All HTTP status codes

HTTP 501 Not Implemented

Server Error

The server doesn't support the functionality required — typically an HTTP method it doesn't implement at all (contrast 405: method known, not allowed here).

Defined in RFC 9110

Common causes

  • Exotic/unsupported HTTP methods
  • Proxies that don't implement a requested transformation

How to fix it

  1. Use a method the server supports

  2. Implement the handler if the endpoint should support it

Related status codes

405 Method Not Allowed

The URL exists but doesn't support the HTTP method used — e.g. POSTing to a GET-only endpoint. The Allow header lists what's permitted.

505 HTTP Version Not Supported

The server doesn't support the HTTP protocol version used in the request.

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