Claude Cowork Broke on Windows After the September 8 Update — Here’s the Exact Fix (KB5129195)
A Windows security update on September 8, 2026 broke the folder-sharing link that Claude Cowork uses to run commands on your machine, so anything that needed Cowork to touch local files or scripts just failed. Microsoft confirmed the cause and shipped an out-of-band fix, KB5129195, on September 14, 2026. According to Microsoft’s own KB5124008 known-issues page, Claude Cowork and the Windows Subsystem for Linux were both named as affected apps — not just a forum rumor. If you’ve already installed that update or anything after it, Cowork’s local command execution should work again. The steps below show you how to check, and what to do if you’re still stuck.
Where to Jump In
- What Actually Broke on September 8, 2026?
- Why Does a Windows Update Break an AI App’s Local Commands?
- How Do You Know If You’re Affected?
- KB5124008 vs KB5129195: The Fix Timeline
- How Do You Fix Claude Cowork on Windows Right Now?
What Actually Broke on September 8, 2026?
Claude Cowork on Windows stopped being able to run local commands the same day Microsoft pushed its September Patch Tuesday update. Chat still worked. Reading and editing files still worked. But anything that needed Cowork to reach into your computer’s drive and execute a command just sat there or failed outright.
When I checked Microsoft’s own support page for the update, the known-issues list named the exact cause: applications that use HCS-managed virtual machines — Claude Cowork among them — lost the ability to share folders from the Windows host into the Linux virtual machine running underneath, through a mechanism called Plan9.
Anthropic’s status page logged it plainly as degraded functionality for Claude Cowork on Windows, beginning September 8, 2026. There was no in-app fix. Restarting Cowork didn’t help. Reinstalling it didn’t help either, because the problem lived inside Windows itself, not inside the app.
This part trips a lot of people up: the bug looked like a Claude problem, so the first instinct was to blame Anthropic. It wasn’t Anthropic’s bug to fix.
Why Does a Windows Update Break an AI App’s Local Commands?
Cowork doesn’t run your local commands directly on Windows. It hands them to a lightweight Linux virtual machine that Windows spins up in the background, using Microsoft’s Host Compute Service (HCS). That VM needs a way to see your actual files, and Plan9 is the file-sharing protocol that lets a Windows folder show up inside the Linux guest.
The September 8 update changed something in that Plan9 path. The virtual machine still started normally. The folder just stopped showing up on the other side. Any command Cowork tried to run against your drive had nothing to read or write to.
Windows Subsystem for Linux (WSL) hit the identical wall, which is one reason this wasn’t a niche Claude-only bug — developers running WSL projects on the same machines reported the same missing-folder symptom that week.
That same KB5124008 update also caused problems elsewhere on the same class of machines. Always On VPN connections dropped for some enterprise users. File History backups silently stopped completing, showing a ‘reconnect your drive’ message even with the drive plugged in. Cowork’s issue wasn’t isolated — it was one symptom of a broader compatibility problem inside that specific patch.
How Do You Know If You’re Affected?
You’re almost certainly hitting this exact issue if all three are true: you’re on Windows 11 24H2 or 25H2, you installed a Windows update on or after September 8, 2026, and Cowork can chat and read files but refuses to run a script, install a package, or touch anything on disk.
Check your installed build before doing anything else. Open Settings → Windows Update → Update history, and look for KB5124008 (builds 26100.9445 or 26200.9445) with nothing newer installed after it. If that’s the last update on your machine, this is almost certainly your problem — not a Cowork account issue, not a broken install.
KB5124008 vs KB5129195: The Fix Timeline
According to Microsoft’s support documentation, the September 8, 2026 update (KB5124008) broke Plan9 folder sharing on Windows 11 24H2 and 25H2 machines, and the September 14, 2026 out-of-band update (KB5129195) fixed it. Here’s how the two compare side by side.
| Update | Release Date | Build (24H2 / 25H2) | Cowork Local Commands | WSL Folder Access |
|---|---|---|---|---|
| KB5124008 | Sept 8, 2026 | 26100.9445 / 26200.9445 | Broken | Broken |
| KB5129195 | Sept 14, 2026 (out-of-band) | 26100.9457 / 26200.9457 | Fixed | Fixed |
Six days, one silent VM-sharing bug, and no in-app workaround in between — that’s the entire window this affected anyone.
How Do You Fix Claude Cowork on Windows Right Now?
Install KB5129195, or any cumulative update released after September 14, 2026. That’s the actual fix — not a Cowork setting, not a reinstall.
Open Settings → Windows Update. Click Check for updates. If KB5129195, or a later build (26100.9457/26200.9457 or newer), doesn’t show up automatically, install it manually from the Microsoft Update Catalog page linked in Sources below.
Restart your PC after the update finishes. HCS-managed virtual machines need a clean restart to rebuild the Plan9 share — relaunching Cowork alone won’t do it.
Reopen Cowork and try a simple local action first, like asking it to list files in a folder, before trusting it with anything that writes or deletes.
If you’re on WSL as well, the same update resolves the missing-folder issue there — you don’t need a separate fix for that.
What If You Can’t Install the Update Yet?
Some managed or work devices can’t take an out-of-band update on demand — IT controls the rollout schedule. If that’s you, there’s no reliable in-app workaround. Uninstalling KB5124008 does restore the old behavior for some users, but that also removes whatever security fixes shipped with it, so it’s a trade-off, not a real fix.
Don’t spend time reinstalling Cowork, clearing its cache, or recreating your workspace. None of that touches the actual cause, and you’ll just lose time you could’ve spent asking IT to fast-track the September 14 update instead.
Avoid running local-command workflows on an unpatched machine in the meantime. A command that silently fails is safer than one you assume ran and didn’t.
Checklist Before You Reopen Cowork
- ✅ Confirm your build number in Settings → Windows Update → Update history
- ✅ Install KB5129195 or newer, then restart — not just relaunch Cowork
- ✅ Test with a harmless local command first, like listing a folder, not deleting one
- ✅ If you’re on WSL too, re-check the same project folder for the same fix
- ✅ If IT controls your updates, flag KB5129195 by name, not ‘the Cowork bug’
FAQ
Does reinstalling Claude Cowork fix the Windows local command error?
No. When I compared reports across the affected week, reinstalling or resetting Cowork made no difference, because the fault was in how Windows shared folders into the Linux virtual machine, not in the Cowork app itself. Only the September 14, 2026 Windows update (KB5129195) resolved it.
Is Windows Subsystem for Linux (WSL) affected by the same bug?
Yes. WSL relies on the same Plan9-based folder sharing that Cowork uses, so projects that read or write files through WSL hit the identical missing-folder symptom after KB5124008, and the same KB5129195 update fixes both.
Will Microsoft push KB5129195 automatically, or do I need to install it myself?
Most home and personal devices receive it automatically through Windows Update within a few days of an out-of-band release. If checking for updates doesn’t show it yet, you can install it directly from the Microsoft Update Catalog rather than wait.
Sources
- Microsoft Support — September 8, 2026 KB5124008 (OS Builds 26200.9445 and 26100.9445)
- Microsoft Support — September 14, 2026 KB5129195 Out-of-band update
- Claude Status — Incident History
