HTTP status codes / 4xx — Client errors
The server cannot produce a representation matching your Accept headers.
Content negotiation failed: the Accept (or Accept-Language/Encoding) header demands something the server cannot generate. Rare, because most servers ignore Accept rather than enforcing it.
curl -i -H "Accept: application/xml" https://httpbin.org/json
Same request, no terminal: paste this into the cURL converter for native code, or straight into ReqPad on your phone.
Loosen or remove the Accept header and compare responses.
Server-side note: Loosen the Accept header client-side, or stop enforcing negotiation server-side unless you really ship multiple formats.
The fastest way to pin down a 406 is to reproduce the exact request and inspect what actually went over the wire — status, headers, timing and body, without your app code in the way. That is what an API client is for; ReqPad does it from your phone, with every request saved to history.
400 Bad Request · 401 Unauthorized · 402 Payment Required · 403 Forbidden · 404 Not Found · 405 Method Not Allowed — or the full reference.
Build the request, send it, read status + headers + timing — on your iPhone. Free to start.