A Windows update breaks Claude Cowork's local sandbox

Your move: Hold off installing KB5124008, KB5124012, or KB5122880 (all Windows 11) for now

A Windows 11 security update released on September 8, 2026 (KB5124008 on 24H2/25H2, KB5124012 on ARM64, KB5122880 on 23H2) stops Claude Cowork's local sandbox from reaching your PC's files. Chat still works, and simple file reading or editing still works, but Cowork can no longer run commands on your computer or reach connected folders. Anthropic says the cause is a change on Microsoft's side, not a Cowork bug, and that Microsoft is working on a fix. There is no in-app workaround yet: restarting or reinstalling the app does not help. Windows 10 is not affected.

Why it matters

If you rely on Cowork's local sandbox on Windows 11 to run commands or work with files on your own machine, hold off installing this update for now, or uninstall it if you already have. Chat-only use of Cowork is unaffected.

Source: Claude Cowork changelog ↗

Published by Agent Log. The summary reports the linked source; “Why it matters” and the verdict are Agent Log's interpretation.

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