User-Agent Reduction

Updates

May 6, 2022: "Phase 4" began rolling out to a small percentage of the Chrome 101 stable population while we evaluate stability.

April 11, 2022: Add a table to map from UA to UA-CH

March 3, 2022: Added frozen Chrome OS platform version to <unifiedPlatform> value

February 18, 2022: Added information on the platforms for which User-Agent reduction is applicable, as well as links to resources.

October 12, 2021: Information on using the Origin Trial with third-party embeds was add to the blog post.

September 16, 2021: Chrome milestones were added to reflect https://blog.chromium.org/2021/09/user-agent-reduction-origin-trial-and-dates.html.

May 24, 2021: The chrome://flags#freeze-user-agent flag was renamed to chrome://flags/#reduce-user-agent in Chrome 93 and the values were updated to align with the plan below (also testable via --enable-features=ReduceUserAgent).

Proposed Rollout Plan

Reduction Preparation

Phase 1: Warn about accessing navigator.userAgent, navigator.appVersion, and navigator.platform in DevTools, beginning in M92.

Phase 2: Chrome 95 to Chrome 100 Launch an Origin Trial for sites to opt into the final reduced UA string for testing and feedback, for at least 6 months.

Reduction Rollout

Phase 3: Chrome 100 Launch a reverse Origin Trial, for instances where a site may need more time for migration, for at least 6 months.

Phase 4: Chrome 101 Ship reduced Chrome MINOR.BUILD.PATCH version numbers (“0.0.0”). Once rolled-out, the reduced UA string would apply to all page loads on desktop and mobile OSes that do not opt into the reverse Origin Trial.

Phase 5: Chrome 107 Begin roll-out of reduced Desktop UA string and related JS APIs (navigator.userAgent, navigator.appVersion, navigator.platform). Once rolled-out, the reduced UA string would apply to all page loads on desktop OSes that do not opt into the reverse Origin Trial.

Phase 6: Chrome 110 Begin roll-out of reduced Android Mobile (and Tablet) UA string and related JS APIs. Once rolled-out, the reduced UA string would apply to all page loads on Android that do not opt into the reverse Origin Trial.

Reduction Completion Phase 7: Chrome 113 reverse Origin Trial ends and all page loads receive the reduced UA string and related JS APIs.

Reduced User Agent String Reference

This reduced format will be available for testing via chrome://flags/#reduce-user-agent in Chrome 93.

Applicable platforms

User-Agent reduction will be applied to the following platforms: Windows, macOS, Linux, Chrome OS, and Chrome on Android. We don't have current plans for User-Agent Reduction on iOS and Android WebView at this time.

Unified Format

The unified format that covers all platforms post-UA Reduction looks like so:

Mozilla/5.0 (<unifiedPlatform>) AppleWebKit/537.36 (KHTML, like Gecko)
Chrome/<majorVersion>.0.0.0 <deviceCompat> Safari/537.36

Desktop

The Chrome Desktop User Agent string currently uses the following format:

Mozilla/5.0 (<platform>; <oscpu>) AppleWebKit/537.36 (KHTML, like
Gecko) Chrome/<majorVersion>.<minorVersion>; Safari/537.36

Post UA-Reduction, the new format will be:

Mozilla/5.0 (<unifiedPlatform>) AppleWebKit/537.36 (KHTML, like Gecko)
Chrome/<majorVersion>.0.0.0 Safari/537.36

Mobile and Tablet

The Chrome Mobile and Tablet User Agent strings use the following format:

Mozilla/5.0 (Linux; Android <androidVersion>; <deviceModel>)
AppleWebKit/537.36 (KHTML, like Gecko)
Chrome/<majorVersion>.<minorVersion> <deviceCompat>
Safari/537.36

Post UA-Reduction, the new format will be:

Mozilla/5.0 (Linux; Android 10; K) AppleWebKit/537.36 (KHTML, like Gecko)
Chrome/<majorVersion>.0.0.0 <deviceCompat> Safari/537.36

Token Reference

UA Token to UA-CH Mapping

Sample UA Strings: Phase 4

In Phase 4 we change the <minorVersion> token to “0.0.0”.

Sample UA Strings: Phase 5

In Phase 5 we change the <platform> and <oscpu> tokens from their platform-defined values to the relevant <unifiedPlatform> token value (which will never change).

Note: There may not be user-visible changes here, unless the user was on a lower version.

Also note that the macOS platform version was already capped to 10_15_7 in Chrome 90 for site compatibility reasons.

Sample UA Strings: Phase 6

In Phase 6, we change the <deviceModel> token to “K” and change the <androidVersion> token to a static “10” string.

Sample UA Strings: Final Reduced State

Reduced navigator.platform values (for all versions)

Reduced navigator.appVersion values

navigator.appVersion is effectively an alias of navigator.userAgent (it’s everything after “Mozilla/”).

Reduced navigator.userAgent values

To avoid confusion and reduce implementation complexity, we aim to follow the same plan for navigator.userAgent.

Alternative: high entropy client hints

All of the information that was contained in the User-Agent string prior to reduction is available through the high entropy client hints, which are available by request through the User-Agent Client Hints request headers, as well as the navigator.userAgentData.getHighEntropyValues Javascript API.

Resources

The following sites show snippets and allow developers to preview what the reduced User-Agent string will look like on different platforms:

• User-Agent Reduction Snippets
• User-Agent Reduction Interactive Demo