GLOSSARY · USER-AGENT STRING
User agent
A user agent is the line of text a browser or app sends with every web request to say what it is: its name and version, the rendering engine, and the operating system and device type. It travels in the User-Agent header, and begins with the historic word Mozilla for compatibility reasons.
Why it matters
Websites use it to serve the right page for your device, and to guess whether a visitor is a person, a search engine or a script. It is also one of the pieces of information that makes a browser recognisable. Because anyone can set it to any value, it is not proof of anything, and a mismatch between it and other signals can look suspicious.
In practice
Modern browsers are reducing how much detail the string contains and offering structured client hints instead. If you use a proxy for testing, remember that a proxy does not change your user agent, so you may need to set one. The parser tool splits a string into browser, engine, system, device and bot flags, and the headers tool shows what your browser sends.
Related terms
- Browser fingerprint: A browser fingerprint is a profile built by combining many small details a browser reveals: screen size, installed fonts, language, time zone, graphics and audio behaviour, supported features and more.
- HTTP proxy: An HTTP proxy is a proxy that understands web requests.
- Proxy: A proxy server is an intermediary that makes requests on your behalf.
- IP address: An IP address is a number that identifies a device or network connection on the internet, in the way a street address identifies a building.