[BUG] Claude Desktop 2.37337.1 (Windows MSIX): no way to install a local .mcpb (DnD falls into chat, no file association, Install Extension no-op)

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?

Regression of #68002, which is closed. It worked on 2026-10-13 before the auto-update to 2.37337.1.

Environment: Windows 11, Claude Desktop 2.37337.1.0 (MSIX Claude_pzs8sxrjxfjjc)

Steps / actual result

Drag a valid .mcpb onto Settings → Extensions. The drop is captured by the chat, not by the Extensions drop zone.
Settings → Extensions → Advanced settings → Install Extension… → select the .mcpb. Nothing installs.
Double-click the .mcpb. The MSIX package registers no .mcpb file association: Settings → Default apps → “Choose defaults by file type” finds nothing for .mcpb. The file instead opened in the Claude Code CLI claude.exe from the VS Code extension.

Expected: any of these opens the extension install dialog.

Impact: end users currently have no supported way to install a local .mcpb on Windows; the only fallback is hand-editing claude_desktop_config.json. This blocks distributing our extension (ABBA/Fiji, 16 tools) to non-technical users.

What Should Happen?

Any of the three standard install paths should open the extension install dialog for the .mcpb:

1. Dropping a .mcpb onto Settings → Extensions is captured by the Extensions drop zone, not by the chat behind it.
2. Settings → Extensions → Advanced settings → Install Extension… loads the preview and offers Install.
3. The Desktop app registers itself as the handler for .mcpb files, so double-clicking one opens Claude Desktop’s install dialog (not the Claude Code CLI) and .mcpb appears under Default apps → file types.

This is how drag-and-drop and Install Extension behaved on the previous build (before 2.37337.1).

Error Messages/Logs

Steps to Reproduce

1. On Windows 11, install or update Claude Desktop to 2.37337.1.0 (MSIX).
2. Get any valid .mcpb bundle (mine passes mcpb validate, manifest v0.4, uv server).
3. Open Settings → Extensions and drag the .mcpb from Explorer onto the page.

→ The drop goes to the chat behind the modal; no install dialog appears.

1. Click Advanced settings → Install Extension… and select the same .mcpb.

→ Nothing installs.

1. In Explorer, double-click the .mcpb.

→ Claude Desktop does not open it. On my machine it launched the Claude Code CLI instead.

1. Open Windows Settings → Apps → Default apps → Choose defaults by file type and search .mcpb.

→ “We couldn’t find anything to show here”: the Desktop package registers no .mcpb association.

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.37337.1.0

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Other

Additional Information

Claude Desktop issue.