Why Claude Cowork Is Not Working on Windows After the September 2026 Update

What is actually failing in Claude Cowork?

The clearest symptom is that Cowork can start but cannot reliably run local commands that depend on access to your Windows drive. The problem can look like a Cowork workspace or sandbox failure even though the Claude service itself is still reachable.

Anthropic's incident notice says the Windows update left Cowork's workspace unable to reach the computer's drive. That means tasks that need local command execution can fail even when the conversation itself still works.

You may therefore see a confusing split: Claude can respond normally, and some file reading or editing can still work, but a task that needs a shell command or deeper local workspace access fails.

Incident status: Anthropic — Claude Status.

The root cause is a Windows Plan9 folder-sharing regression

Microsoft has now documented the underlying operating-system issue. After the September 2026 security update, applications that use HCS-managed virtual machines can have trouble sharing folders from the Windows host into a Linux virtual machine through Plan9.

The virtual machine itself can start normally, but the folders that should be shared from Windows either do not appear or cannot be accessed inside the guest environment. Microsoft says affected applications can report that no Plan9 drive shares were mounted.

Microsoft explicitly names Claude Cowork and Windows Subsystem for Linux (WSL) as affected examples. Standard Hyper-V virtual machines that do not use this Plan9 host-folder-sharing path are not affected by this particular issue.

Microsoft confirmation: Windows 11 release health — Host folder shares might be unavailable in Hyper-V-based Linux VMs.

Which Windows versions are affected?

Microsoft's current affected-platform list covers Windows 11 versions 23H2, 24H2, 25H2, and 26H1. The problem originates from the September 8, 2026 security updates for those supported Windows 11 branches.

Microsoft's 24H2 release-health page documents the issue under build 26100.9445 and update KB5124008. Its 26H1 release-health page documents the corresponding issue under build 28000.2954 and KB5124012. Because Windows servicing packages differ by branch, check your own Windows version and update history rather than assuming every affected PC has the same KB number.

1. Check your Windows version. Press Windows + R, type winver, and confirm whether you are on Windows 11 23H2, 24H2, 25H2, or 26H1.
2. Check Update history. Open Settings → Windows Update → Update history and look for the September 8, 2026 security update for your branch.
3. Compare with Microsoft Release Health. The official issue entry is more reliable than guessing from a Cowork error message alone.

Affected-platform list: Microsoft Windows 11 release health.

Why restarting or reinstalling Claude does not fix it

This is one of the easiest places to waste time. The broken component is not simply a corrupted Claude Desktop installation. It is the Windows host-to-guest folder-sharing behavior Cowork depends on for the affected local-command workflow.

Reinstalling the Claude app does not replace the Windows virtualization and Plan9 components changed by the security update. Restarting the app or the computer can restart the same environment, but it does not remove the underlying Windows regression.

Anthropic therefore states directly that restarting or reinstalling will not help and that there is currently no in-app workaround.

Official incident guidance: Claude Status.

What can still work while Cowork is degraded?

Anthropic says that for most affected users, chat plus file reading and editing still work. The confirmed failure is specifically local command execution caused by the workspace losing access to the host drive.

That means the best temporary approach is to separate tasks by what they require. A writing, summarization, planning, or simple file-editing task may still be usable. A task that needs local shell commands, developer tools, scripts, or command-based processing of files on the Windows drive is much more likely to hit the incident.

Cowork also supports cloud sessions and connected services, but local-file access from web or mobile still depends on the Claude Desktop app when the task needs files on that computer. Do not assume moving the same local-file task to the browser will bypass the confirmed Windows folder-share problem.

Cowork surfaces and local-file behavior: Anthropic — Use Claude Cowork on web, desktop, and mobile and Get started with Claude Cowork.

What should you do right now?

Because there is no confirmed in-app workaround, troubleshooting should focus on avoiding unnecessary changes and identifying whether your task can continue without the broken local-command path.

1. Confirm the incident matches your PC. Check your Windows 11 version and September 8 update history.
2. Check Claude Status. If Cowork on Windows is still listed as degraded, treat the incident as active.
3. Do not reinstall Claude as your first response. Anthropic says reinstalling does not address this Windows-level problem.
4. Continue tasks that do not need local commands. Chat and some file reading/editing may still work for most users.
5. Use another supported environment for urgent command-dependent work if you already have one available. The confirmed incident is specifically tied to Windows, so a separate unaffected machine can be a practical temporary route.
6. Watch for Microsoft's corrective Windows update. Install the official fix once Microsoft marks the issue resolved for your Windows branch.

For company-managed PCs, let your IT team make the update or rollback decision. Organization policies, security requirements, and patch-management systems can matter more than a consumer troubleshooting shortcut.

Should you uninstall the September security update?

Some users may be tempted to remove the September security update because the problem appeared immediately afterward. BeeVaults does not recommend presenting removal of a security update as the default fix.

The update contains security protections in addition to the regression affecting Cowork. Removing it can re-expose vulnerabilities or conflict with an organization's patch policy. Anthropic's official incident notice does not currently provide update removal as its supported in-app workaround; it says Microsoft has developed a fix and is working to release it.

If an IT administrator decides that a temporary rollback is necessary for business-critical work, that decision should be made against Microsoft's guidance for the exact Windows branch, device, and security environment. For a normal user, the safer default is to keep the security update installed and wait for the official corrective update.

How will you know when the problem is actually fixed?

Use two sources rather than relying on a random post saying “it works again.”

1. Anthropic: the Cowork incident on status.claude.com should move from degraded/identified to monitoring or resolved.
2. Microsoft: the Plan9 host-folder-sharing issue on Windows Release Health should identify a resolution and the Windows update that contains it.
3. Your PC: install the corrective update for your branch, restart if Windows requires it, and then test a simple Cowork task that uses a local command against a non-sensitive test folder.

Do not assume a newer Claude Desktop version alone is the fix unless Anthropic explicitly says the application update contains a mitigation. The currently documented root cause is in Windows.

Common questions