406 4xx RFC 9110 §15.5.7

Not Acceptable

Server cannot produce a response matching the Accept headers in the request.

What HTTP 406 means

RFC 9110 §15.5.7. Triggered when the client's Accept header (or Accept-Language, Accept-Encoding) lists no representation the server can produce. Rare in practice; PostgREST uses it for malformed query syntax.

Typical causes

Accept: application/xml when only JSON is supported
PostgREST malformed query