Microsoft patches the Windows bug that cut Claude Cowork off from the local drive

For most people who hit this, Claude Cowork looked healthy from the outside. Chat answered and files opened and saved; only the commands failed. "A Windows update released September 8 has left Claude Cowork on Windows unable to run local commands, because its workspace can no longer reach your computer's drive. For most users, chat and file reading and editing still work," Anthropic wrote in a notice to users ^[5].

Microsoft described the same fault from underneath. Affected virtual machines start normally, the company said, but folders shared from the Windows host using Plan9 do not appear or cannot be accessed inside the guest ^[6]. "Applications or sandbox environments that depend on these shared folders might display an error indicating that no Plan9 drive shares were mounted," Microsoft said ^[6]. WSL broke the same way ^[4].

So the machine passes every obvious check. The VM boots, the tool opens, and the part that stops is the part that reaches the disk. The first ticket goes to the application vendor. Anthropic ended up publishing the Windows advisory ^[5].

Monday's patch, KB5129195, installs itself on affected PCs as OS Builds 26200.9457 or 26100.9457 ^[8]. It fully resolves the Remote Desktop Services and virtual machine faults ^[9]. For USB audio it fixes only the error that blocked multichannel modes, including 8-channel and 3D audio, and peripherals that emit no sound at all still emit none ^[9]. "We are working to address the other symptoms and will update this documentation when more information is available," Microsoft said ^[10]. According to Windows Latest, the patch does not cover the AMD Radeon GPU driver problems also tied to KB5124008 ^[11].

KB5124008 was built to fix nearly 1,000 software bugs and vulnerabilities ^[12]. Of the four problem areas tied to it, Monday's follow-up clears two completely, one partly, and leaves the Radeon reports ^[15]. The gap ran six days, from the Tuesday release to Monday's fix ^[14]. Microsoft's earlier KB5120998 was itself built to fix an update that made the desktop mouse cursor randomly change size ^[13].

Two properties sort the estate. First, whether a tool does its work by executing local commands through a virtual machine or a sandbox, which Cowork and WSL both do ^[4]. Second, whether it fails loudly or keeps serving a smaller version of itself, which is what Cowork did when chat and file editing survived and the shell did not ^[5]. Tools in the second group cost more to diagnose, because the user reports that the assistant got worse and nobody thinks of last Tuesday's build. The check that would have caught this takes a minute on one test machine: run a command in the tool that writes a file to the host drive, and see whether the share is mounted.