Google's revised plans for the User-Agent


2/24/2023 10:30 AM

Google User Agent Client Hints Industry News Web

User-Agent Client Hint (UA-CH) updates from the fourth CMA report

On 31 January 2023, Google and the UK Competition and Markets Authority (CMA) released their fourth report on Privacy Sandbox. The CMA is overseeing the Privacy Sandbox proposals from Google to ensure they are developed in a way that benefits consumers.

We've continually updated our customers on the changes to the User-Agent and User-Agent Client Hints. We recommend preparing your business now for the changes by getting started with 51Degrees Version 4.4.

Learn more about Version 4.4

Page 29: phased rollout

The report featured additional information about Phase 6 of the User-Agent reduction. Phase 6 is significant as it includes reduced device model information within mobile and tablet User-Agents.

Mobile (user on Samsung Galaxy, for example)
Previous UA Mozilla/5.0 (Linux; Android 9; SM-A205U) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/ Mobile Safari/537.36
Phase 6 UA Mozilla/5.0 (Linux; Android 10; K) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/ Mobile Safari/537.36

51Degrees has previously used the Chrome Platform Status roadmap and the Chromium Projects to prepare for the different phases of the User-Agent reduction.

However, the fourth report released on 31 January includes a revised deployment schedule to gradually increase the percentage of Chrome web browsers that will implement the User-Agent reduction. The key dates are now:

  • 14 February: 1% of Chrome users will be able to update to Chrome 110.
  • 14 March: roll-out increases to 10% of users.
  • 28 March: roll-out increases to 50% of users.
  • 11 April: roll-out completes as 100% of users can update to Chrome 110.

Checks are implied before each stage commences to determine if there has been a detrimental impact. Therefore, it's possible the timeframes will extend whilst data is gathered and analyzed.

We welcome the gradual deployment approach. However, the timeline is too short and the tests incomplete for the impact on the vast complexity of the web eco-system to be assessed.

No web view

Google has confirmed independently of the report that web view generated HTTP header User-Agent value's will not be altered as part of the User-Agent reduction. This is welcome news for developers of Android Apps that incorporate the web view.

The report by omission also confirms that web view related information is not available in UA-CH and there is no alternative for web view use cases. This will be a concern for the CMA who require equivalence in UA-CH compared to the User-Agent which is found to be absent. Google will need to fix this before the next report. We would expect the CMA to delay roll out beyond 1% until after design and data equivalence has been achieved.

51Degrees provides information on the web view via the IsWebApp property.

Page 30: latency. A monumental misunderstanding

For those of us interested in the most efficient and fastest web performance (which is so important for search engine ranking and user experience), the report makes for uncomfortable reading.

The following diagram shows the requests and responses between a web browser and server where T0 is the commencement of an initial request for a web page, and T1 is the initial processing of that request. High performance users of 51Degrees initiate device detection at T1 for purposes such as advertising or response optimization.


Google defines additional latency as the time delay introduced in calling the getHighEntropyValues() method in JavaScript to obtain the information now removed from the User-Agent. This is a delay associated with just some of the processing between T2 and T3 in the previous diagram. As expected for any computer operation executed on a single device and written by competent engineers, it completes quickly taking 0.5 milliseconds (according to Google).

The rest of the eco-system defines additional latency as the time associated with the delay in the server receiving the User-Agent information. This is shown in the additional request that needs to be made between T2 and TC in the previous diagram.

We have previously discussed the latency issues within the User-Agent reduction which included a testing website to illustrate the latency impact you can expect from User-Agent Client Hints. We observe this delay to range between 40-100 milliseconds, but it may be much higher depending on network and other conditions.


The report lays bare a monumental misunderstanding. Google has chosen a best-case, limited use case scenario for their headline measurements. The CMA is aware of this now.


Google's view is that the User-Agent reduction “will not have a material impact on third-party websites” however any potential impact “will be addressed and mitigated thanks to the safeguards in place”. These safeguards include testing.

The tests defined do not include independent analysis and CMA approval of the results related to the most significant impact of UA-CH, the impact on competition. Neither Google nor the CMA has the evidence today to determine the impact on SEO or advertising revenue among other competition impacting metrics.

If Google are true to their word concerning “safeguards” we'd expect a more cautious roll out schedule to emerge.

Beyond the User-Agent

While the report is uncharacteristically detailed on the User-Agent due to the proximity of the change, there are plenty of updates relevant to 51Degrees users. Highlights include:

  • CMA subtly steering Associated Sets (a replacement for third-party cookie data sharing) to include an unlimited number of domains and removing other restrictions.
  • Enhanced testing and communication associated with further Privacy Sandbox changes.

What should I do?

Phase 6 has commenced following the timeline mentioned earlier.

You can still make your feedback on the User-Agent reduction and User-Agent Client Hints known to Google. We urge you to make your voice heard before the end of the 1% phase of User-Agent reduction:

We also recommend you read Movement for an Open Web's article on Google's Q4 report for additional details. 51Degrees, as part of Movement for an Open Web, has supported efforts to allow competition to thrive through an open web.

We will continue to keep you updated on changes to the User-Agent reduction and User-Agent Client Hints. If you have any questions, or wish to prepare your business now for the changes, you can contact us or get started with Version 4.4.

Get started with Version 4.4