HTTP 501 Not Implemented

What HTTP 501 means

Usually the HTTP method isn’t recognised by the server at all, such as PROPFIND on a server without WebDAV. If the method exists but isn’t allowed on that URL, use 405. Some APIs return 501 for endpoints that are planned but not built yet.

Retrying

Rarely. The server can’t handle this request, so retrying it unchanged is unlikely to help.

Example response

HTTP/1.1 501 Not Implemented
Date: Wed, 21 Oct 2026 10:30:00 GMT
Content-Type: application/json

How to return 501

res.status(501).json({ error: 'Not Implemented' });

from fastapi import HTTPException

raise HTTPException(status_code=501, detail="Not Implemented")

http.Error(w, http.StatusText(http.StatusNotImplemented), http.StatusNotImplemented)

location /private/ {
    return 501;
}

Related status codes

• 405 Method Not Allowed : The resource exists but doesn’t support the HTTP method used.
• 500 Internal Server Error : The server hit an unexpected error and couldn’t complete the request.
• 505 HTTP Version Not Supported : The server doesn’t support the HTTP version used in the request.

Other 5xx status codes

500 Internal Server Error · 502 Bad Gateway · 503 Service Unavailable · 504 Gateway Timeout · 505 HTTP Version Not Supported · 507 Insufficient Storage · 508 Loop Detected · 511 Network Authentication Required · 520 Web Server Returned an Unknown Error · 521 Web Server Is Down · 522 Connection Timed Out · 523 Origin Is Unreachable · 524 A Timeout Occurred · 525 SSL Handshake Failed · 526 Invalid SSL Certificate

See the full list of HTTP status codes.

Sources

• RFC 9110, section 501
• IANA HTTP Status Code Registry
• MDN: 501 Not Implemented

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