CET regression in Codex Desktop 26.901.41600 blocks all local command execution on Windows

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

26.901.41600

What subscription do you have?

plus

What platform is your computer?

Microsoft Windows NT 10.0.19042.0 x64

What issue are you seeing?

Codex Desktop can no longer execute any local command because its bundled PowerShell runtime terminates with a CET-related fatal error before the requested command starts.

The same Codex installation on the same Windows computer was working normally approximately five hours earlier. It could read local files and execute project commands. Later, without any intentional changes to the Codex configuration, shell configuration, workspace, or Windows security settings, every local command began failing.

This is not specific to a particular repository or command. Even simple read-only operations fail, so Codex cannot inspect local files, read workspace instructions, run Git commands, or execute tests.

Full error message:

Fatal error.
Your Windows doesn't fully support CET.
Please install all available Windows updates.

The failure occurs before the requested command is executed.

Windows PowerShell launched independently outside Codex continues to work. For example, it can execute Get-Location successfully. This suggests that the failure is associated with the PowerShell/.NET runtime bundled with Codex Desktop rather than with the workspace command itself.

All workspace names, paths, and project details have been redacted because they are not required to reproduce the runtime failure.

What steps can reproduce the bug?

Start ChatGPT Desktop on the affected Windows computer.

Switch to Codex mode.

Open any local folder. The folder does not need to contain a Git repository.

Ask Codex to execute a simple read-only command:

Get-Location

Observe that the command does not execute.

Codex reports:

Fatal error.
Your Windows doesn't fully support CET.
Please install all available Windows updates.

Ask Codex to read an ordinary local text file or execute another basic command.

Observe the same failure before the requested operation begins.

Restart ChatGPT Desktop and reopen the local folder.

Observe that local command execution remains blocked.

The failure is reproducible across local tasks and is not tied to a specific repository.

What is the expected behavior?

Codex should execute local commands using a runtime compatible with the installed Windows version.

For the reproduction command, Codex should return the current working directory.

If the bundled PowerShell runtime is incompatible with the current Windows build, Codex should detect the incompatibility and either:

fall back to Windows PowerShell 5.1, cmd.exe, or another compatible shell;

honor a user-selected compatible shell;

or display a clear compatibility warning before starting the local task.

A failure in the bundled runtime should not block every local Codex operation.

Additional information

This appears to be a regression because Codex Desktop worked normally on the same computer approximately five hours before the failure began.

I did not knowingly change:

the local workspace;

the Codex configuration;

the selected shell;

Windows security or exploit-protection settings;

or the project environment.

It is possible that the desktop application or its bundled runtime was automatically updated or refreshed between the working and failing sessions, but I cannot confirm this.

Windows PowerShell outside Codex remains functional. The error only appears when Codex attempts to launch its local command runtime.

Possibly related existing issue:

#39843 — Bundled PowerShell 7.6.4 (.NET 10.0.10) crashes with CET fail-fast on Windows 11 21H2

Start by locating the Windows local-command launch path and how Codex selects its bundled PowerShell runtime; compare the behavior with related issue #39843. Reproduce the failure with Get-Location on the reported Windows build. Done means local commands work with a compatible runtime, or the incompatibility is detected with a clear fallback or warning.