Default User-Agent (UA) String Changed

The user agent (UA) string has changed in Windows Internet Explorer 9 in several ways.

Description of changes

Shortened User Agent String

By default, Internet Explorer 9 sends a new short UA string to improve the overall performance, interoperability, and compatibility. Internet Explorer 9 does not send additions to the UA string that are made by other software that is installed on the computer, such as the Microsoft .NET Framework and many other programs.

There are four primary changes to UA string from Internet Explorer 8:

  • The application version is incremented from "Mozilla/4.0" to "Mozilla/5.0", to match other browsers and make Internet Explorer 9 an interoperable browser.
  • The version token is incremented from "MSIE 8.0" to "MSIE 9.0".
  • The Trident token is incremented from "Trident/4.0" to "Trident/5.0".
  • Internet Explorer 9 sends the following short UA string without additions from other software that is installed on the computer.
Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0)

Because previous long, extended UA strings cause compatibility issues, Internet Explorer 9 sends the short UA string that is shown earlier without Pre-Platform and Post-platform registry value tokens.

Applications and platforms can continue to add to the UA string through the Pre-Platform and Post-Platform registry keys as they did in previous Windows Internet Explorer releases. Internet Explorer 9 does not change existing registry values.

Your websites can still get the extended UA string with the Pre-Platform and Post-Platform tokens through the navigator.userAgent property.

UA String in Compatibility View

Similar to Windows Internet Explorer 8, the Internet Explorer 9 Compatibility View maps to IE7 Standards mode. The UA string for Internet Explorer 9 in Compatibility View takes the following form.

Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0; Trident/5.0)

In Compatibility View, Internet Explorer 9 reports itself as Windows Internet Explorer 7 through the application version number (for example, "Mozilla/4.0") and version token (for example, "MSIE 7.0") for compatibility. The incremented Trident token (from "Trident/4.0" to "Trident/5.0") enables websites to distinguish between when Internet Explorer 9 is running in Compatibility View and when Internet Explorer 8 is running in Compatibility View.

Internet Explorer 9 sends the short UA string by default.

Test how your website responds to the Internet Explorer 9 UA string by checking and changing the UA string through the registry. If your site does not already respond with Internet Explorer-compatible content, update it to recognize Internet Explorer 9 and be future-proof.

Internet Explorer 8 - User-Agent String

The Internet Explorer 8 User-Agent String (Updated Edition)

Understanding User Agent Strings

navigator.userAgent Property



Build date: 9/16/2013