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501 Not Implemented

What HTTP 501 Not Implemented means, how it differs from 405 Method Not Allowed, why servers send it for unknown methods, and how to fix it as a client or a server.

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What 501 means

The server doesn't support the functionality the request needs. In practice that almost always means it doesn't recognize the request method at all. RFC 9110 §15.6.2 defines it, and says 501 is the right answer when the server doesn't recognize the method "for any resource". It's one of the few error codes that is cacheable by default, so a proxy may store the answer.

HTTP/1.1 501 Not Implemented
Content-Type: text/plain
Content-Length: 34

Method PURGE is not implemented.

The key difference from 405 Method Not Allowed: 405 means the server knows the method but this resource doesn't allow it (and the response must carry an Allow header listing the methods that work). 501 means the server has no idea what the method is anywhere. General-purpose servers are required to support GET and HEAD, so a 501 for those points at something very unusual in between.

Common causes

  • Custom or extension methods such as PURGE, BAN, PROPFIND or MKCOL sent to a server without the module that handles them (a cache, WebDAV).
  • A typo in the method, for example from a script that builds the request line itself.
  • An unsupported Transfer-Encoding in the request: some servers answer 501 when they can't decode the body's encoding.
  • Older proxies or embedded servers that only implement GET, HEAD and POST and reject PUT, PATCH or DELETE.
  • APIs that use 501 for "not built yet" endpoints. That's a loose reading of the spec, but you will meet it.

How to fix it

As a client:

  1. Check the method name and spelling, and whether the API documents it.
  2. If the method works directly against the application but not through a proxy, CDN or load balancer, the middle box is answering. Compare the response's Server and Via headers.
  3. For cache purges, use the provider's API rather than a raw PURGE request, unless you configured the cache to accept it.

As a server:

  • Enable the module the method needs (WebDAV in Apache with mod_dav, ngx_http_dav_module in nginx; a PURGE handler in Varnish VCL).
  • For a known method that a route doesn't accept, return 405 with an Allow header instead of 501.
  • In APIs, prefer 404 for routes that don't exist and 405 for wrong methods; keep 501 for genuinely unrecognized methods.

501 and SEO

Crawlers fetch pages with GET and HEAD, so a 501 normally never reaches them. If one does, Google treats it like any other 5xx: crawling slows down, and URLs that keep failing are eventually dropped. A 501 on a normal page fetch usually means a firewall or proxy rule is rejecting requests it shouldn't.

Try other methods against the same URL with the HTTP Request Builder to see which ones the server accepts.