Client Hints and userAgentData

Client Hints exist because the classic user agent string was too coarse and too easy to lie about. Sites now ask for the high-entropy fields directly. Sec-CH-UA, platform, architecture, bitness, model, and full version sit in headers and on navigator.userAgentData. If those fields disagree with navigator.userAgent, the caption failed.

Why a user agent string is not a fingerprint is the sibling. This page is the passport that replaced the caption.

What the page reads

Low-entropy hints are cheap: brand list, mobile bit, platform. High-entropy hints need a permissioned getHighEntropyValues call or a server opt-in. They return architecture, bitness, full version list, model, platform version, and related fields.

A Windows desktop profile should answer like a Windows desktop. MaskWright fingerprints set mobile false. There is no Android emulation path. A model string that sounds like a phone on this host is a contradiction.

The same profile also has to agree with screen metrics, hardwareConcurrency, fonts, and timezone. Hints are not a sealed identity. They are one more witness.

How we set the fields

CDP Emulation.setUserAgentOverride carries userAgentMetadata. The inject script writes the high-entropy values onto navigator.userAgentData. That is the implementation. It has to match the UA string we also set. Two sources, one story.

When the custom Chromium 131 core is missing, the fallback is system Chrome or Edge plus the same CDP and inject. The real binary version and the spoofed full version can still be a deep tell at the TLS or process layer. Hints will not fix JA3. JA3 is not a JavaScript spoof.

Headers and JavaScript have to match

Client Hints arrive as request headers when the server asks, and as navigator.userAgentData in the page. A profile that spoofs only the JS object leaves the next navigation on the real binary. A profile that spoofs only headers leaves a scripted probe on the real object. We set metadata through CDP and the high-entropy values through inject. Both have to name the same brand, the same major, and the same platform.

GREASE brands in the brand list are normal Chromium behavior. A list that looks too clean can be as loud as a list that looks too fake. I will not publish a brand recipe. I will say the list has to look like Chrome on Windows if that is the UA you claimed.

Full version is the field people set to 131 and then forget when the fallback binary is a different Edge build. The process on disk can still disagree at a layer hints cannot see. Hints are still required. They are not sufficient.

Platform version is the quiet field

platformVersion is easy to leave on the host. A UA that says Windows 11 next to a hint that still names a Windows 10 build is a cheap fork. Architecture and bitness are the other quiet pair. x64 on a 32-bit story, or the reverse, is the same class of mistake as a GPU mismatch.

I read those three after brand and major. I do not treat them as optional polish.

Plugins and mimeTypes are another witness we set to an OS-coherent list. They are not Client Hints. They still have to sound like the same Windows Chrome. window.chrome is the other Chromium-shaped witness we inject on Chromium profiles. Hints that say Chrome and a missing window.chrome are a fork. I check that object on Chromium profiles only. Firefox cores are a different story.

Mismatch patterns I actually see

UA says Chrome 131, hints say another major.

Platform hint says Windows, UA says Macintosh.

Mobile bit is true, battery looks like a plugged-in desktop, screen looks like a tower.

Architecture says x86, the rest of the profile says a different class.

Client rects will not save those. Neither will a prettier canvas.

How I use hints

I treat userAgentData as required, not optional polish. I read it next to the UA, not after. I do not ship a profile that only changed the old string.

The parent bundle is what browser fingerprinting actually measures. The Browser fingerprinting notes keep this slug so the high-entropy fields have a home the 2023 crawls never wrote.

FAQ

Are Client Hints the same as the user agent string? No. They are the high-entropy fields that replaced most of what the old string used to carry.

Is spoofing only the JavaScript object enough? No. Headers and userAgentData have to name the same brand, major, and platform.

Does this fix JA3? No. TLS fingerprints belong to the binary. Hints are JavaScript and HTTP headers.