GitHub issue #92984 in anthropics/claude-code, opened 2026-09-09 by tomokuri8, records a report that after the September 2026 Windows 11 cumulative security update KB5124008 was installed and the machine rebooted, the Cowork local sandbox in Claude Desktop for Windows could no longer mount any host drive share, with every 9p mount inside the guest failing with invalid argument, and that uninstalling KB5124008 and rebooting fully restored the sandbox.
Verbatim Excerpts from Cited Records
“After the September 2026 Windows 11 cumulative security update **KB5124008 (OS build 26200.9445)** was installed and the machine rebooted, the Cowork local sandbox (Claude Desktop, Windows) can no longer mount any host drive share.”
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anthropics/claude-code (issue 92984 opening body, What is Wrong section)
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2026-09-09T03:16:16Z
·GitHub Issue ↗
“The VM boots, vsock RPC works, and on the host side `HcsModifyComputeSystem` returns `hr=0x0` for every Plan9 share — but inside the guest every 9p mount fails with `invalid argument`.”
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anthropics/claude-code (issue 92984 opening body, What is Wrong section)
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2026-09-09T03:16:16Z
·GitHub Issue ↗
“**Uninstalling KB5124008 and rebooting fully restores the sandbox.**”
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anthropics/claude-code (issue 92984 opening body, What is Wrong section)
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2026-09-09T03:16:16Z
·GitHub Issue ↗
Counter-record: Issue remains open as of 2026-11-01 with 121 comments. Surveyed comments include multiple independent confirmations in September 2026 on Windows 11 Pro and Home, build 26200.9445, with Claude Desktop 1.49585 and 1.52386, and one September 12 report that removing KB5124008 did not revert the Plan9 binaries on that machine. The counter-record is material: surveyed comments from September 14 report that the out-of-band update KB5129195 fixed the mounts, with one comment identifying vp9fs.dll as the culprit, a status page note marking the incident resolved on September 14, a Windows 10 fix via KB5129236 on September 16, and the latest surveyed comment, dated 2026-11-01, confirming the fix with the October cumulative update KB5127648 on build 26200.9711. The claim documents the original report, not a verified current state, and no maintainer resolution appears in the surveyed thread.
Falsification probe: If issue 92984 is deleted without archive, if the author edits the opening body so the quoted lines no longer match verbatim, or if the author or maintainers post a definitive retraction showing the described failure was mischaracterized, this claim drops to Withdrawn.
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