HTTP 501 Not Implemented
501 Not Implemented means the server does not support the functionality required to fulfil the request — typically an HTTP method it does not recognise or handle at all. It is a server-side error distinct from 405, where the resource exists but rejects a specific method. For crawlers it is an uncommon, server-level failure.
What 501 means
501 Not Implemented is a server-error status indicating the server does not support the functionality required to fulfil the request. The most common trigger is an HTTP method the server does not recognise or implement. Of the request methods, only GET and HEAD are required to be supported by general-purpose servers; others may legitimately return 501.
It signals a capability gap on the server, not a problem with a particular resource.
501 vs 405
501 and 405 both involve methods, but at different levels. 501 means the server does not implement the method anywhere — it does not know how to handle that request type. 405 means the server understands the method but the specific resource does not allow it, and it returns an Allow header listing what is permitted.
For crawlers, the practical risk is a server that does not implement HEAD: some crawlers probe with HEAD, and a 501 there can interfere. Ensure GET and HEAD are supported on content you want crawled.
- 501 = server does not implement the method at all
- 405 = resource exists but disallows that method
- Support GET and HEAD so crawler probes succeed
Operator checklist
Confirm your server implements GET and HEAD for content you want crawled. Investigate 501s a crawler receives, especially on HEAD. Distinguish a 501 (server capability gap) from a 405 (per-resource method rule) when diagnosing.
How it appears in analytics and logs
A 501 means the server lacks the capability to fulfil the request — often an unrecognised method. It is a server-side limitation rather than a per-resource rule; for crawlers it is unusual and indicates the server cannot handle the request type at all.
Diagnostic use case
Diagnose a 501 a crawler receives, and tell an unsupported-method server limitation from a 405 method restriction on a specific resource.
What WebmasterID can help detect
WebmasterID can surface 501s crawlers receive, helping you spot a server that cannot handle a request method crawlers rely on, such as HEAD.
Common mistakes
- Not implementing HEAD, so crawler HEAD probes get 501.
- Confusing 501 (no method support) with 405 (method not allowed here).
- Treating 501 as a per-page issue rather than a server capability gap.
Privacy and accuracy notes
Status codes carry no personal data. WebmasterID reports 501 patterns for crawler traffic without exposing individual visitors.
Related pages
- HTTP 405 Method Not Allowed
405 Method Not Allowed means the server recognises the request method but the target resource does not support it — for example a POST to a GET-only page. For crawlers, which issue GET (and sometimes HEAD), a 405 usually means the route does not allow GET, often a misconfiguration on a URL that should serve a page.
- HTTP 500 and crawl health
500 Internal Server Error is a generic message that something went wrong on the server and it could not complete the request. Occasional 500s happen, but repeated 500s on important URLs harm crawl health: crawlers may slow down and, if errors persist, treat affected pages as unreliable.
- Website observability
Spot server-error responses crawlers receive across paths.
Sources and verification notes
Last reviewed 2026-06-24. Facts are checked against primary/official sources where available; uncertain specifics are marked “Data not yet verified” rather than guessed.