HTTP 501 Not Implemented

Server error code, RFC 9110, section 15.6.2

What it means

The server does not support what the request needs. Most often the method (a verb) is unknown to it.

A real response

This is the request and the answer our test server gave when we asked it to return 501.

GET /c/501 HTTP/1.1
Host: example.com

HTTP/1.1 501 Not Implemented
Date: Sat, 14 Nov 2026 10:00:00 GMT
Content-Type: text/html; charset=utf-8
Content-Length: 11

<p>demo</p>

If you run the site

Use 501 when the server does not recognise the method at all, and 405 when it knows the method but not for this address. In practice, a server returns 501 when old software meets a new method.

If you are a visitor

Probably a problem of the site or tool you are using.

A common mistake

Using 501 as another word for 500.

Registered in the IANA HTTP Status Code Registry as Not Implemented, defined in RFC 9110, section 15.6.2.

This is an independent guide. It is not connected with the IETF, IANA, any browser maker or any company named here. Meanings were checked against the RFC texts on 14 November 2026. Responses shown were recorded on 14 November 2026 from a small test server on the author's own computer.