A User Agent (UA) string is a text identifier sent by your browser to every website you visit. It tells the server what browser, operating system, and device you're using — allowing websites to tailor content, optimize layouts, and ensure compatibility. The string typically includes the browser name, version, rendering engine, and OS details.

UA parsing uses pattern matching and regular expressions to extract meaningful information from the raw string. For example, Chrome/120.0 indicates the browser, Windows NT 10.0 reveals the OS, and Mobile or Tablet keywords identify the device type. A good parser handles hundreds of variations across browsers, bots, and legacy formats.

User Agent strings are easily spoofed — browsers, extensions, and bots can modify them at will. While they're useful for analytics and content negotiation, they should never be used for security purposes. For critical logic, rely on feature detection (e.g., navigator APIs) instead of UA sniffing. That said, UA parsing remains widely used for analytics, device stats, and debugging.

The browser is the application you use (Chrome, Firefox, Safari, Edge), while the rendering engine is the core component that turns HTML/CSS/JS into the visual page. Major engines include Blink (Chrome, Edge, Opera), WebKit (Safari), Gecko (Firefox), and legacy Trident (Internet Explorer). The engine version often matches the browser version for Blink-based browsers.

This is a historical artifact from the browser wars of the 1990s. "Mozilla" was the codename for Netscape Navigator. Over time, every browser adopted this prefix for compatibility — websites would block browsers that didn't claim to be Mozilla-compatible. Today, virtually all modern browsers include Mozilla/5.0 at the start of their UA string as a legacy convention.

You can find your current User Agent by clicking the Auto Detect button above, or by typing navigator.userAgent in your browser's Developer Tools console (F12 → Console). Many online tools also display it, but this parser runs entirely in your browser — your UA never leaves your device.

A UA string typically reveals your browser name and version, OS name and version, device type (desktop/mobile/tablet), and sometimes the CPU architecture (x64, ARM). It does not expose your IP address, location, name, or any personally identifiable information. Combined with other signals (IP, cookies, fingerprinting), it can contribute to a more detailed profile — which is why many privacy-focused browsers offer UA spoofing features.