Windows 11 KB5124008 Claude Cowork fix: what's confirmed so far
Microsoft's own support documentation for the KB5124008 update warns that applications using HCS-managed virtual machines "might experience issues" sharing host folders with Linux VMs through Plan9, and it names Claude Cowork and the Windows Subsystem for Linux (WSL) as two of the affected applications (Microsoft support). The update shipped September 8, 2026, for Windows 11 24H2 and 25H2, and the most recent revision to that documentation, made last week, still describes Microsoft as working on a resolution rather than pointing to a released patch.
If you move files between Windows and a Linux VM through either app, here's what's confirmed, what isn't, and how to check whether you're actually hitting this bug before attributing the failure to another component.
What the Plan9 bug actually does
Microsoft's documentation draws a specific line: HCS-managed virtual machines that use Plan9 to read host folders are affected, while standard Hyper-V virtual machines that don't rely on Plan9 sharing are explicitly excluded from this known issue (Microsoft support). WSL and Claude Cowork sit in the first category. A generic Hyper-V VM you spun up for testing sits in the second, unless it's also configured to share folders through Plan9.
When Plan9 sharing fails, host folders don't show up in the VM, or they appear but can't be opened. Microsoft's known-issues page describes the symptom directly: affected apps "might display an error indicating that no Plan9 drive shares were mounted" (Microsoft support). XDA Developers reported the same Claude Cowork "no Plan9 drive shares mounted" message last week, shortly after Microsoft added the entry to its known-issues page.
A missing share, an unreadable share, and that exact error text are related symptoms, not a guaranteed diagnosis. If your Hyper-V Linux VM shared folders are failing but you're not seeing that specific wording, that alone doesn't rule this bug in or out. Microsoft's entry only confirms what might happen, not the full range of ways the failure could present.
Windows 11 KB5124008 WSL issue: which devices are affected
Microsoft's known-issue entry applies to systems running the update's 24H2 and 25H2 builds, 26100.9445 and 26200.9445, both delivered through KB5124008 (Microsoft support). Knowing you're on 24H2 or 25H2 isn't enough by itself, since devices on either version can be sitting on different cumulative updates depending on when they last checked in.
To confirm, open Settings, then Windows Update, then Update history, and look for KB5124008 by name. If it's not listed, your device doesn't have the specific installation Microsoft's entry identifies, though a later cumulative update could still carry the same defect. The absence of KB5124008 today doesn't guarantee immunity going forward.
Microsoft added the Plan9 entry to its Windows 11, version 25H2 known-issues documentation on September 11, three days after KB5124008 shipped. Anyone who updated before that date could have installed KB5124008 before Microsoft had documented the problem at all.
What Microsoft has confirmed, and what it hasn't
Microsoft's entry says the company is "working on a resolution and will update this documentation when more information is available." That confirms the defect is logged as a known issue and that Microsoft is aware of it. It sets no KB number, no build, and no release date for a fix (Microsoft support).
The known-issues page kept growing after the Plan9 entry appeared. Microsoft added two more entries on September 12: Remote Desktop Services stopping response for some organizations, and USB audio devices failing to start or producing no sound. Both are separate, unrelated known issues tied to the same update, not evidence of one shared root cause.
For the RDS problem, Microsoft directs IT administrators to Microsoft Support for Business for an immediate workaround. The Plan9 entry lists no comparable support contact or workaround for consumers.
How to tell if you're actually hitting this bug
Available reporting on this issue runs through last week, and the most recent version of Microsoft's known-issues page hadn't named a corrective KB, build number, or workaround for Plan9 sharing as of that revision (XDA Developers; Microsoft support). Treat the problem as unresolved and work through which scenario applies before assuming a fix has landed.
Before testing anything, or before reporting the bug to Microsoft, gather these details. This is a reporting checklist, not an official Microsoft diagnostic procedure:
- Windows version (24H2 or 25H2)
- Installed KB, found under Update history
- Whether host folders are missing entirely or visible but unreadable
- The exact error text, including whether it names Plan9 or matches "no Plan9 drive shares mounted"
- Which app is affected: WSL, Claude Cowork, or another HCS-managed VM tool
With KB5124008 installed and that symptom present, try opening a shared folder in WSL or a Claude Cowork workspace and reading or writing a test file. A failure there lines up with what Microsoft has documented, but this is an editorial test readers can run themselves, not a Microsoft-verified diagnostic, and it can't establish that Plan9 is the sole cause of every failure that looks similar.
A different symptom on the same update, like a crash, a network failure, or a permissions error unrelated to file visibility, falls outside this known issue and needs its own troubleshooting path. Without KB5124008 installed, Microsoft's documentation doesn't say other builds are unaffected; it only confirms that this specific entry applies to systems running this update.
Microsoft's Plan9 entry lists no rollback and no workaround. Uninstalling KB5124008 isn't something Microsoft has endorsed as a fix, and XDA Developers reported no verified home fix either as of last week's coverage.
What to do until Microsoft ships a fix
If KB5124008 is installed and the Plan9 symptom matches, document what you're seeing, keep an independent local or cloud copy of anything time-sensitive that normally passes through the affected share, and check the KB5124008 known-issues page periodically rather than searching for a patch that hasn't been announced. That backup step is precautionary editorial advice tied to this specific known issue, not a sign that data loss has actually been reported anywhere.
If the KB isn't installed, or your symptom doesn't match the Plan9 error, treat it as a separate WSL or VM problem and troubleshoot accordingly rather than assuming this known issue is the cause. And don't call a future build a fix just because folder sharing happens to start working again; wait until Microsoft names a corrective update, or until your own read/write test holds up consistently, before treating the issue as resolved.
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