HTTP status codes / 4xx — Client errors
A proxy between you and the origin wants credentials.
Like 401, but issued by an intermediary proxy: it sends Proxy-Authenticate and expects Proxy-Authorization. Seen on corporate networks and authenticated forward proxies.
Configure proxy credentials in your HTTP client or environment (HTTP(S)_PROXY with user:pass).
Server-side note: Set proxy credentials (HTTPS_PROXY=http://user:pass@proxy:8080) or configure them in your HTTP client.
The fastest way to pin down a 407 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.