What Is a User Agent String?
A User Agent string is a line of text that your web browser sends to every website you visit as part of the HTTP request headers. It identifies your browser type and version, operating system, device type, and sometimes additional information like rendering engine and platform details.
The User Agent string is sent in the User-Agent HTTP header with every request your browser makes. Websites use this information to serve appropriate content, apply compatibility workarounds, and gather analytics about their visitors.
Example User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120.0.0.0 Safari/537.36 — this tells the website you are using Chrome on Windows 10 (64-bit).
Why Do User Agent Strings Look So Complex?
Modern User Agent strings are notoriously complex and often misleading. This is due to decades of browser compatibility history:
- In the early web, Netscape Navigator used
Mozilla/as its User Agent prefix. When Internet Explorer arrived, websites would check for "Mozilla" and serve better content. Microsoft had IE identify itself as "Mozilla-compatible." - When new browsers like Safari, Chrome, and Edge appeared, each had to pretend to be previous browsers to receive the correct content from websites that checked User Agent strings.
- This historical baggage means almost every modern browser claims to be Mozilla, Chrome, and Safari simultaneously, regardless of its actual identity.
What Information Does Your User Agent Reveal?
- Browser name and version — Chrome, Firefox, Safari, Edge, Opera, etc.
- Rendering engine — Blink, Gecko, WebKit, etc.
- Operating system and version — Windows, macOS, Linux, Android, iOS.
- Device architecture — 32-bit or 64-bit, ARM or x86.
- Mobile or desktop — Mobile browsers typically include "Mobile" in the string.
User Agent and Privacy
Your User Agent string is part of your browser fingerprint — a set of characteristics that can be used to identify and track you across websites, even without cookies. The more unique your User Agent string, the easier it is to track you.
- Client Hints — Modern browsers are moving toward User-Agent Client Hints, which provide structured, privacy-preserving information instead of a single complex string.
- User Agent reduction — Chrome and other browsers are gradually freezing and reducing the information in the User Agent string to improve privacy.
- Spoofing — You can change your User Agent string using browser extensions or developer tools, though this may break some websites.
Frequently Asked Questions
Can websites see my real browser if I change my User Agent?
Changing your User Agent string only modifies the HTTP header. Sophisticated websites can still detect your real browser using JavaScript feature detection, rendering behavior, and browser fingerprinting techniques that go beyond the User Agent string.
Why does my User Agent say "Chrome" when I use Edge?
Microsoft Edge is built on the Chromium engine (the same engine Chrome uses). Its User Agent includes Chrome's identifier for compatibility. This is common across Chromium-based browsers like Brave, Vivaldi, and Opera.
Does my User Agent string change?
Your User Agent string changes when you update your browser (the version number changes), switch browsers, or update your operating system. It remains the same between regular browsing sessions.