[BUG] Cowork Windows Insider Beta 26220.9472: KB5129195 Plan9 fix is not applicable to this branch, device_bash still dead

### 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?

### Summary

The September 2026 Plan9 regression is fixed on every retail Windows branch by KB5129195 (2026-09-14), but it is NOT fixed on the Windows Insider **Beta** channel, and the fix cannot be installed there. Every existing report on this regression is from a retail build, so this branch appears to have been missed.

`device_bash` remains permanently dead on Insider Beta build **26220.9472**.

### Environment

| | |
|---|---|
| Channel | Windows Insider, **Beta** |
| OS build | **26220.9472** |
| Delivered by | KB5124115, 2026-09-11 |
| Architecture | x64 |
| Claude Desktop | 2.110.0 (also seen on 1.52386.6) |
| First failure | 2026-09-08, continuously since |

### Symptom

Every `device_bash` call fails immediately:

```
sandbox-helper: no Plan9 drive shares mounted under /mnt/.virtiofs-root/shared
```

The shell does not run at all. Even `echo alive` fails, so this is not only a mount failure.

`device_list_dir`, `device_stage_files` and `device_commit_files` continue to work normally against the same connected folders, which matches the other reports.

### Why this is not a duplicate

The existing issues (#92958, #92984, #92985, #93071, #93442, #93629) are all filed from retail builds: 26100.9445, 26200.9445 and 26300.9445. I could not find a single report from a 26220 or 28020 Insider build. The resolution guidance everywhere assumes a retail branch:

- Microsoft's Windows release health entry lists affected platforms as Windows 11 26H1, 25H2, 24H2, 23H2 and Windows 10 22H2, 21H2. Insider builds are not listed.
- Cowork's own changelog entry "Resolved: Cowork on Windows" (2026-09-14) says "On Windows 11 24H2 and 25H2 the fix is KB5129195. No Claude Desktop update is needed."

### The specific problem for this branch

KB5129195 produces builds 26100.9457 (24H2), 26200.9457 (25H2) and 26300.9457 (26H2). KB5129194 produces 28000.2956 (26H1).

**26220.9472 is higher than every one of those, and is on a different branch.** Windows Update does not offer KB5129195, and a manual install from the Update Catalog is not applicable. There is no supported path to the fix from this build.

The Beta channel has flighted nothing since 26220.9472 on 2026-09-11, three days before the out-of-band fix shipped. Historical 26220 flight cadence is a median of 10 days with a range of 4 to 21, so the next Beta build carrying the September servicing content has no announced date.

### Things that do not work, so nobody else wastes time on them

- Restarting Claude Desktop, and a full OS reboot
- The Claude Desktop update to 2.110.0
- `wsl --update`, and enabling the `Microsoft-Windows-Subsystem-Linux` optional feature
- Settings > Recovery > Go back, and the other restore paths
- Uninstalling KB5124008 is not relevant here; this branch received KB5124115, not KB5124008

### What would help

1. Confirm whether the Plan9 fix is expected to reach the 26220 Beta branch through a normal Insider flight, and roughly when.
2. If the Cowork-side Plan9 attach path can be hardened against the HCS change, that would cover Insider branches without waiting on a flight, and would also protect against the next servicing regression.
3. Surface the guest-side `Plan9 mount failed` error in the UI. The app currently reports the folder connected and the VM reachable while the shell is unusable, so the failure looks like a Claude problem rather than a Windows one. This cost several hours of misdirected troubleshooting on the Windows side before the real cause was found.

### Impact

Around twenty scheduled tasks on this machine run unattended against connected folders. They all still report SUCCEEDED at the task level because the trigger fires and the session starts, while the work inside silently degrades or fails. Tasks whose instructions require the shell simply stop producing output. The failures went unnoticed for eight days for exactly that reason.

Workaround in use: rewriting each task to stage files into the cloud container, do the work there, and commit results back. That works, and it is what the file tools are for, but it is slower and it cannot use tooling that only exists on the device.

### What Should Happen?

`device_bash` should run and connected folders should mount under `$HOME/mnt/`, as they did before the 2026-09-08 Windows servicing content, and as they do now on every retail branch that has taken KB5129195.

Concretely, either of these would resolve it:

1. The Plan9 fix reaches the 26220 Insider Beta branch in a normal Insider flight, so Beta users are not left on a broken build with no supported path to the fix.
2. Cowork's Plan9 attach path is hardened so it does not depend on the host HCS behavior that changed, which would fix every branch at once and protect against the next servicing regression.

Separately, when the guest cannot mount the shares, the app should say so rather than reporting the folder connected and the VM reachable while the shell is dead.

### Error Messages/Logs

```shell
Every device_bash call returns:

sandbox-helper: no Plan9 drive shares mounted under /mnt/.virtiofs-root/shared

[Note: mnt/ failed to mount and cannot be reached from this shell. Those connected folders are still reachable via device_list_dir / device_stage_files / device_commit_files, by their paths on this device.] A Windows update released September 8 prevents Claude's workspace from reaching your files. We're tracking this issue. Claude Code is unaffected.
```

### Steps to Reproduce

On a Windows machine enrolled in the Windows Insider **Beta** channel, on build 26220.9472 (KB5124115, 2026-09-11):

1. Connect any local folder to a Cowork session.
2. Confirm the folder is reachable with the file tools, for example `device_list_dir` on that folder. It returns the directory listing normally.
3. Call `device_bash` with any command at all, for example `echo alive`.
4. It fails immediately with:
```
sandbox-helper: no Plan9 drive shares mounted under /mnt/.virtiofs-root/shared
```
5. `get_device_info` reports the device online and `connectedFolders` populated, and the app shows the folder as connected, so nothing in the UI indicates a problem.

To confirm the branch-specific part:

6. Run Windows Update. KB5129195 is not offered, because 26220.9472 is a higher build than the 26100.9457 / 26200.9457 / 26300.9457 builds that KB produces, and is on a different branch.
7. Attempt a manual install of KB5129195 from the Microsoft Update Catalog. It is not applicable to this build.

100% reproducible. Persists across app restarts, a full OS reboot, and the Claude Desktop update to 2.110.0.

### Claude Model

None

### Is this a regression?

Yes, this worked in a previous version

### Last Working Version

Any Windows build before the 2026-09-08 servicing content. Claude Desktop version is not the variable here.

### Claude Code Version

Claude Desktop 2.110.0 (Cowork). Also reproduced on 1.52386.6. Not the Claude Code CLI.

### Platform

Anthropic API

### Operating System

Windows

### Terminal/Shell

Other

### Additional Information

Terminal/Shell is set to "Other" because this is Cowork's own sandbox shell (`device_bash`) inside the Hyper-V Linux guest, not a terminal emulator on the host.

Related retail-branch reports: #92958, #92984, #92985, #93071, #93442, #93629.

References:

- Microsoft, Windows 11 release health, resolved issue: "After installing the September 2026 security update KB5124008, applications that use HCS-managed virtual machines might experience issues when sharing host folder with Linux VMs using Plan9." Originating KB5124008 (26100.9445, 2026-09-08), resolved by KB5129195 on 2026-09-14. Affected applications listed as Claude Cowork and WSL. Affected platforms listed as Windows 11 26H1, 25H2, 24H2, 23H2 and Windows 10 22H2, 21H2. Insider builds are not listed.
- Cowork changelog, "Resolved: Cowork on Windows" (2026-09-14).

Happy to run any diagnostics on this build if that helps, including the coworkd guest log under C:\ProgramData\Claude\Logs\coworkd\, which other reporters have used to capture the guest-side EINVAL.

Start by reproducing device_bash on Windows Insider Beta build 26220.9472 and inspect the coworkd guest log under C:\ProgramData\Claude\Logs\coworkd\. Compare the failure with device_list_dir, device_stage_files, device_commit_files, and get_device_info. Done means the shell mounts connected folders reliably, or the UI clearly reports the guest-side mount failure.

Written by the indexing model from the issue text.