Description: After the September 2026 Windows 11 update, Claude Cowork's local shell may report “no Plan9 drive shares mounted.” Check the affected builds and the official KB5129195 or KB5129194 fix without risking your files.
If Claude Cowork stopped accessing connected folders after the September 8, 2026 Windows update, check for the documented Plan9 folder-sharing regression. Microsoft fixed it on September 14: KB5129195 for Windows 11 24H2/25H2 and KB5129194 for 26H1. As of October 18, 2026, the safest first move is to install the latest applicable cumulative update, not to remove a security patch.[1][2][3]
Why the workspace starts but commands fail
The affected feature is host-folder sharing into an HCS-managed Linux virtual machine through Plan9. The virtual machine may start normally while folders connected from Windows fail to appear inside it. Microsoft's release-health notice specifically names Claude Cowork and Windows Subsystem for Linux (WSL) as examples. This does not mean every WSL installation fails; some Cowork reporters found their ordinary WSL environment still worked.[1][4]
In public issue #92958, users reported that Cowork's local device_bash operations failed with sandbox-helper: no Plan9 drive shares mounted under /mnt/.virtiofs-root/shared. File-listing and staging tools sometimes still worked, making the problem look like an inconsistent app permission. Some reporters kept the Claude Desktop version constant while comparing Windows builds before and after the update. Those are individual A/B observations; the official confirmation concerns the Plan9 regression and its Windows fix.[4][1][2]
Which update applies to your PC?
| Windows 11 release | September update associated with reports | First corrective update | Corrected build |
|---|---|---|---|
| 24H2 | KB5124008 / 26100.9445 | KB5129195 | 26100.9457 |
| 25H2 | KB5124008 / 26200.9445 | KB5129195 | 26200.9457 |
| 26H1 | KB5124012 / 28000.2954 | KB5129194 | 28000.2956 |
These are the initial corrective releases, not the only acceptable builds. Later cumulative updates on the same supported servicing branch can include the fix. Do not force a package meant for 24H2 onto an Insider or 26H2 build.[2][3][5]
Check and recover without resetting anything
- Press
Win + R, enterwinver, and note the Windows release and build. In Settings → Windows Update → Update history, look for the relevant KB. Compare both pieces of information rather than treating a single history entry as definitive.[5] - Reproduce the failure using a non-sensitive test folder. If connected-folder listings work but a local shell operation fails, look for the Plan9 mount error. Remove names, local paths and credentials before sharing logs.[4]
- Use Windows Update to install the newest applicable update. KB5129195 or later is the relevant line for 24H2/25H2; KB5129194 or later for 26H1. Restart when prompted. On managed computers, involve IT.[2][3]
- Retest the same folder in a local Cowork session. Read a harmless test file first; use a disposable file for any write test. Verify scheduled jobs' output and timestamps before rerunning failed work.
- If the error remains, record the OS build, installed KB, Claude Desktop version, local/cloud mode and a redacted error. A WSL boot failure or generic access-denied message may need a different diagnosis.
Why not reinstall, uninstall or wipe?
App restarts and re-adding folders did not help in several user reports. Removing the September security update also removes security fixes, and some builds cannot uninstall it. With a corrective Windows update available, updating is preferable to rolling back. Do not delete WSL distributions, virtual disks or Cowork workspace data to test this issue.[4][2]
Some users temporarily switched from local execution to cloud processing. That changes where data may be processed: check company policy, confidentiality and the product's data settings before moving sensitive files. The official notice documents unavailable shares; it does not establish that existing files were deleted.[1][4]
Bottom line
A broken Cowork local shell after the September 2026 Windows update can be a Windows Plan9 mount problem rather than a Claude app failure. Check winver, apply the latest correct Windows update, and test the same folder safely. The first fixes were KB5129195 (24H2/25H2) and KB5129194 (26H1).
References (1)
[1] Microsoft, KB5124008 known issue: https://support.microsoft.com/en-us/servicing/os/windows-11/2026/09/kb5124008-windows-11-24h2-25h2-security-update [2] Microsoft, KB5129195 OOB fix: https://support.microsoft.com/en-us/servicing/os/windows-11/2026/09/kb5129195-windows-11-24h2-25h2-security-update [3] Microsoft, KB5129194 OOB fix: https://support.microsoft.com/ja-jp/servicing/os/windows-11/2026/09/kb5129194-windows-11-26h1-security-update [4] Public reproduction, issue #92958: https://github.com/anthropics/claude-code/issues/92958 [5] Microsoft, KB5124012 (26H1): https://support.microsoft.com/en-us/servicing/os/windows-11/2026/09/kb5124012-windows-11-26h1-security-update
