The requested resource is capable of generating only content not acceptable according to the Accept headers sent in the request.
What does HTTP 406 mean?
The 406 Not Acceptable code is part of the 4XX class. This is a client error code — the request was faulty, unauthorized, or cannot be fulfilled. The problem is on the request side.
When is it used?
A server returns 406 Not Acceptable 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.