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501 Not Implemented

The HyperText Transfer Protocol (HTTP) 501 Not Implemented server error response code means that the server does not support the functionality required to fulfill the request.

This status can also send a Retry-After header, telling the requester when to check back to see if the functionality is supported by then.

501 is the appropriate response when the server does not recognize the request method and is incapable of supporting it for any resource. The only methods that servers are required to support (and therefore that must not return 501) are GET and HEAD.

If the server does recognize the method, but intentionally does not support it, the appropriate response is 405 Method Not Allowed.

  • A 501 error is not something you can fix, but requires a fix by the web server you are trying to access.
  • A 501 response is cacheable by default; that is, unless caching headers instruct otherwise.

Status

501 Not Implemented

Specifications

Specification Title
RFC 7231, section 6.6.2: 501 Not Implemented Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content

Browser compatibilityUpdate compatibility data on GitHub

The information shown below has been pulled from MDN's GitHub (https://github.com/mdn/browser-compat-data).

Desktop
Chrome Edge Firefox Internet Explorer Opera Safari
501 Yes 12 Yes Yes Yes Yes
Mobile
Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet
501 Yes Yes Yes Yes Yes Yes

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https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/501