[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.

Written by the indexing model from the issue text.