[BUG] Cowork Windows — remote-bash reports "no Plan9 drive shares mounted" after Sept 2026 Windows update; host registers 7 shares successfully

### 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?

Cowork's shell to the local machine fails with:

> no Plan9 drive shares mounted

File read/write through the mounted folder still works. Only script
execution on the host is blocked — the session falls back to the cloud
sandbox. Onset was immediately after a Windows update on 2026-09-08.

The host service registers all shares without error, but remote-bash
receives none.

## Environment

- Claude Desktop 1.49585.0 (MSIX, `Claude_pzs8sxrjxfjjc`)
- Windows 11, 32 GB RAM
- Recent updates: KB5124008 (9/9), KB5124007 (9/8), KB5126052 (9/8)
- Working folder: `C:\Users\cronm\Desktop\Vault\Music` (local NTFS, not
OneDrive, no spaces)

## Evidence

Host side succeeds — `C:\ProgramData\Claude\Logs\cowork-service.log`:

[VM] User token set for Plan9 identity
[VM] Plan9 share added: c -> C:\ (port=9902, readOnly=false)
[VM] Plan9 share added: d -> D:\ (port=9903, readOnly=false)
[VM] Plan9 share added: e -> E:\ (port=9904, readOnly=false)
[VM] Plan9 share added: g -> G:\ (port=9906, readOnly=false)
[VM] Plan9 share added: h -> H:\ (port=9907, readOnly=false)
[VM] Plan9 share added: i -> I:\ (port=9908, readOnly=false)
[VM] Plan9 share added: j -> J:\ (port=9909, readOnly=false)
[VM] Sent plan9Shares notification (7 shares)

No `Failed to add Plan9 share` and no `0x80070005` anywhere in the log —
this is not the access-denied failure in #43896 or #44486.

App side, `%LOCALAPPDATA%\Claude\logs\main.log`:

[remote-bash] user=rcw-... mounts=Music:rw cmdLen=60

## Reproduction matrix

Two external USB drives (J: WD Elements, G: Seagate Backup Plus) are
mapped as shares along with the internal drives. Both conditions must
hold for the shell to work:

| Windows update | J and G attached | Result |
| --- | --- | --- |
| Installed | Attached | Fails |
| Installed | Unplugged | Fails |
| Uninstalled | Attached | Fails |
| Uninstalled | Unplugged | **Works** |

Uninstalling the update and reinstalling it toggles the failure
reproducibly.

## Tried, no effect

- Reboot (multiple)
- `Restart-Service CoworkVMService`
- Full uninstall/reinstall of Claude Desktop
- Fresh Cowork sessions
- `icacls` grant to `NT VIRTUAL MACHINE\Virtual Machines` (access denied;
and logs show no permission failure to fix)

## Expected

Shares registered successfully by the service should be visible to
remote-bash. If a drive cannot be shared, that drive should be skipped
with a warning rather than the whole mount set being unavailable.

## Open question

Three updates installed across 9/8–9/9; the rollback removed the most
recent. Which specific KB triggers this is not established.

Related issues to link in the body: #44486, #43896, #28890.

### What Should Happen?

Shares registered successfully by the service should be visible to
remote-bash. If a drive cannot be shared, that drive should be skipped
with a warning rather than the whole mount set being unavailable.

### Error Messages/Logs

```shell
no Plan9 drive shares mounted

[VM] User token set for Plan9 identity
[VM] Plan9 share added: c -> C:\ (port=9902, readOnly=false)
[VM] Plan9 share added: d -> D:\ (port=9903, readOnly=false)
[VM] Plan9 share added: e -> E:\ (port=9904, readOnly=false)
[VM] Plan9 share added: g -> G:\ (port=9906, readOnly=false)
[VM] Plan9 share added: h -> H:\ (port=9907, readOnly=false)
[VM] Plan9 share added: i -> I:\ (port=9908, readOnly=false)
[VM] Plan9 share added: j -> J:\ (port=9909, readOnly=false)
[VM] Sent plan9Shares notification (7 shares)

[remote-bash] user=rcw-... mounts=Music:rw cmdLen=60

no Failed to add Plan9 share and no 0x80070005 appear anywhere in cowork-service.log.
```

### Steps to Reproduce

1. Windows 11 machine with Claude Desktop 1.49585.0 (MSIX) and two
external USB drives attached (J: WD Elements, G: Seagate Backup Plus),
alongside internal drives C, D, E, H, I.
2. Install the Sept 2026 Windows updates (KB5124008, KB5124007, KB5126052).
3. Open Cowork and add a local folder — C:\Users\cronm\Desktop\Vault\Music.
4. Ask Cowork to run any script on the host, e.g. a Python file in the
added folder.
5. Observe: file read/write works, script execution fails with
"no Plan9 drive shares mounted".
6. Check C:\ProgramData\Claude\Logs\cowork-service.log — all 7 shares
registered successfully, no errors.
7. Uninstall the Windows update AND detach both USB drives, then reboot.
Scripts now run on the host.
8. Restore either condition (reinstall the update, or reattach the
drives) and the failure returns.

### Claude Model

Opus

### Is this a regression?

Yes, this worked in a previous version

### Last Working Version

_No response_

### Claude Code Version

1.49585.0.0

### Platform

Anthropic API

### Operating System

Windows

### Terminal/Shell

PowerShell

### Additional Information

_No response_

Start with C:\ProgramData\Claude\Logs\cowork-service.log and %LOCALAPPDATA%\Claude\logs\main.log, comparing the plan9Shares notification with remote-bash mount handling. Reproduce the update and USB-drive matrix, then trace the notification path; done means registered shares reach remote-bash and an unavailable drive does not block the remaining mount set.

索引モデルが issue の本文から書いたものです。