reliability (Software) issues

[BUG] Cowork (Windows): all Plan9 shares fail with "Plan9 mount failed: invalid argument" after Windows update KB5124008 (26200.9445) — uninstalling the KB fixe

### 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? After the September 2026 Windows 11 cumulative security update **KB5124008 (OS build 26200.9445)** was installed and the machine rebooted, the Cowork local sandbox (Claude Desktop, Windows) can no longer mount any host drive share. The VM boots, vsock RPC works, and on the host side `HcsModifyComputeSystem` returns `hr=0x0` for every Plan9 share — but inside the guest every 9p mount fails with `invalid argument`. Every `device_bash` call then fails with: ``` sandbox-helper: no Plan9 drive shares mounted under /mnt/.virtiofs-root/shared ``` **Uninstalling KB5124008 and rebooting fully restores the sandbox.** Nothing else helped. ### Environment - Claude Desktop **1.49585.0** (MSIX `Claude_1.49585.0.0_x64__pzs8sxrjxfjjc`), CoworkVMService running, VM bundle `2a762adfc2eea13eca0d113a2e6452ff00ae3f62`, SDK 2.1.260 - Windows 11 Pro 24H2, build 10.0.26200 (→ 26200.9445 with KB5124008) - Hyper-V (all features) + Virtual Machine Platform enabled; HypervisorPlatform disabled; VBS/HVCI running - WSL2 (docker-desktop distro) present and **still working** with the KB installed (`wsl -d docker-desktop ls /mnt/host/c` is fine) - Antivirus: Bitdef