What is a User Agent string?
A User Agent (UA) string is a text string that your browser sends in every HTTP request, identifying the browser, its version, the operating system, and sometimes the device type. Websites use it to deliver browser-appropriate content and to understand their audience.
Structure of a User Agent string
Modern UA strings are verbose and contain legacy fields for compatibility. A typical Chrome UA on Windows looks like: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120.0.0.0 Safari/537.36. Despite claiming to be Mozilla, AppleWebKit, and Safari, it is Chrome — a legacy of browser feature-detection arms races in the early web.
Common use cases
- Device detection — serve mobile-optimised layouts to mobile browsers and full versions to desktop.
- Bot detection — crawlers identify themselves in their UA string (e.g., Googlebot, curl, Python-requests).
- Analytics — aggregate UA data reveals which browsers and operating systems your visitors use.
- Support debugging — capturing a user's UA tells you their exact browser version to reproduce issues.
User Agent spoofing and privacy
Browsers increasingly allow users to reduce their UA string. Firefox's "Resist Fingerprinting" mode returns a generic UA. The broader trend toward UA reduction and the newer User-Agent Client Hints API means traditional UA-based detection is becoming less reliable over time.
Frequently asked questions
What is my user agent?
It's shown above, under "Your current request" — pulled directly from the User-Agent header your browser sent with this page request, exactly as your browser reported it, no guessing involved.
Why does my user agent string start with "Mozilla" even though I'm not using Firefox?
Historical legacy — in the mid-1990s, sites checked for "Mozilla" in the User-Agent to decide whether to serve Netscape-specific markup. Every browser since (including Chrome, Safari, and Edge) kept including it so they wouldn't get served broken pages meant for older browsers. It's a compatibility fossil, not a meaningful signal about which browser you're actually using.
I need a different (or random) user agent for testing, not my own — is that what this tool does?
No — this shows the User-Agent your own browser is genuinely sending right now. If you need to send requests with a different or randomized User-Agent string (for cross-browser testing, scraping, or automation), that's a separate need — see the Browser & Crawler User Agents reference for real example strings you can use.