Windows app: every completed turn fails with "This turn couldn't render"

Summary

In the Codex Windows app, every completed turn ends with "This turn couldn't render".

The request itself appears to work: the UI shows Thinking, then briefly streams a few tokens, but when the turn completes the rendered message is replaced by the error card.

This affects old threads, new threads, project chats, and a completely new chat with no project context.

Environment

• Codex Windows app version: 26.814.5167.0
• Package: OpenAI.Codex_26.814.5167.0_x64__2p2nqsd0c76g0
• Codex CLI on the same machine works correctly.

Reproduction

1. Open Codex Windows app.
2. Start a completely new chat with no project/repository context.
3. Send a trivial prompt such as: Antworte ausschließlich mit dem Wort Hallo.
4. Observe Thinking and some streamed output.
5. When the turn completes, the message changes to "This turn couldn't render".

The same happens for existing conversations: thread/project metadata is visible in the sidebar, but every individual turn fails to render.

Troubleshooting already tried

• Fully restarted the app.
• Tested multiple existing chats and projects.
• Tested a brand-new chat without project context.
• Renamed the app's LocalCache folder under the MSIX package data directory so Codex had to rebuild it from scratch.
• Confirmed that Codex created a fresh LocalCache and fresh desktop logs after restart.
• Problem remains unchanged after the cache reset.

Expected behavior

Completed turns should remain visible after streaming finishes.

Actual behavior

The turn is replaced by:

This turn couldn't render

with a Try again button.

Additional observation

Because streaming starts successfully and the CLI works on the same machine, this looks more like a Windows desktop rendering/client regression than a general API/auth/network failure.

Logs are available from the freshly recreated Codex desktop log directory if useful for diagnosis.

Reproduce the failure in the Codex Windows app using a new chat and a trivial prompt, then inspect the freshly recreated desktop logs in the app's log directory. Compare the successful streaming phase with the completion point to identify why the rendered turn is replaced by the error card. Done means completed turns remain visible and existing, new, and project chats render normally.