FedCM—Support Structured JSON Responses from IdPs

Category: Miscellaneous
Type: New or changed feature
Status: Proposed (Chrome Proposed)
Intent stage: None

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.

Motivation

see https://github.com/w3c-fedid/idp-registration/issues/13 for more discussion on the motivation

Standards & signals

Specification: https://github.com/w3c-fedid/FedCM/pull/771
Firefox: 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
Safari: No signal
Web developers: Positive
Tracking bug: https://issues.chromium.org/346567168

Explainers: https://github.com/w3c-fedid/FedCM/blob/main/explorations/structured_data_support.md

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