User Agent Parser

About the User Agent Parser

A User Agent (UA) string is a short piece of text that a browser, HTTP client, or bot sends with every request to identify itself to the server. The server can use this information to serve different content, log traffic by browser or device type, or block unwanted bots. UA strings follow a loose convention dating back to the Netscape era and have become famously complex as browsers adopted each other's tokens for compatibility reasons.

Anatomy of a User Agent string

A typical Chrome UA on Android looks like: Mozilla/5.0 (Linux; Android 8.0; Pixel 2 Build/OPD3.170816.012) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120 Mobile Safari/537.36

• Mozilla/5.0 — a historical compatibility token; nearly every browser sends this
• (Linux; Android 8.0; Pixel 2 Build/...) — platform block: OS, version, and device model
• AppleWebKit/537.36 — the rendering engine; Chrome-based browsers use Blink, a WebKit fork
• Chrome/120 — the actual browser name and version
• Mobile Safari/537.36 — a Safari compatibility token included by mobile Chrome

Why UA parsing is unreliable

UA strings are self-reported and not verified. Browsers can spoof any UA they like, and many do for compatibility. Chrome on iOS sends a Safari-like UA because iOS requires all browsers to use the WebKit engine. Bots frequently send fake UAs to avoid detection. For critical feature detection in a web app, use JavaScript feature detection (e.g. typeof fetch !== 'undefined') rather than UA sniffing.

Common use cases

• Debugging unexpected browser behaviour from a log entry
• Identifying which device or browser generated an error report
• Checking whether a log entry is from a real user or a crawler
• Verifying what UA your own HTTP client or scraper is sending

Privacy

Parsing runs entirely in your browser — no UA strings are sent to any server. Learn more about how Dev-Utilities handles privacy.