HTTP Status Codes

501 Not Implemented

The server does not support the functionality required — for example an HTTP method it does not recognise at all. Different from 405, which knows the method but forbids it here.

What it means

501 Not Implemented means the server lacks the capability to fulfil the request, such as an unknown or unsupported method. Whereas 405 says 'this resource does not allow that method', 501 says 'this server does not implement that method at all'.

Why it matters

  • It cleanly reports missing server capability versus a per-resource restriction.

Common mistakes

  • Using 501 where 405 (method known but not allowed here) is correct.

How WebInspect checks this

  • The API Inspector reports method handling and distinguishes 501 from 405 behaviour.