501 Not Implemented

501 Not Implemented indicates that the server does not support the functionality required to fulfill the request. It is different from 405 Method Not Allowed: 405 means the server knows the method but doesn't allow it for this resource, while 501 means the server doesn't support the method at all.

Common in proxy servers, API gateways, or servers that only implement a subset of HTTP methods.

• Server doesn't implement the HTTP method used (e.g. PATCH on an older server)
• API endpoint not yet built (placeholder returning 501)
• Proxy or gateway forwarding to a server that doesn't support the method
• WebDAV-specific methods (LOCK, PROPFIND) on non-WebDAV servers

• Check which HTTP methods are supported via an OPTIONS request
• Use a supported method (GET, POST) as an alternative
• If you're the API owner, implement the missing method or return 405 for known-unsupported methods
• Check server documentation for supported HTTP methods