Windows desktop: XLSX attachments leave Send spinning without an error; same files work on web

What version of the Codex App are you using (From “About Codex” dialog)?

Release 26.1027.30824; Windows package OpenAI.Codex 26.1027.3384.0 (x64)

What subscription do you have?

Pro100

What platform is your computer?

Windows x64

What issue are you seeing?

Messages containing Excel (.xlsx) attachments cannot be sent in the Windows desktop app. The file appears in the composer, but after clicking Send, the send control keeps spinning and the message is not submitted. No error is displayed. The state persists for over a several minutes.

This reproduces with both an existing workbook and a newly generated minimal XLSX file (3,491 bytes, one worksheet, six populated cells, no formulas or macros). It happens in both an existing chat and a new chat. Text-only messages and other attachments work. The same Excel file uploads successfully in the browser version of ChatGPT.

What steps can reproduce the bug?

1. Open the Windows desktop app.
2. Open an existing chat or start a new one.
3. Attach a small .xlsx workbook.
4. Add a short text prompt, such as Test.
5. Click Send.
6. Observe that the attachment and text remain in the composer with a spinning Send control, without an error.
7. Fully quit and restart the application, then repeat. The issue persists.
8. Upload the same file in ChatGPT in a web browser. It works.

What is the expected behavior?

The Excel attachment should finish processing and the message should be submitted. If processing or uploading fails, the app should show an actionable error and restore the composer to a usable state.

Additional information

Checks already performed:

• Added a text prompt alongside the attachment.
• Fully restarted the application.
• Reproduced in a new chat.
• Reproduced with a minimal 3.5 KB workbook.
• Confirmed the test workbook opens and its XLSX archive passes an integrity check.
• Confirmed the same file works in the browser version.
• Checked the official Windows update feed on October 30, 2026: the installed package version matched the latest published package.

Possibly related reports: #58181, #58233, #58313. The root cause has not been established.