The User-Agent request header is a characteristic string that lets servers and network peers identify the application, operating system, vendor, and/or version of the requesting user agent.

Warning: Please read Browser detection using the user agent for why serving different Web pages or services to different browsers is usually a bad idea.

Header type Request header
Forbidden header name no


User-Agent: <product> / <product-version> <comment>

Common format for web browsers:

User-Agent: Mozilla/5.0 (<system-information>) <platform> (<platform-details>) <extensions>



A product identifier — its name or development codename.


Version number of the product.


Zero or more comments containing more details. For example, sub-product information.

Firefox UA string

For more on Firefox- and Gecko-based user agent strings, see the Firefox user agent string reference. The UA string of Firefox is broken down into 4 components:

Mozilla/5.0 (platform; rv:geckoversion) Gecko/geckotrail Firefox/firefoxversion
  1. Mozilla/5.0 is the general token that says that the browser is Mozilla-compatible. For historical reasons, almost every browser today sends it.
  2. platform describes the native platform that the browser is running on (Windows, Mac, Linux, Android, etc.) and if it is a mobile phone. Firefox OS phones say Mobile — the web is the platform. Note that platform can consist of multiple ";"-separated tokens. See below for further details and examples.
  3. rv:geckoversion indicates the release version of Gecko (such as "17.0"). In recent browsers, geckoversion is the same as firefoxversion.
  4. Gecko/geckotrail indicates that the browser is based on Gecko. (On the desktop, geckotrail is always the fixed string 20100101.)
  5. Firefox/firefoxversion indicates that the browser is Firefox and provides the version (such as "17.0").


Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:47.0) Gecko/20100101 Firefox/47.0
Mozilla/5.0 (Macintosh; Intel Mac OS X x.y; rv:42.0) Gecko/20100101 Firefox/42.0

Chrome UA string

The Chrome (or Chromium/Blink-based engines) user agent string is similar to Firefox's. For compatibility, it adds strings like KHTML, like Gecko and Safari.


Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.103 Safari/537.36

Opera UA string

The Opera browser is also based on the Blink engine, which is why it almost looks the same as the Chrome UA string, but adds "OPR/<version>".


Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.106 Safari/537.36 OPR/38.0.2220.41

Older, Presto-based Opera releases used:

Opera/9.80 (Macintosh; Intel Mac OS X; U; en) Presto/2.2.15 Version/10.00
Opera/9.60 (Windows NT 6.0; U; en) Presto/2.1.1

Microsoft Edge UA string

The Edge browser is also based on the Blink engine. It adds "Edg/<version>".


Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/91.0.4472.124 Safari/537.36 Edg/91.0.864.59

Safari UA string

In this example, the user agent string is mobile Safari's version. It contains the word "Mobile".


Mozilla/5.0 (iPhone; CPU iPhone OS 13_5_1 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/13.1.1 Mobile/15E148 Safari/604.1

Crawler and bot UA strings


Mozilla/5.0 (compatible; Googlebot/2.1; +
Mozilla/5.0 (compatible; YandexAccessibilityBot/3.0; +

Library and net tool UA strings




HTTP Semantics
# field.user-agent

Browser compatibility

BCD tables only load in the browser

See also