[BUG] [Cowork] present_files cards not rendered + overlayfs shadow blocks write to mounted folder

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?

Two bugs in Cowork mode (research preview):
Bug 1 — present_files cards not rendered in UI
mcp__cowork__present_files returns file paths successfully (no error), but no download/open card is rendered in the chat UI. The user has no way to access files created inside the sandbox through the intended UI mechanism.
Bug 2 — mkdir inside /sessions/.../mnt/ creates an overlayfs shadow that silently breaks writes to the user's mounted Mac folder
When Claude runs mkdir for a directory that matches the name of the user's selected workspace folder (e.g., [workdir]), it creates a local directory in the overlayfs upper layer that shadows the real Mac mount (lower layer). All subsequent bash/python writes go to the upper layer only and never reach the Mac filesystem — with no error or warning. The user sees files "disappear" and Claude falsely believes the write succeeded.

What Should Happen?

Bug 1: present_files should render interactive file cards in the Cowork chat UI so the user can open or save files directly.
Bug 2 (two options):

Option A: Prevent mkdir from creating directories that shadow active mount points inside /sessions/.../mnt/, or surface a clear warning when this happens.
Option B: Provide a mcp__filesystem__write_binary_file tool (or extend write_file to support binary/base64 content) so Claude can write .docx, .xlsx, .pptx and other binary files directly to the user's Mac folder via MCP — bypassing the overlayfs limitation entirely.

Error Messages/Logs

No explicit error shown. Both operations appeared to succeed:

present_files returned file paths without error
bash cp / Python shutil.copy2 reported success (File 1: 15865 bytes)
mcp__filesystem__list_directory on /Users/[username]/[workdir]/[vigilant-admiring-lamport] showed empty directory
mcp__filesystem__get_file_info returned ENOENT: no such file or directory

Steps to Reproduce

Open Cowork, select a Mac folder (e.g. /Users/[username]/[workdir]) as workspace
Ask Claude to create a .docx file and save it to the workspace folder
Claude runs mkdir /sessions/.../mnt/[workdir] — this creates a shadow directory
Claude writes files into the shadow directory — no error returned
Files do not appear in /Users/[username]/[workdir]/ on the Mac
Claude calls mcp__cowork__present_files with the file paths — no UI card appears

Claude Model

Sonnet (default)

Is this a regression?

I don't know

Last Working Version

N/A

Claude Code Version

2.1.81 (Claude Code)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Claude desktop app (not CLI) — no terminal involved on user side

Additional Information

Workaround applied: wrote a Node.js self-extracting script (restore.js) with base64-encoded .docx content to the Mac via mcp__filesystem__write_file (text-only). User must run node ~/출장/vigilant-admiring-lamport/restore.js manually in Terminal.
The overlayfs shadow was confirmed by listing the /sessions/.../mnt/ directory via Python (os.listdir), which showed the upper-layer directory hiding the real mount.
mcp__filesystem__write_file is text-only; no binary write tool is available to Claude as an alternative.
Session transcript available for reference.