ChatGPT for Excel attachments consistently fail with streaming error after desktop update (Windows 26.818.3698.0)

Summary

After the latest Codex / ChatGPT desktop update on Windows, ChatGPT for Excel can no longer submit conversations containing file or image attachments. Each attachment submission ends with a visible streaming error.

The problem started immediately after the Codex update and persists across network changes, including a mobile hotspot.

Environment

• Platform: Windows x64
• Codex / ChatGPT desktop package: 26.818.3698.0
• Bundled/local Codex CLI observed: codex-cli 0.149.0-alpha.4
• Surface: Microsoft Excel with the official ChatGPT for Excel add-in
• Excel add-in connection itself is healthy: a workbook session can connect and expose its supported workbook tools.

Steps to reproduce

1. Open Microsoft Excel.
2. Open and sign in to the ChatGPT for Excel add-in.
3. Start a conversation in the add-in.
4. Attach a document or image.
5. Submit the conversation.
6. The request fails with a streaming error.

Expected behavior

The attachment should upload and the response should stream normally.

Actual behavior

Submitting a conversation with a file or image attachment consistently produces a streaming error. Changing to a mobile hotspot does not change the result.

Troubleshooting already attempted

• Restarted Excel and the Codex / ChatGPT desktop app
• Signed out and signed back in
• Reinstalled / reset the relevant apps and add-in
• Cleared local cache
• Tested with only the current Codex / ChatGPT desktop app running
• Tested on a different network using a mobile hotspot

None of these steps changed the behavior.

Relevant sanitized desktop log evidence

The desktop log enables a remotely supplied feature:

Features enabled enabledFeatures="..., apps_mcp_path_override, thread_tools, ..."

The bundled runtime then reports:

codex_features: unknown feature key in config: thread_tools
app_server_connection.closed code=1073807364
Codex CLI process exited classifiedAsExpected=false
fatal_error_broadcasted
Socket error EPIPE

After a clean restart, the new log again shows thread_tools in the enabled desktop feature list.

The user's local config.toml does not contain thread_tools. In addition:

• codex features list does not list thread_tools.
• A read-only CLI override test with --disable thread_tools returns Unknown feature flag: thread_tools.

Working hypothesis

This is an inference from the logs, not yet a proven causal chain: a remotely enabled desktop feature may be incompatible with the bundled 0.149.0-alpha.4 runtime, destabilizing the app-server/streaming path used when Excel submits attachments.

Please check:

1. Feature-flag compatibility for desktop build 26.818.3698.0 and bundled CLI 0.149.0-alpha.4.
2. The ChatGPT for Excel attachment upload / streaming path on Windows.
3. Whether the affected account rollout can be disabled or corrected server-side.

Potentially related feature-mismatch report: #26850.

Privacy note

This report intentionally omits account IDs, email addresses, local usernames, home paths, conversation contents, document names, authentication data, and full raw logs. Sanitized logs can be provided if the maintainers request them.