[BUG] Cowork (macOS): connected folder still mounted read-only in sandbox despite #76344 being closed as completed

### Preflight Checklist

- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%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?

### What's Wrong?

The issue described in #76344 (connected folders no longer writable-mounted into the workspace sandbox) persists as of September 9, 2026. That issue was closed as completed on July 17, but the behavior is unchanged on my machine.

**Key observations from my session:**

1. The FUSE mount reports `rw` (read-write), but the filesystem owner is `user_id=0` (root) while the sandbox process runs as `uid=1017`. The folder permissions are `drwx------` (owner-only), so the sandbox user cannot write despite the mount allowing it.

2. Both `Write` (file tool) and `bash` (shell) fail with `Permission denied` when targeting the connected folder.

3. The `outputs` directory **is** writable — the permission issue is specific to connected folders.

4. **Root-level folders like Desktop and Downloads work fine** as connected folders (writing succeeds). The problem appears when connecting subfolders (e.g., `~/Desktop/Claude Masterclass Materials/`).

5. The behavior is **intermittent** — the same folder was writable earlier today in a different session, without any app version change.

**Mount output confirming the issue:**

```
/proc/self/fd/3 on /sessions/.../mnt/Claude Masterclass Materials type fuse (rw,nosuid,nodev,relatime,user_id=0,group_id=0,default_permissions,allow_other)
```

The mount says `rw` but `user_id=0` + `default_permissions` + `drwx------` on the directory means only root can write. The sandbox user (uid=1017) is denied.

### Environment

- **Product:** Claude Cowork (Desktop app, macOS)
- **OS:** macOS Darwin 25.3.0, Apple Silicon
- **Date:** September 9, 2026
- **Related issue:** #76344 (closed as completed Jul 17, but behavior persists)

### Is this a regression?

Yes — this is the same regression described in #76344, which was closed but not fully resolved.

### What Should Happen?

Connected folders should be writable in the sandbox, as they were before the July 8 regression and as root-level folders (Desktop, Downloads) still are. The FUSE mount permissions should match the sandbox user's uid, or the directory permissions should allow the sandbox user to write.

### Error Messages/Logs

```shell

```

### Steps to Reproduce

1. Open Cowork on macOS
2. Connect a subfolder (not Desktop/Downloads directly, but a folder inside them — e.g., `~/Desktop/My Project/`)
3. Start a new session
4. Attempt to create a file in the connected folder via `Write` tool or `bash`
5. Observe `Permission denied`
6. Verify with `mount` — the mount says `rw` but `user_id=0` with `default_permissions` blocks the sandbox user

### Claude Model

None

### Is this a regression?

Yes, this worked in a previous version

### Last Working Version

_No response_

### Claude Code Version

2.1.260.

### Platform

Anthropic API

### Operating System

macOS

### Terminal/Shell

Terminal.app (macOS)

### Additional Information

_No response_

Reproduce the failure by connecting a subfolder on macOS, then compare writes to that folder with root-level Desktop or Downloads and the writable outputs directory. Inspect the FUSE mount details, especially user_id=0, default_permissions, and directory ownership, across successful and failing sessions. Done means connected subfolders are writable by the sandbox user through both Write and bash.

索引モデルが issue の本文から書いたものです。