HTTP status codes / 4xx — Client errors
Switch protocols (see the Upgrade header) and try again.
The server refuses to serve the request over the current protocol and names the required one in the Upgrade header — e.g. forcing a newer TLS or HTTP version.
Check the Upgrade header in the response and reconnect using that protocol.
The fastest way to pin down a 426 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.