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User-Agent header

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since ⁨July 2015⁩.

The HTTP 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: See Browser detection using the user agent for reasons why serving different content to different browsers is usually a bad idea.

Header type Request header
Forbidden request header No

Syntax

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

Common format for web browsers:

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

Directives

<product>

A product identifier — its name or development codename.

<product-version>

Version number of the product.

<comment>

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

User-Agent reduction

The information exposed in the User-Agent header has historically raised privacy concerns — it can be used to identify a particular user agent, and can therefore be used for fingerprinting. To mitigate such concerns, supporting browsers provide a reduced set of information in their User-Agent header, and in related API features such as Navigator.userAgent, Navigator.appVersion, and Navigator.platform.

For example, whereas previously the User-Agent string for Chrome running on Android might have looked like this:

Mozilla/5.0 (Linux; Android 16; Pixel 9) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/143.0.12.45 Mobile Safari/537.36

After the User-Agent reduction update, it now looks like this:

Mozilla/5.0 (Linux; Android 10; K) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/143.0.0.0 Mobile Safari/537.36
  • The platform version is always a fixed value, in this case, Android 10.
  • The device model is always a fixed value, in this case, K.
  • The Chrome major version number shows correctly, but the minor version numbers are always shown as zeros — 0.0.0.

Servers that need more information can request it via User-Agent client hints. After the initial connection, the server can send an Accept-CH response header detailing the data items they want, and the client can then send the data back via Sec-CH-UA-* headers. This information can also be accessed via the User-Agent Client Hints API.

For more detailed information, including a guide to retrieving more information as required, see User-Agent reduction. You can also find examples of reduced User-Agent strings in the following sections.

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:gecko-version) Gecko/gecko-trail Firefox/firefox-version
  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. Note that platform can consist of multiple ;-separated tokens. See below for further details and examples.
  3. rv:gecko-version indicates the release version of Gecko (such as "17.0"). In recent browsers, gecko-version is the same as firefox-version.
  4. Gecko/gecko-trail indicates that the browser is based on Gecko. (On the desktop, gecko-trail is always the fixed string 20100101.)
  5. Firefox/firefox-version indicates that the browser is Firefox and provides the version (such as "17.0").

Desktop examples:

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. It adds "CriOS/<version>" on iPhone.

Desktop examples:

Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/143.0.0.0 Safari/537.36

Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/143.0.0.0 Safari/537.36

Android phone example:

Mozilla/5.0 (Linux; Android 10; K) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/143.0.0.0 Mobile 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>" on desktop and Android, and "OPT/<version>" on iPhone. For preview versions, Opera also includes a description of the particular browser edition in parentheses, for example (Edition developer).

Desktop examples:

Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/139.0.0.0 Safari/537.36 OPR/124.0.0.0 (Edition developer)

Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/139.0.0.0 Safari/537.36 OPR/124.0.0.0 (Edition developer)

Android phone example:

Mozilla/5.0 (Linux; Android 10; K) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/140.0.0.0 Mobile Safari/537.36 OPR/92.0.0.0

Microsoft Edge UA string

The Edge browser is also based on the Blink engine. It adds "Edg/<version>" on desktop platforms, "EdgA/<version>" on Android, and "EdgiOS/<version>" on iPhone.

Desktop examples:

Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/143.0.0.0 Safari/537.36 Edg/143.0.0.0

Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/143.0.0.0 Safari/537.36 Edg/143.0.0.0

Android phone example:

Mozilla/5.0 (Linux; Android 10; K) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/141.0.0.0 Mobile Safari/537.36 EdgA/141.0.0.0

Safari UA string

Safari is based on the WebKit engine, but its UA string is also similar to the Blink-based browsers. It tends to include a Version/xxx string before the actual engine build version to indicate the browser release version, which unlike Blink-based browsers is different. In the case of iPhone (Mobile) Safari, the string also includes Mobile.

Note: At the time of writing, non-Apple iPhone browsers (such as Firefox, Chrome, and Edge) are still based on WebKit, therefore their UA strings are similar to the Safari UA string.

Desktop example:

Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/26.0 Safari/605.1.15

iPhone example:

Mozilla/5.0 (iPhone; CPU iPhone OS 18_6 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/26.0 Mobile/15E148 Safari/604.1

Pre-user-agent reduction examples

This section provides some examples of UA strings in earlier browser versions, prior to the introduction of user-agent reduction:

Google Chrome:

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

Microsoft Edge:

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

Opera:

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 a structure like this:

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

Crawler and bot UA strings

Examples

Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)

Mozilla/5.0 (compatible; YandexAccessibilityBot/3.0; +http://yandex.com/bots)

Library and net tool UA strings

Examples

curl/7.64.1

PostmanRuntime/7.26.5

Specifications

Specification
HTTP Semantics
# field.user-agent

Browser compatibility

See also

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