[BUG] Cowork Windows: sandbox-helper fails to unmount host share, VM service won't start (Win 11 Pro)

Bug Description

Cowork on Windows fails to start with two distinct errors that occur in sequence:

1. Initial error — CLI output is not valid JSON due to sandbox-helper debug output contaminating stdout:

CLI output was not valid JSON. This may indicate an error during startup.
Output: sandbox-helper: failed to unmount host share (virtiofs at /mnt/.virtiofs-root: invalid argument, plan9 at /mnt/.virtiofs-root/shared: invalid argument)

1. After restarting Claude Desktop — The workspace fails with:

VM service not running. The service failed to start.

Steps to Reproduce

1. Open Claude Desktop on Windows 11 Pro
2. Try to use Cowork
3. First attempt shows the sandbox-helper: failed to unmount host share error
4. Restarting Claude Desktop changes the error to "VM service not running. The service failed to start."
5. Cycle repeats indefinitely

Troubleshooting Already Attempted

• Deleted vm_bundles/claudevm.bundle (rootfs.vhdx, sessiondata.vhdx, smol-bin.vhdx) to force a fresh re-download — same errors after rebuild
• Restarted Hyper-V services (vmms, HvHost) — both confirmed running, error persists
• Terminated all claude, cowork-svc, vmwp processes and restarted — no improvement
• Confirmed Hyper-V is fully enabled and functional (vmms: Running/Automatic, HvHost: Running/Manual)

Environment

| Detail | Value |
|--------|-------|
| OS | Microsoft Windows 11 Pro 10.0.26200 (Build 26200) |
| Architecture | AMD64 |
| Claude Desktop version | 1.1.2998.0 (MSIX/AppX package) |
| Claude Code CLI version | 2.1.39 |
| Hyper-V vmms service | Running (Automatic) |
| Hyper-V HvHost service | Running (Manual) |
| Installation type | Microsoft Store / AppX (Claude_pzs8sxrjxfjjc) |

Expected Behavior

Cowork should start the VM workspace successfully without sandbox-helper unmount errors.

Actual Behavior

The sandbox-helper fails to unmount host shares using both virtiofs and plan9 protocols with "invalid argument", which corrupts the JSON output stream and prevents the workspace from initializing. After restart, the VM service fails to start entirely.

Possibly Related Issues

• #25206 — VM starts but crashes within 5 minutes, service unrecoverable (Win 11 Pro)
• #24974 — sessiondata.vhdx not created on Windows
• #24945 — VM boots but API unreachable on Windows
• #12056 — Sandbox debug output contaminating JSON stream