Client Hints explained: how your browser advertises your device to websites
Client Hints is a browser API that lets websites ask for detailed device information — screen size, memory, user agent data — in a structured way. Here is why it matters for privacy.
What Client Hints does
Client Hints is a set of HTTP headers that websites can use to request specific information about your device and browser. Instead of sending all data to every website, the browser only sends what each site asks for.
The API was designed to replace the bloated User-Agent string with a more efficient, structured approach. However, it also creates a new fingerprinting surface: the combination of hints a browser sends can be unique.
Why it matters for privacy
Client Hints can expose device characteristics that were previously only available through JavaScript fingerprinting. This includes screen resolution, device memory, and user agent details — all of which contribute to your browser fingerprint.
The privacy concern is that Client Hints can be used to identify you even when you block cookies. The hints are sent automatically with requests, so websites can collect them without any visible interaction.
- Sec-CH-UA: user agent information (browser, version, platform)
- Sec-CH-UA-Mobile: whether the device is a mobile device
- Sec-CH-UA-Platform: operating system name
- Viewport-Width and Width: screen and content dimensions
What your browser can and cannot tell you
Your browser cannot directly show you which Client Hints a website has requested. However, you can inspect network requests in developer tools to see which CH headers are being sent.
Firefox limits Client Hints to same-origin requests by default, reducing cross-site tracking. Chrome supports Client Hints more broadly. Using a privacy-focused browser or a content blocker that strips these headers is the most effective defense.