User Agent Parser

Why does every browser claim to be Mozilla?

History. In the 1990s servers checked for "Mozilla" before serving frames, so every competing browser added it to get the good version of pages. The same pattern repeated: Chrome added "Safari" because sites were sniffing for WebKit, Edge added "Chrome" because sites were sniffing for Chrome. Nothing was ever removed, because removing it breaks whatever is still checking.

The result is that a modern Chrome UA claims to be Mozilla, KHTML, Gecko and Safari simultaneously. It is layers of compatibility fiction, and reading it requires knowing the order the claims were added in.

So how does the parser tell Edge from Chrome?

By checking most-specific first. Edge's string contains the complete Chrome string plus Edg/124, so a parser that tests for Chrome before Edge reports Chrome for every Edge user — and it will look correct in testing, because Chrome users get the right answer. The detection tables here are ordered deliberately: Edge, Opera, Samsung Internet, Vivaldi and Brave are all tested before Chrome, and Chrome before Safari.

The same problem appears in operating systems. iPadOS 13 and later report "Macintosh", so iPad has to be checked before macOS or every iPad is reported as a Mac.

Why does it say "Windows 10 or 11" instead of picking one?

Because the string does not say. Windows 11 still reports Windows NT 10.0 — Microsoft did not increment it. Any tool that confidently tells you "Windows 11" from a UA string alone is guessing, usually from unrelated hints that break. Saying so is more useful than being confidently wrong.

Distinguishing them requires Client Hints (Sec-CH-UA-Platform-Version), which is a different mechanism the browser sends separately.

Can I trust this for access control or licensing?

No. A User-Agent is a string the client chooses to send, and anyone can send anything — changing it takes one line of code or a browser setting. Treat the result as a hint for support tickets and rough analytics.

If you need to know whether a capability exists, feature-detect it in JavaScript instead. That answers the real question and cannot be spoofed, because it tests the thing itself rather than a claim about it.

Why is the field prefilled?

With your own browser's User-Agent, because that is the string most people arrive wanting to see — checking what a site sees when you visit, or grabbing your UA to paste into a bug report. Replace it with any other string to parse that instead.

Is my User-Agent sent to a server?

No. Parsing is regex matching in this tab; nothing is transmitted. Your browser does of course send its real User-Agent to every site you visit, including this one, as part of ordinary HTTP — but this page does not record, store or forward it.

Why not use an off-the-shelf parsing library?

It would be around 20 KB for one tool, and the hard part is not the code. UA parsing is a table of patterns in the right order, plus knowing which compatibility claims each browser makes. The table is here in the source, commented, so when a new browser appears the fix is one line rather than waiting for a release.