"Plan9 mount failed: invalid argument" in Claude Cowork on Windows after KB5124008. Install Microsoft's KB5129195

Error (Claude Desktop, Cowork local sandbox on Windows 11; every shell call fails):

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

The host side looks healthy (HcsModifyComputeSystem returned: hr=0x0 for every share), but inside the sandbox none of the shared folders mount.

Cause: the September 2026 Windows security update KB5124008 (OS build 26200.9445) broke folder sharing into Linux virtual machines over Plan9, which is how Cowork reaches your folders. Microsoft lists it as a known Hyper-V issue. WSL2 drives kept working, so it looks like a Cowork fault, but it is not.

Fix:

1. Install KB5129195 (out-of-band, 14 Sept 2026, OS builds 26200.9457 and 26100.9457) from Windows Update, or any later cumulative update, and restart. Microsoft's note: "Some applications that use HCS-managed virtual machines experienced issues when sharing host folder with Linux VMs using Plan9."
2. Check with winver: build 26200.9457 or 26100.9457 or later.
3. If you cannot install it: uninstalling KB5124008 (wusa /uninstall /kb:5124008) and restarting also restores Cowork, per three reporters. That removes a security update, so prefer step 1.

No Claude Desktop change is needed; reporters saw it fixed with the same app version (1.49585.0).

Versions: Windows 11 24H2 and 25H2 with KB5124008. Fixed by KB5129195. Not covered: Windows 10 22H2 reports (issues 93358 and 94868) and a Release Preview build 26300.9539 (issue 95910) still failing; KB5129195 is for Windows 11 24H2 and 25H2 only.

Sources:

• Microsoft, KB5129195: https://support.microsoft.com/en-us/servicing/os/windows-11/2026/09/kb5129195-windows-11-24h2-25h2-security-update
• https://github.com/anthropics/claude-code/issues/92984 (uninstall confirmations 9 and 10 Sept 2026; KB5129195 confirmed 17 Sept 2026)

Verified: the fix and builds from Microsoft's own update notes; the error text and before/after from reporters. Not verified: Anthropic has not commented on the issue; not run here (needs Windows 11).