Data & identity · also called UA-CH, Client Hints
User-Agent Client Hints
User-Agent Client Hints are a web standard, championed by Chrome, where browsers share only basic details by default and send detailed device and browser information only when a site asks.
The traditional user agent string broadcasts browser version, OS version and device model with every request, which is handy for device fingerprinting. Client Hints change the default. Browsers send a few low-detail "hints" (such as brand, major version, mobile or not, and platform), and sites that need more, such as full version or device model, must explicitly ask, via HTTP headers or a JavaScript API.
Chrome reduced its user-agent string in phases during 2022 and 2023 and pushed Client Hints as the replacement. Other Chromium browsers followed, while Safari and Firefox have not adopted the full API. OpenRTB added fields so ad requests can carry structured client-hint data.
For ad tech, this means device and browser detection for targeting, reporting and fraud detection must request and forward hints properly, or risk losing detail. Verification vendors use the consistency between hints, the legacy UA string and actual browser behaviour as a signal: mismatches can reveal spoofed or automated browsers.
Think of it like this
Client Hints are like a guest who only gives their first name at the door, and shares their full details only if the host has a good reason to ask.
An example
An SSP starts requesting high-entropy hints for device model on Chrome for Android, restoring detailed device reporting to buyers that the reduced user agent had removed.
Related terms
User agent
A user agent is the text string a browser or app sends with each request describing its browser, version, operating system and device, used for compatibility, analytics and fraud detection.
Device fingerprinting
Device fingerprinting is identifying or characterising a device from the combination of its technical attributes, such as browser, fonts, screen, graphics and network traits, instead of a stored cookie or ID.
Signal loss
Signal loss is the reduction in data available to target, measure and optimise advertising, caused by browser cookie blocking, mobile ID restrictions, privacy laws, consent choices and ad blocking.
Device spoofing
Device spoofing is faking the identity or type of device requesting an ad, such as a server posing as a smart TV or iPhone, to earn higher prices or dodge detection.
Bid request
A bid request is the message an SSP or exchange sends to potential buyers describing an available ad impression, inviting them to bid on it in a real-time auction.
Sources: WICG: User-Agent Client Hints specification, MDN: HTTP Client Hints, IAB Tech Lab OpenRTB