[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:
- 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)- After restarting Claude Desktop — The workspace fails with:
VM service not running. The service failed to start.Steps to Reproduce
- Open Claude Desktop on Windows 11 Pro
- Try to use Cowork
- First attempt shows the
sandbox-helper: failed to unmount host shareerror - Restarting Claude Desktop changes the error to "VM service not running. The service failed to start."
- 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,vmwpprocesses 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
12 Comments
Found 3 possible duplicate issues:
This issue will be automatically closed as a duplicate in 3 days.
🤖 Generated with Claude Code
Confirming this issue on Windows 10 with v1.1.2998.0.
Additional related errors I'm experiencing:
sdk-daemon not connected- occurs after 1-5 successful Cowork requestsfailed to write length: An operation was attempted on something that is not a socketFailure pattern:
Regression:
v1.1.2685.0 worked reliably in temporary workspace mode (despite greyed-out folder UI). v1.1.2998.0 is significantly worse - unstable even without folder selection.
Impact:
Consuming significant usage limits on failed attempts. Cowork essentially unusable for any real work.
+1 for urgent fix or rollback to previous version.
Already reported, nice to see the updates coming out for windows daily :-)
I can see this was also on the Mac version in the beginning.
Hope to get it resolved no matter what I do I cant use Cowork any more.
<img width="1441" height="262" alt="Image" src="https://github.com/user-attachments/assets/78a7deed-f096-4a62-8bce-5c94ac7fcf19" />
Exact the same here
I was able to get around this by going to File -> Exit then restarting Claude Windows app. Then it worked for me.
yes, exiting it from the taskbar tray menu and restarting resolves it.
I'm experiencing both this issue and an additional VPN-related problem:
Issue 1 - Cowork hang/crash (this issue):
Can't use File → Exit, must kill via Task Manager
Restart works but is temperamental (inconsistent success rate)
Still on latest version with daily updates
The simple exit/restart workaround mentioned by @jbfranks and @afnan007 doesn't work reliably for me - the app won't exit gracefully when Cowork is in this failed state.
Issue 2 - VPN triggers "no connection" error (separate but related):
When VPN is active, Cowork fails with a connection error due to routing table conflicts. I've developed a PowerShell repair script that fixes this by:
Repairing routing table conflicts caused by VPN interaction
Restarting the CoworkVMService
More reliable than manual restart in my testing
Repair tool available here: https://github.com/AlbertSteptoe/Cowork-repair-tool
The temperamental restart behavior suggests there may be a race condition or timing issue during VM service recovery.
+1 exactly the same problem here with Win11 Pro.
going to File -> Exit resolved the issue for me.
on Windows 11 Pro, 64Gb memory, AMD processor
The albertsteptoe repository it's down so I wasn't able to see the "Repair Tool"
Closing for now — inactive for too long. Please open a new issue if this is still relevant.
This issue has been automatically locked since it was closed and has not had any activity for 7 days. If you're experiencing a similar issue, please file a new issue and reference this one if it's relevant.