Intent to Experiment: FedCM Button Mode API and Use Other Account API

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Yi Gu

Feb 26, 2024, 5:46:10 PMFeb 26
to blink-dev

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This will be added as an extension.


We plan to experiment with two new extensions for the Federated Credential Management (FedCM) API:

  • Button Mode API

    • The button mode lets websites trigger FedCM directly when a user clicks a button (like a "Sign-in with IdP" button). This means FedCM will always display a visible user interface for login, in contrast to the widget mode where no UI is shown if a user’s login status is logged out. 

    • When the FedCM API is used in "button mode" and a user isn't logged in, they'll be taken to the IdP login screen (in a pop-up window). Since this happens in response to a clear user action, the UI might even be more prominent (e.g., centered and modal) compared to the more subtle UI of widget mode.

  • Use Other Account API

    • With this API, an Identity Provider can request the browser to show a button that allows users to choose other accounts.

Blink component


Search tags


TAG review 

TAG review status



Interoperability and Compatibility

These are extensions to the FedCM API. Apple and Mozilla have both expressed a positive opinion on the initial FedCM API [1]. They have not yet shipped but Mozilla is prototyping [2]. If a user agent chooses not to implement these extensions, the sign-in flow should not be affected in that user agent because developers can fallback to the existing federated sign-in mechanisms.



Gecko: No signal

WebKit: No signal

Web developers: Positive ( These features are being developed to address existing feedback for the FedCM API.

Other signals:


Similar to the FedCM API, we deliberately leave the bulk of the work to the IdP to ensure that minimal RP change is needed. 

This feature, specifically, is one that can be currently controlled by IdP (via JS SDK for “button mode”, via server-side config for “use other account”), so we expect activation to have a similar profile as FedCM: immediately enabled to websites (without redeployment) by IdPs making use of it (by redeploying their JS SDKs).


The button mode shares most of the security properties from the widget mode. e.g. honoring CSP, CORS, using security headers, not asking users to type in the browser UI etc.

It’s worth noting that the pop-up window has the same web platform properties as what one would get with,””,”popup,noopener,noreferrer”) that loads the login_url. It is important to note that there's no communication allowed between the website and this pop-up (e.g. no postMessage, no window.opener). We have shipped LoginStatus API and Error API in FedCM that use this type of pop-up window.

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

Gather data on whether a browser mediated sign in flow on a critical user journey is well received by users and developers. We'd like to see how the proposed UI/API play out and iterate on them to ship the API in its best shape.

Ongoing technical constraints



Same as FedCM in general – console messages in devtools and general JS debugging

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


FedCM API is not available in WebView

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

Not yet.

We will continue adding tests before the experiment starts.

Flag name on chrome://flags

FedCmButtonMode, FedCmUseOtherAccount

Finch feature name



Non-finch justification


Requires code in //chrome?


Tracking bug

Launch bug

Estimated milestones

OriginTrial desktop last


OriginTrial desktop first


OriginTrial Android last


OriginTrial Android first


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.

Mike Taylor

Feb 26, 2024, 7:41:32 PMFeb 26
to Yi Gu, blink-dev

LGTM to experiment from M124 to M127 inclusive.

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Mike Taylor

Feb 26, 2024, 7:43:01 PMFeb 26
to Yi Gu, blink-dev

That said, please request approvals from the various review gates in your chromestatus entry before experimenting.

Yoav Weiss (@Shopify)

Apr 5, 2024, 9:36:24 AMApr 5
to blink-dev, Mike Taylor, Yi Gu
On Tuesday, February 27, 2024 at 1:43:01 AM UTC+1 Mike Taylor wrote:

That said, please request approvals from the various review gates in your chromestatus entry before experimenting.

On 2/26/24 7:41 PM, Mike Taylor wrote:

LGTM to experiment from M124 to M127 inclusive.

On 2/26/24 5:45 PM, Yi Gu wrote:
With my API owner hat off, this is not sufficient as an explainer, and makes it hard for me to assess the technical complexity of participating in the OT.
I think it'd be good to elaborate on the exact flows that button mode enables. (e.g. what happens when the user is not logged in to the IdP? Does the browser automatically opens a separate window to handle that log in? Is this something that the IdP should handle? If so, how?)
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Yi Gu

Apr 8, 2024, 11:07:07 AMApr 8
to Yoav Weiss (@Shopify), blink-dev, Mike Taylor, Yi Gu
Hi Yoav,

Thanks for the suggestion! We have a flowchart in that thread to try to explain what a browser would do in case a user is not logged in to the IdP but maybe it's not very clear. The answer to your questions is yes. IdP controls which login page the new window would load (by reusing the required "login_url" field from LoginStatus API). However they cannot choose another way to handle user logged-out in the middle of a FedCM button flow. In addition, we provided this deck of mocks w.r.t. how Chrome plans to implement the Button flow. We may change some of the UI affordance during OT and will publish a blog post with instruction and proposed UX soon.

Please let us know if we could make it more clear for developers.


Yoav Weiss (@Shopify)

Apr 9, 2024, 12:33:28 AMApr 9
to Yi Gu, blink-dev, Mike Taylor
That's indeed very useful to get a better understanding of the overall flow, thanks! :)
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