user agent client hints browser
User-Agent Client Hints is not just a new list of headers, it is the direct answer to the same problem that drove the reduction of the traditional User-Agent: a server that needs a detailed piece of browser data today has to ask for it explicitly, instead of getting it for free on every request.
Sent on every request by default, no Accept-CH needed; it includes a fake brand ("Not A(Brand") on purpose, to stop sites from doing rigid string sniffing instead of checking the list.
The value is a boolean in RFC 8941 format (?0 or ?1), also sent automatically; there is no third value for tablet.
Only shows up after the explicit Accept-CH request; it is the same kind of data User-Agent Reduction hid from the traditional string.
It is an HTTP header sent by the browser with every request, in the format: "BrowserName/Version (OS; Architecture) Compatibility/1.0 Engine/Version". The syntax is defined by RFC 7231 and was originally created so servers could adapt responses for different clients.
No. Chrome and Chromium-based browsers, like Edge and Opera, implement the full set. Firefox and Safari do not send Client Hints at all so far, they prefer trimming the traditional User-Agent instead of adopting a separate hints system, so any analysis relying only on Client Hints misses a relevant slice of traffic.
Not automatically. Low-entropy hints arrive as usual, but high-entropy hints only reach a third-party resource if the main page explicitly delegates that via the Permissions-Policy header; without that delegation, the iframe gets only the basics, even if its own server asked for more through Accept-CH.
Yes, in practice. Since Firefox and Safari do not send Client Hints, any system that needs to cover the major browsers still depends on traditional string parsing as its base, using the hints as a precision add-on only when they are available, not as a full replacement.