Extension fails to launch on MSIX-installed Claude Desktop (uv editable build error)

[kaokula]

Environment

• OS: Windows 11 Enterprise 26200
• Claude Desktop installed via Microsoft Store (MSIX package, family Claude_pzs8sxrjxfjjc)
• Windows-MCP Extension v0.7.1
• uv 0.10.6, Python 3.13

Symptom

Claude Desktop UI shows: MCP Windows-MCP: Server disconnected. Could not attach to MCP server Windows-MCP.

Log excerpt

From %APPDATA%\Claude\logs\mcp-server-Windows-MCP.log:

× Failed to build `windows-mcp @ file:///C:/Users/.../Claude%20Extensions/ant.dir.cursortouch.windows-mcp`
├─▶ The build backend returned an error
╰─▶ Call to `setuptools.build_meta.build_editable` failed (exit code: 1)

    [stdout]
    running egg_info

    [stderr]
    error: error in 'egg_base' option: '.' does not exist or is not a directory

Root cause

The Extension's manifest.json command is uv --directory ${__dirname} run windows-mcp, which triggers an editable rebuild via setuptools every launch. Under MSIX filesystem virtualization (Microsoft Store install of Claude Desktop), setuptools' egg_info cannot resolve . correctly because the path Claude Desktop passes (%APPDATA%\Roaming\Claude\...) is a virtualized view of %LOCALAPPDATA%\Packages\Claude_pzs8sxrjxfjjc\LocalCache\.... Native subprocesses spawned with the virtual path don't see consistent filesystem state.

Workaround

Calling the pre-built venv exe directly (skipping uv run) works, set via a manual mcpServers entry in claude_desktop_config.json pointing to:

%LOCALAPPDATA%\Packages\Claude_pzs8sxrjxfjjc\LocalCache\Roaming\Claude\Claude Extensions\ant.dir.cursortouch.windows-mcp\.venv\Scripts\windows-mcp.exe

Suggested fix

Change the manifest to invoke ${__dirname}/.venv/Scripts/windows-mcp.exe directly (with a one-shot install on first activation), or revert to the hatchling build backend (which v0.6.0 used) so editable rebuilds don't go through setuptools' egg_info code path. Either avoids the MSIX-related setuptools failure.