[Windows] OpenAI.Codex fails to launch with ERROR_UNTRUSTED_MOUNT_POINT when installed on a secondary AppX volume

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

26.911.7940.0

What subscription do you have?

ChatGPT Plus

What platform is your computer?

Microsoft Windows NT 10.0.26200.0 | x64

What issue are you seeing?

Summary

OpenAI.Codex fails to launch when installed on a valid secondary AppX/MSIX volume.

The same package version launches successfully when installed on the system AppX volume.

The Windows error is:

The path cannot be traversed because it contains an untrusted mount point.

This appears to be related specifically to the package being installed on a secondary AppX volume.

Environment

• Windows 11 x64
• OpenAI.Codex version: 26.911.7940.0
• Installation method: Microsoft Store / MSIX
• System AppX volume: C:\Program Files\WindowsApps
• Secondary AppX volume: P:\WindowsApps
• Secondary volume:
  • NTFS
  • Fixed disk
  • Healthy
  • Not BitLocker encrypted
  • Registered successfully in Get-AppxVolume

What steps can reproduce the bug?

Reproduction steps

1. Register/use a secondary NTFS volume as the default AppX volume.
2. Set the secondary AppX volume as default:

$PVolume = Get-AppxVolume | Where-Object {$_.PackageStorePath -eq "P:\WindowsApps"}
Set-AppxDefaultVolume -Volume $PVolume
3. Install OpenAI.Codex from the Microsoft Store.
4. Launch OpenAI.Codex.
5. Windows shows:
The path cannot be traversed because it contains an untrusted mount point.
Inspect the installed package:
$pkg = Get-AppxPackage OpenAI.Codex

Get-Item $pkg.InstallLocation -Force |
Format-List FullName,Attributes,LinkType,Target

Result:
FullName   : C:\Program Files\WindowsApps\OpenAI.Codex_26.911.7940.0_x64__2p2nqsd0c76g0
Attributes : Directory, ReparsePoint
LinkType   : Junction
Target     : {P:\WindowsApps\OpenAI.Codex_26.911.7940.0_x64__2p2nqsd0c76g0}
The package is therefore physically located on the secondary AppX volume and exposed through a junction from the system AppX path.

Control test
The package was then uninstalled.
The system AppX volume was set as default:
$CVolume = Get-AppxVolume | Where-Object {$_.IsSystemVolume -eq $true}
Set-AppxDefaultVolume -Volume $CVolume
OpenAI.Codex was reinstalled from the same installer / Store source.
Package inspection then showed:
FullName   : C:\Program Files\WindowsApps\OpenAI.Codex_26.911.7940.0_x64__2p2nqsd0c76g0
Attributes : Directory
LinkType   :
Target     : {}
In this configuration, OpenAI.Codex launches successfully.
Additional checks
The secondary AppX volume appears healthy:
FileSystem        : NTFS
DriveType         : Fixed
HealthStatus      : Healthy
OperationalStatus : OK
P:\WindowsApps:
- is a normal directory
- is not itself a junction or symbolic link
- is owned by NT SERVICE\TrustedInstaller
- has normal-looking AppX permissions
- is registered in Get-AppxVolume
- has been used successfully for other Microsoft Store / AppX applications
BitLocker is disabled on the secondary volume.
The problem appears specific to OpenAI.Codex when installed on the secondary AppX volume.
Expected behavior
OpenAI.Codex should launch normally when installed on a valid secondary AppX volume.
Actual behavior
OpenAI.Codex fails to launch with:
ERROR_UNTRUSTED_MOUNT_POINT
when the package resides on the secondary AppX volume.
Workaround
Install OpenAI.Codex on the system AppX volume (C:).
Notes
The same OpenAI.Codex version was tested in both configurations.
The only meaningful difference was the AppX volume on which the package was physically installed.

### What is the expected behavior?

OpenAI.Codex should launch normally when installed on a valid secondary AppX volume.

### Additional information

The same OpenAI.Codex version was tested in both configurations.
The only meaningful difference was the AppX volume on which the package was physically installed.

Start by reproducing the launch failure with the documented PowerShell AppX-volume setup, then compare the secondary-volume and system-volume installations. No source files or entry points are named, so trace the Windows packaging and launch path until the secondary-volume behavior is isolated; done means OpenAI.Codex launches successfully from a valid secondary AppX volume without ERROR_UNTRUSTED_MOUNT_POINT.