Question 1

What does a User-Agent string actually tell you?

Accepted Answer

A User-Agent (UA) string is a text header your browser sends with every HTTP request. It identifies, in order of usefulness: the rendering engine (Blink, WebKit, Gecko, Trident), the browser family and version (Chrome 126, Safari 17, Firefox 128), the operating system (Windows NT 10.0, Mac OS X 15, Android 14, iOS 17), and the device class (iPhone, iPad, Macintosh, Linux x86_64). Bots identify themselves too — Googlebot, Bingbot, GPTBot and ClaudeBot all have recognizable UA strings. This parser walks the string with the same precedence real analytics tools use and shows you every field it extracts, plus the raw segments so you can verify the logic.

Question 2

Why does every browser claim to be Mozilla?

Accepted Answer

Historical compatibility. The UA format began with 'Mozilla/5.0' in the 1990s Netscape era, and every browser since — Chrome, Safari, Edge, even modern Firefox — keeps the prefix because some old sites snubbed anything that did not start with it. Chrome also includes 'Safari/' because it inherited the WebKit lineage, and 'like Gecko' to satisfy even older sniffers. The real signal is further in the string: 'Chrome/126' or 'Gecko/128' is what identifies the actual engine. This is why UA parsing is pattern-matching archaeology rather than simple reading — and why client-side feature detection (navigator.userAgentData or platform checks) is often more reliable than the UA string for modern apps.

Question 3

How do I identify mobile devices from a User-Agent?

Accepted Answer

Look for the device tokens: 'iPhone' with 'Mobile/15E148' means iOS Safari; 'Android' with 'Mobile' means a phone; 'Android' without 'Mobile' means a tablet (most Android tablets omit the Mobile token); 'iPad' is self-identifying. The order matters: an iPhone UA contains both 'Mobile' and 'Macintosh'-compatible tokens, so naive checks that look for 'Mobile' only get half the picture. This parser combines the device token, the Mobile marker, and the OS to classify phone vs tablet vs desktop, and it shows you which rule fired so you can audit the decision.

Question 4

How reliable is User-Agent based browser detection?

Accepted Answer

It is a heuristic, not a guarantee. Users can spoof their UA (privacy browsers, developer tools), and Chrome 110+ ships a reduced UA that hides the exact minor version. For most analytics purposes the UA is still fine — you care about the browser family and OS platform, which are accurate. For security-sensitive decisions (blocking, feature gating), never trust the UA — use feature detection and the modern navigator.userAgentData API where available. This tool is for understanding and debugging UA strings, not for building security boundaries.

User-Agent Parser

🟢📖 Reading a User-Agent Like an Analyst