406 Not Acceptable

406 Not Acceptable is returned when the server cannot produce a response matching the list of acceptable values defined in the request's proactive content negotiation headers (Accept, Accept-Encoding, Accept-Language).

The server may return a list of available representations in the response body so the client can choose an appropriate one.

• Client requests XML but server only produces JSON
• Accept-Language specifies an unsupported locale
• Client requires a specific encoding that the server doesn't support
• API versioning mismatches via Accept headers

• Ensure your API documents supported content types clearly
• Implement proper content negotiation for all response formats
• Return a default format (e.g. JSON) if no match is found rather than 406
• Use Vary header to indicate which request headers affect the response