[BUG] Filesystem extension silently fails to install after Squirrel → MSIX upgrade on Windows

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Summary

After upgrading from the Squirrel-based installer to the MSIX package, the Filesystem desktop extension cannot be installed via the Connectors directory. Clicking the Install button triggers a download but the installation never completes — no error is shown and the extension does not appear as installed.

Environment

• OS: Windows 11
• Claude Desktop version: 1.12603.1.0 (MSIX)
• Package family: Claude_pzs8sxrjxfjjc
• Upgrade path: Squirrel installer → MSIX package

What Should Happen?

Expected behavior

The Filesystem extension installs successfully and appears as an available connector.

Actual behavior

The .mcpb file is downloaded to a temp folder and copied to a dxt-install-* directory, but installation never completes. Multiple retries leave behind orphaned dxt-install-* folders.

Error Messages/Logs

### Root cause (from logs)

On every app startup, `main.log` shows:


[MSIX] Filesystem not virtualized — C:\Users\<user>\AppData\Local\Packages\Claude_pzs8sxrjxfjjc\LocalCache\Roaming\Claude absent (likely Squirrel upgrade)


The MSIX virtualization path is never created by the app on startup, so the extension installer has no valid write target. Manually creating the directory is overwritten/ignored on next launch.

The install sequence in the log:

[download:N] Starting download from .../ant.dir.ant.anthropic.filesystem/download
[warn] Installing unsigned extension from C:\...\dxt-download-XXXX.mcpb

…followed by silence. No success or failure log entry ever appears after `Installing unsigned extension`.

Steps to Reproduce

Steps to reproduce

1. Install Claude Desktop via the Squirrel installer (older method)
2. Upgrade to the MSIX version (e.g. via Microsoft Store or new installer)
3. Open Claude Desktop → Code tab → + button → Connectors
4. Select Filesystem → click Install
5. Nothing happens — no confirmation, no error, no extension installed

Claude Model

Not sure / Multiple models

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

1.12603.1.0

Platform

Other

Operating System

Windows

Terminal/Shell

Other

Additional Information

Additional notes

• extensions-blocklist.json has 0 entries — not a blocklist issue
• Windows Defender shows no blocks related to Claude or .mcpb files
• Manually creating the virtualization path (AppData\Local\Packages\Claude_pzs8sxrjxfjjc\LocalCache\Roaming\Claude) does not persist across app restarts
• Other app functionality (Code sessions, remote connectors) works normally
• The same issue affects repeated install attempts; each attempt creates a new orphaned dxt-install-* folder in both AppData\Roaming\Claude and the virtualization path