[BUG] Cowork Windows - sandbox-helper fails to mount host share (virtiofs/Plan9) with OneDrive-synced folders
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?
<html>
<body>
<!--StartFragment--><html><head></head><body><h1>[BUG] Cowork Windows - sandbox-helper fails to mount host share (virtiofs/Plan9) with OneDrive-synced folders</h1>
<h2>Preflight Checklist</h2>
<ul>
<li>[x] I have searched <a href="https://github.com/anthropics/claude-code/issues?q=is%3Aissue+state%3Aopen+label%3Abug">existing issues</a> and this hasn't been reported yet</li>
<li>[x] This is a single bug report (please file separate reports for different bugs)</li>
<li>[x] I am using the latest version of Claude Code</li>
</ul>
<h2>What's Wrong?</h2>
<p>Cowork fails at VM startup with a sandbox mount error. The workspace never reaches API connection; it crashes during the host-share mounting phase. My working directories are stored on OneDrive, which may be the root cause since OneDrive uses the Windows Cloud Files API (placeholder/dehydrated files) rather than standard NTFS file access.</p>
<p><strong>Error output:</strong></p>
<pre><code>CLI output was not valid JSON. This may indicate an error during startup.
Output: sandbox-helper: failed to unmount host share (tried both virtiofs and Plan9): invalid argument
</code></pre>
<p>The sandbox-helper attempts both virtiofs and Plan 9 (9p) filesystem protocols to mount the shared folder from the Windows host into the Linux VM. Both fail with "invalid argument," suggesting the underlying filesystem (OneDrive's cloud files layer) is incompatible with the host-share mechanism.</p>
<p>Regular Chat mode works without issues. Only Cowork is affected.</p>
<h2>What Should Happen?</h2>
<p>Cowork should either:</p>
<ol>
<li>Successfully mount OneDrive-synced folders via virtiofs or Plan 9, or</li>
<li>Detect OneDrive paths and provide a clear error message guiding the user to select a local (non-OneDrive) working directory</li>
</ol>
<h2>Steps to Reproduce</h2>
<ol>
<li>Have a Windows 11 Home system with OneDrive enabled and documents stored in OneDrive-synced folders</li>
<li>Open Claude Desktop (latest version, Feb 2026)</li>
<li>Switch to Cowork tab</li>
<li>Start any task</li>
<li>Observe: sandbox-helper mount failure before the workspace can start</li>
</ol>
<h2>Suspected Root Cause</h2>
<p>OneDrive's Cloud Files API presents files as placeholder/dehydrated stubs on disk. The virtiofs and Plan 9 host-share protocols likely expect standard NTFS file access and cannot traverse OneDrive's filter driver. The mount call returns EINVAL ("invalid argument") because the filesystem at the shared path isn't a conventional local filesystem.</p>
<p>This may also affect other cloud storage providers that use similar virtual filesystem drivers (e.g., Dropbox Smart Sync, Google Drive streaming).</p>
<h2>Troubleshooting Attempted</h2>
<ul>
<li>Verified Windows Features: Virtual Machine Platform ✅, Windows Hypervisor Platform ✅ (Hyper-V not available on Home edition)</li>
<li>Restarted Claude Desktop multiple times</li>
<li>No VPN active</li>
<li>Windows Defender Firewall: Claude allowed on both Private and Public networks</li>
<li>Have not yet tested with a non-OneDrive local folder (will update if this resolves the issue)</li>
</ul>
<h2>Environment</h2>
Component | Detail
-- | --
OS | Windows 11 Home, Version 25H2, Build 26200.7840
Processor | 13th Gen Intel Core i7-13700HX (2.10 GHz)
RAM | 32 GB
System type | 64-bit, x64-based processor
Claude Desktop | Latest version (downloaded Feb 2026)
OneDrive | Active, documents stored in OneDrive-synced folders
Windows Feature Experience Pack | 1000.26100.291.0
<h2>Claude Model</h2>
<p>Opus</p>
<h2>Is this a regression?</h2>
<p>No, this never worked</p>
<h2>Claude Code Version</h2>
<p>Claude Desktop latest version (Windows, downloaded Feb 2026). This is Cowork on Claude Desktop, not Claude Code CLI.</p>
<h2>Platform</h2>
<p>Anthropic API</p>
<h2>Operating System</h2>
<p>Windows</p>
<h2>Additional Context</h2>
<ul>
<li>Windows 11 Home does not include Hyper-V; only Virtual Machine Platform and Windows Hypervisor Platform are available. It is unclear whether Cowork's VM layer requires full Hyper-V.</li>
<li>Possibly related to #24918, #24945, #24962 (other Windows Cowork VM failures), though those report API connectivity issues rather than this specific mount failure.</li>
<li>The "tried both virtiofs and Plan9" message suggests the sandbox-helper has a fallback path, but both protocols fail against the OneDrive filesystem driver.</li>
<li>A potential fix or workaround might involve copying files to a local staging directory before mounting into the VM, rather than sharing the OneDrive path directly.</li>
</ul></body></html><!--EndFragment-->
</body>
</html># [BUG] Cowork Windows - sandbox-helper fails to mount host share (virtiofs/Plan9) with OneDrive-synced folders
Preflight Checklist
- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue+state%3Aopen+label%3Abug) 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?
Cowork fails at VM startup with a sandbox mount error. The workspace never reaches API connection; it crashes during the host-share mounting phase. My working directories are stored on OneDrive, which may be the root cause since OneDrive uses the Windows Cloud Files API (placeholder/dehydrated files) rather than standard NTFS file access.
Error output:
CLI output was not valid JSON. This may indicate an error during startup.
Output: sandbox-helper: failed to unmount host share (tried both virtiofs and Plan9): invalid argumentThe sandbox-helper attempts both virtiofs and Plan 9 (9p) filesystem protocols to mount the shared folder from the Windows host into the Linux VM. Both fail with "invalid argument," suggesting the underlying filesystem (OneDrive's cloud files layer) is incompatible with the host-share mechanism.
Regular Chat mode works without issues. Only Cowork is affected.
What Should Happen?
Cowork should either:
- Successfully mount OneDrive-synced folders via virtiofs or Plan 9, or
- Detect OneDrive paths and provide a clear error message guiding the user to select a local (non-OneDrive) working directory
Steps to Reproduce
- Have a Windows 11 Home system with OneDrive enabled and documents stored in OneDrive-synced folders
- Open Claude Desktop (latest version, Feb 2026)
- Switch to Cowork tab
- Start any task
- Observe: sandbox-helper mount failure before the workspace can start
Suspected Root Cause
OneDrive's Cloud Files API presents files as placeholder/dehydrated stubs on disk. The virtiofs and Plan 9 host-share protocols likely expect standard NTFS file access and cannot traverse OneDrive's filter driver. The mount call returns EINVAL ("invalid argument") because the filesystem at the shared path isn't a conventional local filesystem.
This may also affect other cloud storage providers that use similar virtual filesystem drivers (e.g., Dropbox Smart Sync, Google Drive streaming).
Troubleshooting Attempted
- Verified Windows Features: Virtual Machine Platform ✅, Windows Hypervisor Platform ✅ (Hyper-V not available on Home edition)
- Restarted Claude Desktop multiple times
- No VPN active
- Windows Defender Firewall: Claude allowed on both Private and Public networks
- Have not yet tested with a non-OneDrive local folder (will update if this resolves the issue)
Environment
| Component | Detail |
|---|---|
| OS | Windows 11 Home, Version 25H2, Build 26200.7840 |
| Processor | 13th Gen Intel Core i7-13700HX (2.10 GHz) |
| RAM | 32 GB |
| System type | 64-bit, x64-based processor |
| Claude Desktop | Latest version (downloaded Feb 2026) |
| OneDrive | Active, documents stored in OneDrive-synced folders |
| Windows Feature Experience Pack | 1000.26100.291.0 |
Claude Model
Opus
Is this a regression?
No, this never worked
Claude Code Version
Claude Desktop latest version (Windows, downloaded Feb 2026). This is Cowork on Claude Desktop, not Claude Code CLI.
Platform
Anthropic API
Operating System
Windows
Additional Context
- Windows 11 Home does not include Hyper-V; only Virtual Machine Platform and Windows Hypervisor Platform are available. It is unclear whether Cowork's VM layer requires full Hyper-V.
- Possibly related to #24918, #24945, #24962 (other Windows Cowork VM failures), though those report API connectivity issues rather than this specific mount failure.
- The "tried both virtiofs and Plan9" message suggests the sandbox-helper has a fallback path, but both protocols fail against the OneDrive filesystem driver.
- A potential fix or workaround might involve copying files to a local staging directory before mounting into the VM, rather than sharing the OneDrive path directly.
What Should Happen?
Cowork should access the directory and subdirectories
Error Messages/Logs
CLI output was not valid JSON. This may indicate an error during startup. Output: sandbox-helper: failed to unmount host share (tried both virtiofs and Plan9): invalid argumentSteps to Reproduce
- Start Claude Desktop in Windows Home OS
- Select directory or folder (on OneDrive)
- Enter Prompt and click the go arrow
Claude Model
Opus
Is this a regression?
I don't know
Last Working Version
_No response_
Claude Code Version
Cowork
Platform
Anthropic API
Operating System
Windows
Terminal/Shell
Windows Terminal
Additional Information
_No response_
7 Comments
Found 3 possible duplicate issues:
This issue will be automatically closed as a duplicate in 3 days.
🤖 Generated with Claude Code
Please make sure this is a duplicate. This is a complete show stopper for the windows' use of cowork.
So I am not saying this is a good solution from a security perspective, but it got me up and running and fixed this issue! Upgrade the permission to allow the "Everyone" user group edit, modify, etc the folder in question.
<img width="1972" height="1406" alt="Image" src="https://github.com/user-attachments/assets/7240e8e7-a833-4f73-a695-37b3b35bb09e" />
Hmm thank you for the idea - I tried this and it did not work. Still getting
CLI output was not valid JSON. This may indicate an error during startup. Output: sandbox-helper: failed to unmount host share (tried both virtiofs and Plan9): invalid argument
The update to Claude 1.1.2998 (1f1d4d) 2026-02-12T20:50:41.000Z seems to have fixed the issue!
not fixed! regressed to this today only, been fine all week.
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.