Intent to Experiment: Topics API

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Josh Karlin

Mar 25, 2022, 5:31:45 PM3/25/22
to blink-dev, Yao Xiao, Philippe de Lurand Pierre-Paul

Contact emails,

Developers interested in the Topics API can also join the Topics API announcements group for updates and announcements.





The intent of the Topics API is to provide callers (including third-party ad-tech or advertising providers on the page that run script) with coarse-grained advertising topics that the page visitor might currently be interested in. These topics will supplement the contextual signals from the current page and can be combined to help find an appropriate advertisement for the visitor.

Blink component


Search tags

Topics, browsingTopics, FLoC

TAG request


Interoperability and Compatibility

Gecko: issue – requested March 17th 2022

WebKit: webkit-dev thread – requested March 17th 2022

Web developers: Lots of interest in issues in the explainer. Concerns center around utility. Balancing privacy/utility and making tweaks to the algorithms so that we get this right is what the OT is all about. 

Other signals:

WebView Application Risks

Does this intent deprecate or change behavior of existing APIs, such that it has potentially high risk for Android WebView-based applications?


Goals for experimentation

The intent is for experimenters to try Topics out and see how it performs for them. We expect there to be bumps along the road toward optimizing privacy and utility trade-offs. We also expect it may be necessary to change parameters and algorithms along the way (e.g., we’re already having discussions around adding Topics in request headers and weighting topics by frequency). So please experiment and provide feedback on the issue tracker to help us improve it.

Experiment Configuration

The origin trial for this experiment will be shared among various Privacy Sandbox APIs. Our goal is to allow for coordinated experiments to be run by multiple different sites, across multiple APIs in one OT.

This shared origin trial, Privacy Sandbox Ads APIs, will be a third-party origin trial. To ensure that developers can run coordinated experiments without concern for exceeding page load usage thresholds, this Origin Trial will be available for a subset of users by default. Therefore, it will be necessary to feature test to ensure that the API surface you want to use is available after providing your OT token. A second advantage of this configuration is that different experimenters will experiment with the same users, which enables coordination for APIs like FLEDGE across third parties.

Reason this experiment is being extended


Ongoing technical constraints


There are no particular debugging APIs made available or dev-tools integrations for this OT.

Will this feature be supported on all six Blink platforms (Windows, Mac, Linux, Chrome OS, Android, and Android WebView)?

All but webview.

Is this feature fully tested by web-platform-tests?

Not yet, but those tests are coming.

Flag name


Requires code in //chrome?


Launch bug

Estimated milestones

We hope to start the Origin Trial sometime during M101 beta. We plan to continue the Origin Trial until at least M104 to give developers time to test the API and provide feedback. Once we are confident that the APIs are working properly, we will transition the OT from beta to stable channel.

Link to entry on the Chrome Platform Status

Links to previous Intent discussions

Intent to prototype:

This intent message was generated by Chrome Platform Status.

Philip Jägenstedt

Mar 30, 2022, 11:50:18 AM3/30/22
to Josh Karlin, blink-dev, Yao Xiao, Philippe de Lurand Pierre-Paul
LGTM to experiment

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Philip Jägenstedt

Mar 30, 2022, 11:58:17 AM3/30/22
to Josh Karlin, blink-dev, Yao Xiao, Philippe de Lurand Pierre-Paul
I was not clear about the connection between this intent and 2 other intents that are under review now.

While my LGTM stands, please make sure that the total package of 3 make sense :)

Josh Karlin

May 2, 2022, 3:43:02 PM5/2/22
to Philip Jägenstedt, blink-dev, Yao Xiao, Philippe de Lurand Pierre-Paul
Update: we've discovered a stability issue with the Topics API and fixed the code, but the Topics API has been disabled in the Privacy Sandbox Ads APIs Origin Trial for users that aren't running a version of Chrome with the fix applied. It will take a couple of weeks for the updated canary/dev/beta releases to propagate to the majority of users.



Josh Karlin

May 23, 2022, 3:24:50 PM5/23/22
to Philip Jägenstedt, blink-dev, Yao Xiao, Philippe de Lurand Pierre-Paul
Hi all. The Topics API experiment is back up and running. 50% of canary & dev users are now in the experiment group. We'll add 50% of beta within the next couple of days. While I have your attention I'd like to also point you to our new chrome://topics-internals page (available on canary and dev for M103) that will provide handy developer information about the topics that have been selected in each epoch and it also allows you to query the site classifier to understand the mapping of sites to topics. More improvements to the internals page are in consideration as well.

Sorry about the delay,


Josh Karlin

Jun 3, 2022, 1:17:41 PM6/3/22
to Philip Jägenstedt, blink-dev, Yao Xiao, Philippe de Lurand Pierre-Paul
The Topics API experiment is ramped up to 50% of the beta population as well now. This ramp-up began on May 27th. Remember that it takes a week or more after usage of the API for a user to start returning topics.  

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