Your analytics say 12% of visitors use Safari on a Mac, but the support inbox is full of iPad screenshots. Or a "browser" has requested your pricing page 40,000 times since lunch. Either way, the user agent string is where you start digging, and it's a stranger thing than it looks.
What a user agent is
A user agent string is a line of text that a client, usually a browser, sends in the User-Agent header of every HTTP request. It names the application, its version and the operating system it runs on. Servers use it for analytics, for serving different content to different clients, and for spotting bots.
In theory that's simple. In practice, 30 years of browsers pretending to be other browsers have made these strings long, inconsistent and full of misleading tokens. That's why it's worth knowing how they're built before you write a parser.
Structure of a user agent string
A current Chrome user agent on Windows looks like this:
Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120.0.0.0 Safari/537.36Broken into its parts:
| Token | Meaning |
|---|---|
Mozilla/5.0 | Historical compatibility token sent by almost every browser |
(Windows NT 10.0; Win64; x64) | Platform: 64-bit Windows. Windows 11 also reports NT 10.0, so the string can't tell 10 and 11 apart. |
AppleWebKit/537.36 | Engine token, kept for compatibility (Chrome uses Blink) |
(KHTML, like Gecko) | More compatibility claims |
Chrome/120.0.0.0 | Browser and major version. Chrome now reports only the major version and zeros. |
Safari/537.36 | Safari compatibility token |
Why so much noise? In the early web, sites often served their best pages only to browsers they recognised. Each new browser copied the tokens of the popular ones so it wouldn't get the lesser version. The result is that Chrome introduces itself as Mozilla, WebKit, Gecko and Safari before admitting it's Chrome.
Detecting the browser
The order of checks matters, because many browsers include the tokens of the browser they are built on:
Chrome: Mozilla/5.0 ... Chrome/120.0.0.0 Safari/537.36
Firefox: Mozilla/5.0 ... Gecko/20100101 Firefox/121.0
Safari: Mozilla/5.0 (Macintosh; ...) AppleWebKit/605.1.15 ... Version/17.2 Safari/605.1.15
Edge: Mozilla/5.0 ... Chrome/120.0.0.0 Safari/537.36 Edg/120.0.0.0
Opera: Mozilla/5.0 ... Chrome/120.0.0.0 Safari/537.36 OPR/106.0.0.0Edge and Opera are Chromium-based and include Chrome/, so check for Edg/ and OPR/ first and fall back to Chrome. Every Chromium browser also includes Safari/, so only treat a string as Safari when it has no Chrome/. Safari’s own version is in the Version/ token. Some browsers, such as Brave, send exactly the same string as Chrome and can't be told apart from the user agent alone.
Detecting bots and crawlers
Search engines and other crawlers send their own user agents:
Googlebot: Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)
Bingbot: Mozilla/5.0 (compatible; bingbot/2.0; +http://www.bing.com/bingbot.htm)
GPTBot: Mozilla/5.0 AppleWebKit/537.36 (KHTML, like Gecko); compatible; GPTBot/1.0; +https://openai.com/gptbot
Script: python-requests/2.31.0Spotting bots keeps your analytics honest and helps with rate limiting and security. Well-behaved bots name themselves and link to documentation. Anyone can copy those strings, though, so verify important crawlers another way: Google and Bing support a reverse DNS lookup on the requesting IP. Malicious bots usually send an ordinary browser user agent, so the string alone won't catch them.
Mobile, tablet or desktop
Mobile browsers usually include the word Mobile and a platform such as iPhone or Android. Android tablets typically leave out Mobile.
iPhone Safari: Mozilla/5.0 (iPhone; CPU iPhone OS 17_2 like Mac OS X) AppleWebKit/605.1.15 ... Mobile/15E148 Safari/604.1
Android Chrome: Mozilla/5.0 (Linux; Android 10; K) AppleWebKit/537.36 ... Chrome/120.0.0.0 Mobile Safari/537.36Two caveats. Since iPadOS 13, Safari on iPad sends a desktop Mac user agent by default, so most iPads look like Macs. And Chrome on Android now reports a fixed Android 10; K instead of the real version and device model.
Because user agents carry less detail than they used to, Chromium browsers offer User-Agent Client Hints (Sec-CH-UA headers and navigator.userAgentData) as a structured alternative. For layout, prefer CSS media queries and feature detection over user agent sniffing.
How to use the user agent parser
- Open the user agent parser and paste a string from your logs, or select Use my user agent to fill in your current browser’s.
- Select Parse. The result shows the browser and version, the operating system and version, the device type and the rendering engine.
- Select Copy to copy the result as JSON.
A few details about what the parser reports:
- Browsers detected: Microsoft Edge, Opera, Chrome, Safari and Firefox. Anything else shows as Unknown.
- Operating systems detected: Windows (with the version name, such as 7, 8.1 or 10/11), macOS, iOS, Android and Linux.
- Engines: Blink, WebKit or Gecko.
- Device type is Mobile, Tablet, Desktop or Bot. A string is marked as a bot when it contains
bot,crawler,spiderorcrawling, so scripts such aspython-requestsshow as Desktop.
It's useful for checking analytics discrepancies, testing bot detection rules and making sense of server logs. Parsing happens in your browser.