[BUG] Windows Cowork workspace fails to start — CreateVirtualDisk 0xC03A0014 system-wide, sessiondata.vhdx never created

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Claude Desktop's Cowork/workspace tab fails to start on Windows with:

Failed to start Claude's workspace — failed to set session disk path: session disk not found:
...\vm_bundles\claudevm.bundle\sessiondata.vhdx

The root cause is a system-level Windows virtual-disk stack failure: CreateVirtualDisk/OpenVirtualDisk return
0xC03A0014 ("A virtual disk support provider for the specified file was not found") on EVERY path and volume
(C:/F:/G:), for VHDX and VHD, dynamic and fixed, and even when opening an existing rootfs.vhdx. The session disk file
is never created.

This is a different root cause from #83703 (that one is EFS/APPX encryption on the MSIX LocalCache; our data dir is
AppData\Local\Claude-3p, not encrypted).

What Should Happen?

The workspace should start: CoworkVMService should successfully create the 10GB dynamic sessiondata.vhdx via
CreateVirtualDisk, and the VM should boot.

Error Messages/Logs

CoworkVMService log (C:\ProgramData\Claude\Logs\cowork-service.log):
  [VHDX] Creating dynamic VHDX: C:\Users\Never\AppData\Local\Claude-3p\vm_bundles\claudevm.bundle\sessiondata.vhdx
  (size: 10737418240 bytes)
  [Server] Warning: failed to create session disk: CreateVirtualDisk failed: 0xc03a0014 (err: The system cannot find the
  file specified.)

  App log (cowork_vm_node.log):
  [VM:start] Startup failed: Error: failed to set session disk path: session disk not found: C:\...\sessiondata.vhdx

  Independent P/Invoke test of CreateVirtualDisk (same params the service uses) reproduces 0xC03A0014 on all volumes.

Steps to Reproduce

1. Open Claude Desktop (MSIX, x64) → click the Cowork / workspace tab.
2. Observe: "Failed to start Claude's workspace — failed to set session disk path: session disk not found:

...\sessiondata.vhdx"

1. Optional: run a CreateVirtualDisk P/Invoke call for a 10GB dynamic VHDX on any local path — returns 0xC03A0014.

Environment detail: Windows 11 Enterprise, Insider Beta, build 28020.2623; no third-party AV; Safe Mode also
reproduces.

Claude Model

Not sure / Multiple models

Is this a regression?

Yes, this worked in a previous version

Last Working Version

N/A — never worked

Claude Code Version

N/A for this bug (Claude Desktop, not CLI). Desktop version: 1.26832.0.0

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

PowerShell

Additional Information

Related: #83703, #56542, #32088

Full diagnosis (submitted to help the team — this differs from #83703):

• System-level VHD provider failure, NOT EFS/APPX. CreateVirtualDisk/OpenVirtualDisk fail 0xC03A0014 on every volume

(C/F/G), every format (VHDX/VHD), every type (dynamic/fixed), and even opening an existing VHDX.

• vhdmp.sys driver loads (sc start works) but never creates its device object: \\.\VHD not openable, no "MsVhdHba"/VHD

Loopback Controller device node in PnP tree, zero vhdmp entries in setupapi.dev.log.

• Four root bus enumerators (UMBus, VDRVROOT, CompositeBus, NdisVirtualBus) are in Error 22 (disabled) state.
• Reproduces in Safe Mode (rules out third-party drivers: no WSL/Docker/VMware; Windows Defender real-time protection

is off).

• DISM/SFC clean, feature reinstall, full OS upgrade 26220→28020, pnputil driver reinstall, KB2013544 FsDepends fix —

none helped.

Request: an app-side workaround that doesn't require the VHD provider for session disk creation (e.g., plain file /
raw storage / relocated location), or clearer surfacing of this Windows-level failure.