01What is a user agent?
The user agent is an HTTP header your browser sends with every request to introduce itself: name, version, rendering engine and operating system. Sites use it to adapt their content (mobile version, supported features), analytics to classify visitors, and servers to tell humans from robots.
Its baffling syntax — every browser claims to be “Mozilla/5.0” — is a legacy of the 1990s, when impersonating the competitor avoided being served a degraded version. The user agent is also part of your browser fingerprint: combined with your IP address and the other headers, it helps recognize you.
02Understanding the results
- Browser
- The application rendering the pages. Detected last in the string: Chrome appears in almost every user agent, Edge and Opera append themselves after it.
- Engine
- The component interpreting HTML and CSS: Blink (Chrome, Edge, Opera), Gecko (Firefox), WebKit (Safari and every iOS browser).
- OS
- Windows, macOS, Linux, Android, iOS… Note: “Windows NT 10.0” means both Windows 10 and 11, and macOS is frozen at 10.15.7 in recent browsers.
- Device
- Desktop, mobile, tablet, bot or command-line tool — inferred from the Mobile, iPad, Android, bot… markers.
- Client Hints
- Chrome's successor to the user agent: more precise Sec-CH-UA headers, sent on the site's request rather than systematically.
03From the command line
curl gets its own user agent analyzed as plain text — pass ?ua= to analyze another string. The JSON API returns the structured fields (open CORS):
$ curl its-my-ip.com/ua ua: curl/8.4.0 client: curl 8.4.0 device: cli $ curl "its-my-ip.com/api/ua?ua=Mozilla/5.0%20(Windows%20NT%2010.0)..." { "browser": "Chrome", "os": "Windows", "device": "desktop", … }
04Frequently asked questions
Why does every user agent start with Mozilla/5.0?
Historical legacy: in the 1990s, sites served advanced features to Netscape (“Mozilla”) only. Internet Explorer declared itself Mozilla-compatible to receive them, then every new browser imitated the previous one — Chrome also claims to be Safari and KHTML. The prefix became a fossil, kept so nothing breaks.
Can my user agent identify me?
Not on its own: millions of people share yours. But combined with other signals (IP, language, timezone, screen resolution, fonts, canvas), it contributes to an often unique fingerprint. That is why browsers froze the user agent: precise details (device model, minor version) are no longer sent systematically.
Why does my user agent show the wrong macOS or Windows version?
Browsers deliberately froze it to reduce tracking and avoid breaking old scripts: macOS shows “10_15_7” whatever your real version, and “Windows NT 10.0” covers both Windows 10 and 11. The exact version is only available through Client Hints, sent only if the site requests them.
Can I change my user agent?
Yes: browser developer tools (device mode), dedicated extensions, or curl's -A flag replace it freely. Useful to test a site's mobile version or check what is served to Googlebot. Beware, some sites detect the inconsistency between the declared user agent and the browser's actual behavior.
What replaces the user agent?
User-Agent Client Hints (Sec-CH-UA headers), championed by Chrome: by default the browser only sends the essentials (name, major version, platform, mobile or not), and the site must explicitly request details (full version, model, architecture). Firefox and Safari do not implement them and keep freezing their user agent instead.