User-Agent Client Hints
User-Agent Client Hints are HTTP headers (the Sec-CH-UA family) that let a site request specific browser, platform, and version detail rather than reading it all from one passive string. They underpin UA reduction: the raw user agent is shrinking, and finer detail moves to opt-in hints. This page explains the model.
What Client Hints are
User-Agent Client Hints are a set of request headers, led by the Sec-CH-UA family, that expose browser brand, platform, and version detail. Instead of every request broadcasting a long passive string, the browser sends a low-entropy hint by default and reveals more only when a site explicitly requests it.
This is a deliberate shift from passive (always-present UA string) to active (requested hints) browser information.
- Sec-CH-UA family of request headers
- Low-entropy by default; high-entropy hints are requested
- Active, opt-in model rather than a passive string
Why the raw UA is shrinking
Browsers, led by Chrome's UA reduction, are trimming detail from the legacy user-agent string to reduce passive fingerprinting and the brittleness of UA sniffing. The information has not disappeared; it has moved to Client Hints that a site can request when it has a real need.
The practical consequence: do not rely on the raw user agent for precise version or platform data going forward. Match on stable patterns for coarse classification, and use Client Hints when you genuinely need fine detail.
How it appears in analytics and logs
Reduced detail in a raw user agent is expected, not a fault. Sites that need precise browser or platform data are meant to request it via Sec-CH-UA Client Hints rather than parse the legacy string.
Diagnostic use case
Understand why the raw user-agent string is losing detail and where that detail now lives, so detection and analytics adapt to Client Hints instead of breaking.
What WebmasterID can help detect
WebmasterID classifies clients from server-side signals and is robust to UA reduction, so shrinking raw user-agent strings do not degrade its browser/automation categorisation.
Common mistakes
- Assuming the raw user agent will keep exposing full version and platform detail.
- Treating reduced UA detail as a bug rather than intended behaviour.
- Collecting high-entropy hints you do not need, working against the privacy goal.
Privacy and accuracy notes
Client Hints are designed to reduce passive fingerprinting by making detail opt-in rather than always broadcast. WebmasterID stores coarse categories, not raw fingerprintable detail from real visitors.
Related pages
- Chrome user agent and its quirks
Chrome's user-agent string is full of historical artefacts: it claims AppleWebKit and Safari for compatibility even though Chrome uses the Blink engine. Google has also reduced the detail Chrome exposes in the UA, moving fine-grained information into User-Agent Client Hints. This page explains the pattern and the quirks.
- Browser user agents: how to read them
A browser user-agent string packs several tokens into one line: a legacy Mozilla prefix, a rendering-engine signature, the platform, and the browser itself. This page explains each part so you can read a UA without over-reading it, because the contents are client-controlled and can be copied by any client.
- Privacy-first analytics
Analytics that rely on coarse categories, not fingerprintable detail.
Sources and verification notes
Last reviewed 2026-06-24. Facts are checked against primary/official sources where available; uncertain specifics are marked “Data not yet verified” rather than guessed.