5xx Server Error
Cacheable by default
501 Not Implemented
The server does not support the functionality required to fulfil the request.
What causes it
- An unrecognised HTTP method such as PATCH on an old server, or an unimplemented route.
How to fix it
- Unlike 405, this means the method is unsupported by the server as a whole, not just for this URL.
At a glance
- Class
- 5xx Server Error
- Reference
- RFC 9110 §15.6.2
- Standardised
- Yes
- Retry the request?
- Only if idempotent
Other 5xx server error codes
- 500 Internal Server Error
- 502 Bad Gateway
- 503 Service Unavailable
- 504 Gateway Timeout
- 505 HTTP Version Not Supported
- 506 Variant Also Negotiates
- 507 Insufficient Storage
- 508 Loop Detected
- 510 Not Extended
- 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