[BUG] Cowork Windows - Downloads folder fails to mount: "sandbox-helper: failed to unmount host share (virtiofs and Plan9): invalid argument"

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?

Cowork on Windows fails to mount the user's Downloads folder (C:\Users\<username>\Downloads) as a host share. When selecting the Downloads folder and attempting to run any task, the VM fails with a sandbox-helper error.

Critically, other folders at the same directory level work perfectly. Creating a subfolder inside Downloads and selecting that subfolder also works. The issue is isolated to the Downloads folder root itself.

This strongly suggests the issue is specific to Windows "Known Folders" / Shell folders — the Downloads directory has special OS-level attributes (e.g., FOLDERID_Downloads, shell namespace integration) that may interfere with the VM's host share mounting mechanism.

Key Observations

| Test Case | Result |
|-----------|--------|
| Select C:\Users\<username>\Downloads directly | ❌ Fails with virtiofs/Plan9 mount error |
| Select a subfolder inside Downloads (e.g., Downloads\整理) | ✅ Works perfectly |
| Select any other regular folder (e.g., Desktop, custom folders) | ✅ Works perfectly |
| Create a new folder at C:\Users\<username>\Downloads整理 and move files there | ✅ Works perfectly |

What Should Happen?

Cowork should be able to mount and access the Windows Downloads folder (C:\Users\<username>\Downloads) the same way it handles other user directories. The Downloads folder is a standard user directory and should not be blocked by the VM's host share mounting mechanism.

Error Messages/Logs

Error 1 (initial attempt - folder scan failure):
Attempted to access Downloads folder, encountered I/O errors.
Cowork reported I/O errors when trying to list/scan files in the Downloads folder.
The mounted directory showed I/O errors, and all file operations (listing, counting, size check) failed.

Error 2 (after restarting app and reconnecting):
failed to write length: An operation was attempted on something that is not a socket.

Error 3 (after starting new chat session):
このタスクは正しく読み込めませんでした
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

Steps to Reproduce

1. Install Claude Desktop (latest Windows version, downloaded Feb 11, 2026)
2. Open Cowork tab
3. Select C:\Users\<username>\Downloads as the working folder (confirmed path via Properties → 場所 tab; NOT redirected to OneDrive)
4. Attempt to run any task (e.g., "Organize this folder by file type")
5. Result: Task fails with sandbox-helper: failed to unmount host share error
6. Contrast: Create any regular folder (e.g., C:\Users\<username>\test) → select it in Cowork → works perfectly

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.39 (Claude Code)

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Other

Additional Information

Troubleshooting Already Attempted

• Fully quit and restarted Claude Desktop
• Ran Claude Desktop as Administrator (管理者として実行)
• Verified Downloads path is standard C:\Users\<username>\Downloads (not OneDrive-redirected)
• Confirmed Windows Defender Controlled Folder Access (制御されたフォルダーアクセス) is OFF
• Confirmed other folders mount and work correctly
• Confirmed subfolders within Downloads work correctly
• Started new chat sessions — error persists

Suspected Root Cause

Windows treats the Downloads folder as a "Known Folder" with special shell attributes (KNOWNFOLDERID, junction points, indexing). The sandbox-helper attempting to mount via virtiofs or Plan9 may fail because the folder has special NTFS attributes or reparse points, or because Windows Search Indexer holds locks on the folder.

Workaround (for other users)

Move all files from Downloads to a regular folder (e.g., C:\Users\<username>\Downloads整理), then select that folder in Cowork. This works reliably.

Environment Details

• Product: Cowork on Claude Desktop (not Claude Code CLI)
• OS: Windows 11 (※ fill in your version from 設定 → システム → バージョン情報)
• Claude Desktop Version: (※ fill in from app settings)