Thanks so much for making this easier to understand. Helps a lot, and as a 
result, I now understand that the design has a problem:

JSON allows a single string value as a valid payload; 
e.g.  JSON.parse(`"foo"`). This seems to be a problem for the design, which 
relies on authors sending objects instead of strings as a reliable 
discriminator. How can you validate that this isn't going to create issues 
in the wild?

Best,

Alex

On Friday, September 26, 2025 at 1:41:51 PM UTC-7 Suresh Potti wrote:

> Updated the explainer. Pls review.
>
> Thanks,
>
> Suresh
>
>  
>
> *From:* Alex Russell <[email protected]> 
> *Sent:* Wednesday, September 10, 2025 8:44 PM
> *To:* blink-dev <[email protected]>
> *Cc:* Yoav Weiss <[email protected]>; [email protected] <
> [email protected]>; Suresh Potti <[email protected]>; 
> Chromestatus <[email protected]>
> *Subject:* [EXTERNAL] Re: [blink-dev] Intent to Ship: FedCM—Support 
> Structured JSON Responses from IdPs
>
>  
>
> You don't often get email from [email protected]. Learn why this 
> is important <https://aka.ms/LearnAboutSenderIdentification> 
>
> I like the change, but the linked "explainer" doesn't cover the ground we 
> expect to see. Can you please draft a separate document for this feature 
> and address questions raised in the GH thread in that doc? 
>
>  
>
> Thanks,
>
>  
>
> Alex
>
> On Tuesday, September 9, 2025 at 5:33:34 AM UTC-7 Yoav Weiss wrote:
>
> LGTM1
>
> This seems like a small yet useful addition.
>
>  
>
> On Sat, Sep 6, 2025 at 5:51 AM Chromestatus <
> [email protected]> wrote:
>
> Contact emails 
>
> [email protected] 
> Explainer 
>
>
> https://github.com/w3c-fedid/idp-registration/issues/13#issuecomment-3254858070
>  
> Specification 
>
> https://github.com/w3c-fedid/FedCM/pull/771 
> Summary 
>
> Allows Identity Providers (IdPs) to return structured JSON objects instead 
> of plain strings to Relying Parties (RPs) via the id_assertion_endpoint. 
> This change simplifies integration for developers by eliminating the need 
> to manually serialize and parse JSON strings. It enables more dynamic and 
> flexible authentication flows, allowing RPs to interpret complex responses 
> directly and support varied protocols like OAuth2, OIDC, or IndieAuth 
> without out-of-band agreements.
>
>  
> Blink component 
>
> Blink>Identity>FedCM 
> <https://issues.chromium.org/issues?q=customfield1222907:%22Blink%3EIdentity%3EFedCM%22>
>  
> Web Feature ID 
>
> fedcm <https://webstatus.dev/features/fedcm> 
> TAG review 
>
> https://github.com/w3ctag/design-reviews/issues/1147 
> TAG review status 
>
> Issues open 
> Risks 
>
>  
> Interoperability and Compatibility 
>
> None
>
>
>
> *Gecko*: No signal comments from Ben Vandersloot in 
> https://github.com/w3c-fedid/meetings/blob/main/2025/2025-07-29-FedCM-notes.md#status-of-cr-blockers,
>  
> No strong opinions 
>
> *WebKit*: No signal 
>
> *Web developers*: Positive 
>
> *Other signals*: This was requested by Identity providers. 
> Ergonomics 
>
> n/a
>
>  
> Activation 
>
> n/a
>
>  
> Security 
>
> n/a
>
>  
> 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?*
>
> n/a, FedCM not supported in WebView
>
>  
> Debuggability 
>
> Same as other FedCM features. The network view in devtools would be 
> especially helpful for debugging this feature.
>
>  
> Will this feature be supported on all six Blink platforms (Windows, Mac, 
> Linux, ChromeOS, Android, and Android WebView)? 
>
> No 
>
> FedCM in general is not supported on webview. Supported on all other blink 
> platforms.
>
>  
> Is this feature fully tested by web-platform-tests 
> <https://chromium.googlesource.com/chromium/src/+/main/docs/testing/web_platform_tests.md>
> ? 
>
> Yes 
>
>
> https://wpt.fyi/results/fedcm/fedcm-flexible-token?label=experimental&label=master
>
>  
> Flag name on about://flags 
>
> None 
> Finch feature name 
>
> FedCmNonStringToken 
> Rollout plan 
>
> Will ship enabled for all users 
> Requires code in //chrome? 
>
> False 
> Tracking bug 
>
> https://issues.chromium.org/346567168 
> Estimated milestones 
>
> Shipping on desktop
>
> 143
>
> Shipping on Android
>
> 143
>
>  
> Anticipated spec changes 
>
> *Open questions about a feature may be a source of future web compat or 
> interop issues. Please list open issues (e.g. links to known github issues 
> in the project for the feature specification) whose resolution may 
> introduce web compat/interop risk (e.g., changing to naming or structure of 
> the API in a non-backward-compatible way).*
>
> none 
> Link to entry on the Chrome Platform Status 
>
> https://chromestatus.com/feature/5153509557272576?gate=5128781719273472 
>
> This intent message was generated by Chrome Platform Status 
> <https://chromestatus.com/>. 
>
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> <https://groups.google.com/a/chromium.org/d/msgid/blink-dev/68bbafb9.050a0220.257801.01b2.GAE%40google.com?utm_medium=email&utm_source=footer>
> .
>
>

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