[BUG] Windows MSIX/Cowork: in-app Claude Code writes silently virtualized (succeed but don't persist), and in-place updates deadlock on the Cowork VM service's file handles

Summary

When a **Claude Code session runs inside the MSIX-packaged Claude Desktop app on Windows, the session inherits the app's package identity. MSIX write-virtualization then silently redirects the session's writes to %LOCALAPPDATA%, %APPDATA%, and HKCU into the package's virtual store — and because reads are copy-on-write, the same session reads the redirected values back and reports success**. The real machine is never modified. There is no error at any point.

This is distinct from the existing MSIX issues in this repo, which fail loudly (install HRESULTs, Cowork EXDEV rename failures) or concern the read path (MCP config resolved from a virtualized location). This is a silent write-path failure: the session cannot detect it from the inside, and it reports the operation as done.

Impact

Any in-app Claude Code task that:

• installs software to %LOCALAPPDATA%,
• sets autostart (an HKCU\...\Run value, a Startup-folder shortcut, a scheduled task), or
• otherwise writes to HKCU / %APPDATA% / %LOCALAPPDATA%

…succeeds against the package sandbox only. From the session's perspective every write and read-back verification passes. After a reboot (or from any out-of-package process), none of it is present on the real machine. Two operations that "verified live" in-session were both gone after a restart in my case — one autostart entry that was supposed to be added, and one that was supposed to be removed (the removal only tombstoned inside the overlay; the real entry survived).

Repro

1. Install Claude Desktop for Windows as the default MSIX build (package family Claude_pzs8sxrjxfjjc).
2. Launch a Claude Code session inside the desktop app (so it inherits package identity).
3. From that session, write to a real-machine location, e.g.

reg add "HKCU\Software\Microsoft\Windows\CurrentVersion\Run" /v Foo /d "C:\some\app.exe" /f
or write a file to %LOCALAPPDATA%\Foo\bar.txt.

1. Read it back from the same session — it appears present; the command reports success.
2. From an out-of-package process (a normal terminal, or wmic process call create "cmd /c reg query ..."), query the same key/path — it is absent. The write actually landed in

%LOCALAPPDATA%\Packages\Claude_pzs8sxrjxfjjc\LocalCache\... and the package's private registry hive.

1. Reboot → none of the changes are on the real machine.

The copy-on-write read semantics make this especially hard to catch: an in-session read of a path the container hasn't touched falls through to the real machine (looks correct), while a read of a path it has written/deleted shows the overlay (looks correct too). So "read it back to verify" always passes and never reveals the redirect.

Environment

• Windows 11 Home, build 26200 (25H2)
• Claude Desktop MSIX 1.20186.7.0, package family Claude_pzs8sxrjxfjjc
• Get-AppxPackage -Name Claude → SignatureKind: Developer, InstallLocation: C:\Program Files\WindowsApps\Claude_1.20186.7.0_x64__pzs8sxrjxfjjc
• The package manifest uses flexible virtualization with only a few app-specific exclusions (browser native-messaging-host keys, Office WEF, LocalAppData\Claude-3p); general-purpose HKCU\...\Run and arbitrary %LOCALAPPDATA% writes are not excluded, so they are virtualized.

Confirmed workaround

Installing the non-MSIX (Squirrel) build instead — winget install Anthropic.Claude (InstallerType exe, per-user, from downloads.claude.ai), or the direct Claude-<hash>.exe from Anthropic's CDN — yields an unvirtualized per-user install at %LOCALAPPDATA%\AnthropicClaude, where in-app Code writes hit the real locations. (This matches #48362, which was resolved the same way.)

Suggested resolution (any of)

1. Document this prominently for Claude Code users on Windows — the MSIX build silently no-ops filesystem/registry writes an in-app coding session makes.
2. Provide a supported way for in-app Code sessions to run de-virtualized (e.g. spawn the Code toolchain out of the package via the desktop-app breakaway policy PROC_THREAD_ATTRIBUTE_DESKTOP_APP_POLICY = ENABLE_PROCESS_TREE).
3. Recommend the Win32/Squirrel install for users who run Claude Code from the desktop app.

---

Second symptom, same MSIX/Cowork root: in-place updates deadlock on the Cowork VM service's file handles

Separately triageable, but it shares the packaging root, so filing together.

Symptom. An auto-update installs, then the app locks up reporting a file in use under C:\Program Files\WindowsApps\Claude_<ver>_x64__pzs8sxrjxfjjc\. Recovered only by a reboot; it has recurred across three consecutive updates on this machine. (While investigating, the user also hit the WindowsApps package folder being inaccessible even to an administrator — that part is expected TrustedInstaller behavior, not corruption, but it compounds the "something is badly wrong" impression during a failed update.)

Root cause. CoworkVMService (runs as localSystem, WIN32_PACKAGED_PROCESS) hosts a Hyper-V/HCS utility VM and holds open file handles on binaries inside the package directory — including ...\app\resources\cowork-svc.exe and the VM bundle. An in-place MSIX update must replace those exact files, but Windows cannot replace a file that is in use → the update deadlocks. A reboot stops the service and tears down the VM, releasing the handles, and the staged update then completes on next launch.

Evidence (C:\ProgramData\Claude\Logs\cowork-service.log, this machine):

• HCS/VM initialization: [HCS] vmcompute.dll loaded successfully, [HCS] computecore.dll loaded successfully, [HCS] Procs initialized, HCS ready.
• Service binary path: [Server] Service: C:\Program Files\WindowsApps\Claude_1.20186.7.0_x64__pzs8sxrjxfjjc\app\resources\cowork-svc.exe.
• A VM session is created on app launch: [Server] Created new VM session for DM:... immediately followed by [IdleCheck] ... starting idle timer now.

Notable — this hits users who never use Cowork. The reporting user has never intentionally invoked Cowork, yet CoworkVMService auto-starts and opens a VM session on every app launch, so the update-lock is not gated on actual Cowork usage.

Suggested resolution (any of).

1. Have the updater stop CoworkVMService and quiesce its VM before replacing packaged files.
2. Don't auto-start / auto-open a VM session when Cowork is unused, so idle installs don't hold package handles.
3. As above, the Win32/Squirrel build sidesteps this entirely (per-user install, no WindowsApps, no packaged VM service to lock).

Same convergence as the write-virtualization symptom: the Cowork packaged VM service is the common thread behind both.

Related (not duplicates)

• #48362 (closed) — Code sessions silently fail to persist via EXDEV inside MSIX, resolved by switching to the Win32 installer. Same root cause, loud (EXDEV) symptom; this report is the silent write-redirect variant.
• #70393, #26073 — MSIX-virtualized read path for MCP config.
• #68070 — MSIX fails to launch where the Squirrel/winget build works on the same machine.
• #76980 — Squirrel→MSIX migration orphans Start-Menu/desktop shortcuts (adjacent: stale shortcuts pointing into the sandbox).
• #65887 — CoworkVMService stops between sessions and cannot be restarted by the app (update-lock section).
• #55946 — MSIX 3P mode: rootfs.vhdx commit fails, app self-terminates (update-lock section).
• #36440, #76982 — Cowork EXDEV cross-device link failures under MSIX virtualization (update-lock section).