Windows: Failed to start Claude's workspace - HCS Access Denied on sessiondata.vhdx

Bug Description

Claude Desktop App fails to start the sandboxed workspace (Hyper-V VM) with an "Access is denied" error (0x80070005) on the sessiondata.vhdx file.

Error Message

Failed to start Claude's workspace
HCS operation failed: failed to start VM
HRESULT 0x80070005: Access is denied.

The root cause from the HCS event log:

Account does not have permission to open attachment C:\Users\<user>\AppData\Local\Packages\Claude_pzs8sxrjxfjjc\LocalCache\Roaming\Claude\vm_bundles\claudevm.bundle\sessiondata.vhdx (Lun 2). Error: 'Access is denied.' (0x80070005).

Environment

• OS: Windows 11 Pro 10.0.26200
• Claude App: Windows Store version (package: Claude_pzs8sxrjxfjjc)
• Hyper-V: Enabled
• VM name: cowork-vm-26c6cf54

Steps to Reproduce

1. Open Claude Desktop App on Windows 11
2. Start a conversation that triggers the sandboxed workspace (computer use / cowork)
3. VM fails to start with the access denied error

Expected Behavior

The workspace VM should start successfully.

Actual Behavior

HCS fails to power on the Synthetic Storage device because the user account lacks permission to open sessiondata.vhdx.

Additional Context

• Restarting Claude or the computer does not resolve the issue.
• The "Reinstall the workspace" option is offered in the UI but hasn't been tested yet.
• This may be related to Windows file/NTFS permissions on the VHDX file, or Hyper-V service account permissions after a Windows update.

Possible Workarounds

• Manually granting read/write permissions on the vm_bundles directory to the Hyper-V service accounts
• Reinstalling the workspace via the Claude app UI