Teams interop: all meeting chat operations return 403 Forbidden for ACS users since ~Aug 24 2026 (calls work; Teams web anonymous can chat in same meeting)

リポジトリのファイルやテストは特定されていません。まず、記載されている ACS パッケージバージョン、CallWithChatComposite、およびドキュメント化された POST エンドポイントを使って失敗を再現します。ACS の動作を匿名の Teams Web 参加者の動作と比較し、その後、Azure サポートケースとリンクされたクロスポストを使って、リポジトリの変更が可能かどうかを判断します。完了の条件は、解決が確認されるか、所有者について maintainer から案内されることです。

Describe the bug; what happened?

Since approximately Aug 24, 2026, every ACS (anonymous interop) user joining Microsoft Teams meetings in our tenant receives 403 Forbidden on ALL meeting chat operations — ChatThreadClient.sendMessage and sendTypingNotification both fail with:

CommunicationError: {
  "Code": "Forbidden",
  "Message": "The initiator doesn't have the permission to perform the requested operation."
}

— and the user receives no incoming thread messages. Calling, roster, screen share, and reactions all work normally. The CallWithChatComposite shows "You are no longer in this chat thread" immediately after joining. No changes were made on our side (no deploys, no tenant/policy changes).

Key discriminator: in the SAME meeting, an anonymous participant joining via the Teams web client (display name only, unverified — no email OTP) can send meeting chat messages normally. The failure exists only on the ACS interop path.

What are the steps to reproduce the issue?

1. Create a Teams meeting (either via Graph app-created onlineMeetings or by a human organizer in the Teams desktop app with default options — both reproduce).
2. Mint an ACS identity + token (voip, chat scopes) and join the meeting with createAzureCommunicationCallWithChatAdapter + CallWithChatComposite (minimal quickstart, no custom code).
3. Wait for call state Connected (fully admitted).
4. Send a chat message.

Result: POST /chat/threads/19%3Ameeting_...%40thread.v2/messages returns 403 (on both api-version=2024-06-05-preview and 2025-03-15).

What behavior did you expect?

ACS interop users can send/receive meeting chat while in the call, per the documented Teams external-user capabilities.

If applicable, provide screenshots:

Sample failing request: Ms-Cv: IGMF4wkAI0+MIaxuFQ0kJw.0, 2026-08-31 16:04:38 UTC, resource in Canada region. Happy to share more Ms-Cv values and a live repro meeting privately.

In what environment did you see the issue?

• @azure/communication-react: 1.32.0
• @azure/communication-calling: 1.42.1
• @azure/communication-chat: 1.6.0-beta.7
• OS & Device: Windows 11 PC (also reproduced tenant-wide across devices/users in production)
• Browser: Google Chrome (latest); production users on multiple browsers

Is there any additional information?

Tenant configuration verified as Global Admin, all permissive:

• MeetingChatEnabledType = Enabled on ALL CsTeamsMeetingPolicy instances
• ACS federation configured: EnableAcsUsers = True, resource immutable ID present in AllowedAcsResources
• EnableAcsFederationAccess = True on all external access policies
• BlockedAnonymousJoinClientTypes empty
• EnableExternalAccessRestrictionsForChatParticipants / EnableMutualFederationForChatParticipants both False
• per-meeting allowMeetingChat = enabled

A test policy with AnonymousUserAuthenticationMethod = None assigned to the organizer did not help (a fresh meeting created after assignment still fails). This is NOT the known join-timing issue — the failure persists for the entire meeting while Connected.

A Severity-A Azure support case is open in parallel. Also posted on Microsoft Q&A: https://learn.microsoft.com/en-us/answers/questions/5990218/

Cross-posted with full details on the UI library repo: https://github.com/Azure/communication-ui-library/issues/6074 (filed there first; posting here as this appears to be a service-side issue in the ACS↔Teams interop chat gateway rather than an SDK/UI defect).