HTTP 501 — Not Implemented
5xx Server Error
What it means
The server does not support the functionality required to fulfil the request.
What to do about it
Usually means an HTTP method the server does not recognise at all. Check for a proxy sending an unusual method.
Where it sits
501 belongs to the 5xx family: The server failed to fulfil an apparently valid request. The fix is on the server side.
HTTP/1.1 501 Not ImplementedChecking it yourself
# see the status code and headers only
curl -sI https://example.com/path
# follow redirects and print each hop
curl -sIL -o /dev/null -w "%{http_code} %{url_effective}\n" https://example.com/path
# JavaScript
const r = await fetch(url);
console.log(r.status, r.statusText);How this code behaves
| Cacheable by default | Yes — per RFC 9110, this status is cacheable unless headers say otherwise. |
| Safe to retry | Yes, with backoff. This is a transient condition; retrying the same request is reasonable. |
| Effect on search indexing | Repeated server errors reduce crawl rate and eventually drop pages from the index. |
Returning 501 correctly
# nginx
return 501;
# Express
res.status(501).json({ error: 'Not Implemented' });
# Go
w.WriteHeader(501)
# Python (Flask)
return jsonify(error='Not Implemented'), 501;Frequently asked questions
What does HTTP 501 mean?
The server does not support the functionality required to fulfil the request.
How do I fix a 501 error?
Usually means an HTTP method the server does not recognise at all. Check for a proxy sending an unusual method.
Is 501 a client or server problem?
A server problem. The request was valid; the server failed to fulfil it. Nothing the client changes will help.