, not the path.