501 Not Implemented
The server does not support the functionality required to fulfill the request — classically, an HTTP method it doesn't implement at all (as opposed to 405: method known, not allowed on this route). In practice it also appears from gateways for unsupported features.
What usually causes it
- Exotic HTTP methods against servers that never implemented them
- Gateways/proxies encountering transfer features they don't support
- Stub endpoints deliberately returning 501 during phased rollouts
How to debug and fix it
- Check the method spelling and whether the API actually documents it.
- If you own the server and use 501 as 'coming soon', document it — clients treat 501 as permanent and won't retry.
Easily confused with
Debugging an API? Build and inspect requests, check response headers, or read the full status-code debugging guide.