What Your User Agent Says About You (and Why It Lies)
How to read a user-agent string, why every browser claims to be Mozilla, and when faking your UA is genuinely useful — a practical guide.
Every page you open gets a short introduction from your browser before any content loads. It looks something like this:
Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36
(KHTML, like Gecko) Chrome/126.0.0.0 Safari/537.36That’s a user-agent string, and if you read it literally, it claims to be Mozilla, WebKit, Chrome, and Safari at the same time. The What Is My User Agent tool shows you yours and translates it into something a human can use: browser, version, engine, operating system, device type.
Why every browser pretends to be Mozilla
In the mid-90s, webmasters served frames and other fancy features only to Netscape Navigator, whose UA started with “Mozilla”. Internet Explorer wanted those features too, so Microsoft simply put “Mozilla” in its own string. Every browser since has kept up the act, because dropping it broke sites that sniffed for the word.
Chrome went further. It launched in 2008 claiming to be Safari (both used WebKit back then), and kept the claim after switching to its own Blink engine in 2013. So the string above decodes as: a real Chrome 126 on Windows, wearing three borrowed name tags. The only token that matters is Chrome/126.
What you can actually learn from it
Despite the theatre, the string still answers practical questions:
- Which exact browser version a user runs — the first thing support asks for when a page misbehaves
- Whether a visitor in your server logs is a person or Googlebot
- Why a site is serving you its mobile layout (the word “Mobile” in the string is usually the trigger)
- What an unfamiliar device in your router or analytics actually is
I use it most for bug reports. “It’s broken on my mom’s computer” becomes actionable the moment she pastes her UA string and it turns out to be Firefox 78 from 2020.
Where it lies to you on purpose
Two caveats worth knowing before you trust the output.
Windows 11 does not exist in user-agent strings. Microsoft never bumped the token, so both Windows 10 and 11 report Windows NT 10.0. Any parser, including ours, can only tell you “10 or 11”.
macOS is frozen too. Safari and Chrome have reported Mac OS X 10_15_7 since 2020, no matter how new the machine is. Chrome also zeroes out its minor version — that 126.0.0.0 is deliberate. Browsers did this to fight fingerprinting: the more precise the string, the easier it is to single you out among other visitors. The precise data moved to Client Hints, which sites now have to request explicitly.
Faking it is a feature, not a hack
Changing your UA is built into every major browser. In Chrome DevTools, open Network conditions, untick “Use browser default”, and pick an iPhone — the site you’re debugging will serve its mobile version to your desktop. Firefox has the same via about:config, and extensions can switch strings per site.
This is how developers test mobile layouts without reaching for a phone, and how you can occasionally get past a “this site works best in Chrome” banner in a different browser. The site has no way to check you’re telling the truth.
Nothing leaves your browser
The tool reads navigator.userAgent with JavaScript and parses it locally. No request, no log, no stored copy — you can load the page, go offline, and keep pasting strings from your server logs to decode them.
Open What Is My User Agent, see what your browser tells every site, and copy it with one click the next time a support form asks.