Glossary

User agent

The text a browser sends with every request to say what it is: browser, version and operating system. Chrome also sends structured Client Hints, and both have to match what the browser can actually do.

Also called: UA, user-agent string, Client Hints.

A typical one reads Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/156.0.0.0 Safari/537.36. Sites use it to serve the right version of a page, and anti-fraud systems treat it as a claim to verify.

Changing it is easy, which is why it proves little on its own. Detectors compare it with the features the browser really supports. Castle showed that a profile claiming Chrome 134 on macOS without the BarcodeDetector API, which Chrome on macOS has had since version 88, gives itself away. Since September 2026 Chrome ships a new version every 2 weeks, so a user agent that is months old also stands out.

In an antidetect, the user agent should match the browser kernel the profile runs on. Claiming another browser family doesn't change the TLS handshake underneath, which still identifies the real engine.