What it is
User-Agent Client Hints are the fields Chromium moved out of the frozen UA string: platform and platform version, CPU architecture and bitness, the full browser version, the device model, and whether the browser calls itself mobile. Firefox and Safari do not implement them, which is itself a data point.
How websites read it
const hints = await navigator.userAgentData?.getHighEntropyValues([
'platform', 'platformVersion', 'architecture', 'bitness', 'uaFullVersion',
]);
// headers: Sec-CH-UA, Sec-CH-UA-Mobile, Sec-CH-UA-Platform (+ high-entropy after Accept-CH)Three low-entropy hints (brands, mobile, platform) travel as headers with every request. The high-entropy ones are handed out on demand: a script asks getHighEntropyValues(), or a server opts in with an Accept-CH header and receives them on the next request.
Why it identifies you
The hints say what the reduced UA no longer does — that you are on Windows 11 rather than "Windows NT 10.0", on ARM rather than x86, on build 152.0.7977.76 rather than 152.0.0.0. They are also the reference every other OS-shaped signal gets compared with.
How risk-control systems use it
Three comparisons cost nothing: the header copy against the JavaScript copy, the platform against the UA string, and the architecture against what floating-point behaviour reveals about the real CPU. A hint set that is missing on a browser that should have it — Chrome claiming to be Chrome, but with no userAgentData — is just as telling.
Common mistakes
Rewriting the UA string and forgetting the hints; an "x86" architecture hint on an Apple-silicon GPU; a platformVersion no Windows build ever shipped; hints that change between the header and the script call.
Mango is accepting waitlist registrations. These capabilities describe its first release; client access opens with invitations.
How Mango Browser handles it
CustomIn Mango’s Chromium profiles, Client Hints and the User-Agent share the same profile identity. Firefox profiles retain their engine’s behavior; missing User-Agent Client Hints should not be treated as a failed check for Firefox.
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