User-Agent Parser

Parse User-Agent strings to detect browser, OS and device type

What is it and how does it work?

A User-Agent string is a text identifier sent by browsers, bots, and HTTP clients to identify themselves to servers. It encodes the browser name and version, the rendering engine, the operating system, and sometimes device type — all packed into a single opaque string like "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/124.0 Safari/537.36". Parsing this string manually is notoriously error-prone because the format evolved chaotically over decades of browser history.

This tool decodes any User-Agent string into structured fields: browser name and version, engine, OS and version, device type (desktop, mobile, tablet, bot), and whether the client is a known crawler. It's useful for debugging analytics data, understanding server logs, testing how your site detects devices, and verifying the agent string emitted by custom HTTP clients or scrapers.

Common use cases

• Analytics debugging: a spike in "unknown browser" sessions in your analytics — paste the raw UA from server logs to identify the actual client.
• Bot detection: paste a suspicious crawler UA to verify whether it's a legitimate search engine bot (Googlebot, Bingbot) or an undisclosed scraper.
• Mobile testing: check whether your testing device sends a mobile UA or accidentally sends a desktop string (common in emulators with wrong settings).

Frequently asked questions

Why does every browser UA string contain "Mozilla/5.0"?

Historical compatibility. Early browsers competed by claiming to be "Mozilla compatible" to access sites that served richer content only to Netscape (Mozilla). The prefix stuck and every modern browser still includes it even though it's meaningless — the real identity comes from the rest of the string.

Can I rely on UA strings for device detection?

UA strings can be spoofed easily and their format is not standardised. For layout decisions, CSS media queries and the Device Memory / Network Information APIs are more reliable. UA parsing is best used for analytics and logging, not for gating features.

What is UA Reduction (Google's UA Freezing)?

Google is progressively reducing Chrome's UA string to protect privacy. The OS version, device model and minor browser version are being replaced with fixed tokens. This means UA-based device detection becomes less precise over time; use User-Agent Client Hints (the Sec-CH-UA header family) for detailed info in modern Chrome.

What does "like Gecko" mean in most UA strings?

It's another historical artefact — websites used to serve different content to Gecko-based browsers (Firefox). By claiming "like Gecko", browsers ensured they received the same richer pages. The token carries no technical meaning in modern UAs.

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