What HTTP 501 means
RFC 9110 §15.6.2. Server doesn't support the functionality. Different from 405 (which knows the method but not for this resource). Rare in practice.
Typical causes
- Unsupported HTTP method (e.g., custom verb)
- Feature not yet built
501 5xx RFC 9110 §15.6.2 Not Implemented
Server doesn't recognize the request method or lacks the ability to fulfill it.