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 Server doesn't recognize the request method or lacks the ability to fulfill it.
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.