HTTP request headers
The headers your browser sends, their order, and whether the Accept, Accept-Encoding and header order match the engine your User-Agent claims.
What to do about it
Proxy headers
Use the proxy in CONNECT (tunnel) mode or a VPN. A plain HTTP proxy that appends Via or X-Forwarded-For announces itself on every request.
Accept, Sec-Fetch, TE, order
These cannot be configured in the browser. Run the engine the User-Agent names; a Chromium build claiming Firefox is caught by the Accept header alone.
Client Hints
Sec-CH-UA must agree with navigator.userAgentData and with the User-Agent: brand list, version and platform. Missing hints on a modern Chromium over HTTPS are a signal too.
Accept-Language
Keep the header and navigator.languages identical and plausible for the IP country. Browser-level language settings drive both; extensions that rewrite headers drift.
How this test works
The server records the headers of the request that loaded this page — the same ones any site receives — and, when the edge terminates TLS, their exact order on the wire. Each engine-defined header is compared with what the browser family named in the User-Agent emits at that version; the JavaScript side (navigator.userAgent, userAgentData, languages) is compared with the header side.