Further extensions to the request are required for the server to fulfill it.
What does HTTP 510 mean?
The 510 Not Extended code is part of the 5XX class. This is a server error code — the request was valid but the server failed to fulfil it. The problem is server-side.
When is it used?
A server returns 510 Not Extended in situations matching the definition above. You will encounter it inspecting network requests, reading logs, or testing an API. Whether it signals a problem depends on its class: 2xx and 3xx are normal, while 4xx and 5xx usually need attention on the client or server respectively.