What is my user agent?

A user agent is a line of text the browser sends with every request, naming itself, its version, its rendering engine and the operating system. A typical one for Chrome on Windows reads Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/141.0.0.0 Safari/537.36. Only "Chrome/141" and "Windows NT 10.0" carry real information; the rest is kept for compatibility with sites that check for older browsers.

Because of that history, almost every browser claims to be several others. Edge adds "Edg/", Opera adds "OPR/", and both still say Chrome and Safari. Parsers have to check for the most specific name first, which is a common source of bugs in analytics and browser detection code.

Browsers have reduced the user agent to limit fingerprinting. Chromium browsers report the version as 141.0.0.0 rather than the full build. Every browser on Windows 11 reports Windows NT 10.0. Every browser on a Mac reports macOS 10.15.7, whatever the real version. Chrome on Android reports Android 10 and a device model of "K" for every phone.

Chrome, Edge, Opera and Samsung Internet expose the real values through User-Agent Client Hints, which a site has to ask for. This page asks, and shows the result. Firefox and Safari do not support client hints, so for those browsers the user agent is all there is.

Changing the user agent, in browser developer tools or a test framework, tests code that reads the string: server-side redirects to a mobile site, download pages that pick an installer, analytics, and feature blocks for old browsers. It does not make one browser behave like another, because the engine underneath is unchanged.

For device and browser bugs, test on the real browser or a cloud device rather than a spoofed user agent. Code that still parses the user agent for decisions is worth a test of its own, because the reduced strings above already break naive version checks.