[BUG] Cowork file bridge I/O error with long Korean/Unicode filenames (VirtioFS/WinFSP)

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?

The Cowork file bridge (VirtioFS/WinFSP) returns OSError [Errno 5] Input/output error when a mounted folder contains files with long Korean/Unicode filenames. Even a single file with a long Korean filename causes this error. Short filenames (e.g., test01.jpg) work perfectly.

Environment:

• OS: Windows 11 Pro 25H2 (Build 26200)
• Computer: KBY-VIVOBOOK17
• User: KoLIS
• Product: Claude Desktop - Cowork mode (Research Preview)
• Related issue: #27594

Detailed test results:

| Test Case | Filename | Result |
|-----------|----------|--------|
| Short ASCII name | test01.jpg | Works |
| Long Korean name with special chars []!, | [1월25일_일요일 무료특강](채널공개예정)코인으로...screenshot.jpg | I/O Error |
| Long Korean name, special chars removed except _()- | Same file renamed without []!, | I/O Error |
| Long Korean name, ALL special chars removed | Same file renamed with no special chars at all | I/O Error |

Conclusion: The issue is caused by long filenames containing Korean/Unicode characters, NOT by special characters. The VirtioFS/WinFSP bridge cannot handle long Unicode filenames.

Impact: This makes Cowork unusable for Korean users, as Korean filenames are typically longer in bytes due to UTF-8 encoding (3 bytes per Korean character vs 1 byte per ASCII character). This is extremely common for Korean users.

What Should Happen?

The Cowork file bridge should handle files with long Korean/Unicode filenames without I/O errors. Korean filenames commonly exceed 50+ characters (150+ bytes in UTF-8), and this should be fully supported.

Error Messages/Logs

$ ls "/sessions/.../mnt/KoLIS--새 폴더/"
ls: reading directory '/sessions/.../mnt/KoLIS--새 폴더/': Input/output error

# File that WORKS:
-rwx------ 1 user user 114120 Jan 25 11:59 test01.jpg

# File that FAILS (same image, just renamed with long Korean name):
# [1월25일_일요일 무료특강](채널공개예정)코인으로 4억날리고 개인회생까지간 30대직장인 양산형쇼츠로 월1억찍고 퇴사 연좌제걱정없는 완전자동화비결 전부공개 월억로드월억남 3-40-21 screenshot.jpg

Steps to Reproduce

1. Open Claude Desktop with Cowork mode on Windows 11 Pro 25H2
2. Create an empty folder (e.g., C:\Users\KoLIS\새 폴더)
3. Place a single image file with a short name (e.g., test01.jpg) in the folder
4. Mount the folder in Cowork → File is accessible (WORKS)
5. Remove the file, rename it to a long Korean name (e.g., [1월25일_일요일 무료특강](채널공개예정)코인으로...screenshot.jpg)
6. Place the renamed file back in the folder
7. Re-mount the folder in Cowork
8. Try ls or os.listdir() → I/O error (FAILS)

Key finding: The exact same file works with a short name but fails with a long Korean name. This isolates the issue to filename handling in the VirtioFS/WinFSP bridge.

Claude Model

None

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

Claude Desktop - Cowork mode (Research Preview)

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

PowerShell

Additional Information

This issue has also been reported to Anthropic Support (Conversation ID: 215473197974130). The support AI agent confirmed this is a known limitation and escalated to the human support team.

Related issue: #27594 (general Cowork file bridge I/O error)

This is a critical i18n (internationalization) issue that affects all non-ASCII language users, not just Korean. Japanese, Chinese, and other CJK language users would likely experience the same problem due to multi-byte UTF-8 encoding of filenames.