User agent
The user agent is the string your browser sends to identify itself, its version, and the operating system; sites parse it before anything else.
What a user agent is
The user agent string is a line of text sent in the User-Agent header with every HTTP request and exposed to scripts as navigator.userAgent. A typical desktop value looks like Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/130.0.0.0 Safari/537.36. Almost every token in it is historical baggage: modern browsers claim to be Mozilla, WebKit, and Safari at once so that old server rules keep working. The useful parts are the operating system, the browser name, and the major version.
Browsers have been freezing and simplifying the string for several years. Chromium-based browsers report a fixed Windows NT 10.0 on every Windows version and zero out minor version numbers, moving the detail to client hints instead.
Why it matters
Servers read the user agent to choose a layout, to log statistics, and to decide whether the visitor is a browser at all. Command-line tools, crawlers, and HTTP libraries announce themselves with their own strings, and our anonymity score treats a non-browser user agent as an automation signal worth 20 points. Beyond that, the string is the anchor that every other fingerprint signal is checked against: a user agent that claims Windows while the platform, fonts, and canvas rendering say Linux is a contradiction, and so is a mobile user agent on a 2560-pixel-wide screen.
For antidetect profiles, the user agent is the first thing to set and the easiest to get subtly wrong, usually by choosing a version no longer in circulation or by pairing it with client hints that disagree.
How to check
The user agent tool shows your string, parses it into browser, version, engine, operating system, and device type, and flags known bot or tool signatures. You can also paste any other string to parse it. The HTTP headers tool shows the header next to the client hints headers so you can compare them, and the browser fingerprint page checks the string against the JavaScript-reported platform and screen.
Common misconceptions
Changing the user agent does not change the browser. Feature detection, rendering behavior, and JavaScript engine quirks still reveal the real one, which is why a spoofed string without a matching profile tends to lower trust rather than raise it. And a blank or unusual string is not anonymous; it is rare, and rare is memorable.