Introducing IE9’s User Agent String

This post introduces IE9’s User-Agent (UA) String and it builds on previous blog posts and documentation.

An important change for site developers to know is that IE9 will send the short UA string by default. This change improves overall performance, interoperability and compatibility. IE9 will no longer send additions to the UA string made by other software installed on the machine such as .NET and many others.

Some folks will notice that the IE9 Platform Preview sends IE8’s UA string. We will include the new IE9 UA string in an upcoming Platform Preview update. The reason we’re writing about IE9’s UA string now is to give site developers an early preview of these important changes and time to verify that any current UA logic continues to work with the new IE9 UA string.

IE9’s default UA string

There are four changes to IE8’s UA string that site developers need to be aware of:

  1. Application version is incremented from ‘Mozilla/4.0’ to ‘Mozilla/5.0’ to match other browsers (explained well in the great History of the user-agent string post). This change signals that IE9 is an interoperable browser.
  2. Version token is incremented from ‘MSIE 8.0’ to ‘MSIE 9.0’.
  3. Trident token is incremented from ‘Trident/4.0’ to ‘Trident/5.0’.
  4. IE9 will send the following short UA string without additions made by other software installed on the machine:

breakdown of the different parts of the UA string

IE9 will send the short UA string by default

We’ve received many reports  on compatibility issues due to long, extended UA strings. IE9 will send the short UA string detailed above without pre and post platform registry value tokens. This is interoperable with other browsers, and improves compatibility and network performance.

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 IE releases. IE9 will not change existing registry values.

Websites will continue to be able to get the extended UA string with pre and post platform tokens through the navigator.userAgent property.

IE9’s Compatibility View UA string Similar to IE8, IE9’s Compatibility View will map to IE7 Standards Mode, and IE9’s UA string when in Compatibility View will be:

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

In Compatibility View, IE9 reports itself as IE7 through the application version number (Mozilla/4.0) and version token (MSIE 7.0) . This is done for compatibility. An incremented Trident token, from ‘Trident/4.0’ to ‘Trident/5.0’, allows websites to distinguish between IE9 running in Compat View and IE8 running in Compat View.

Request for site developers

Test how your site responds to the new IE9 UA string (check and change the UA string through the registry). If your site doesn’t already respond with IE-compatible content, we’d love to see it updated now to recognize IE9 and be future-proof.

Marc Silbey
Program Manager