[BUG] mcpb installed MCP servers not discoverable by Claude

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?

When installing an MCP server via a .mcpb file through Developer Extensions, the server shows as installed and running in the UI, but Claude cannot discover or call any of its tools.

This was originally reported in the mcpb repo (modelcontextprotocol/mcpb#290) and closed as completed, but the only resolution documented was a manual install workaround. The mcpb install path itself is still broken.

What Should Happen?

Claude should be able to discover and call tools from mcpb-installed servers. This works correctly when the same server is added manually via the developer config, so the issue is specific to the mcpb install path.

This worked in Claude Desktop in June.

Error Messages/Logs

None — the server appears active in the UI with no visible errors.

Steps to Reproduce

1. Download an .mcpb release file (e.g. chessagine-mcp v0.7.7)
2. Install it via Developer Extensions
3. Enable the connector in connector settings
4. Ask Claude to use or list the server's tools
5. Compare to manually adding the same server via developer config — tools are discoverable that way but not via mcpb

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

This worked in Claude Desktop in June.

Claude Code Version

Claude Desktop 1.18286.0

Platform

Other

Operating System

Windows

Terminal/Shell

Other

Additional Information

Multiple users have reported this. Related issues:

• modelcontextprotocol/mcpb#290
• jalpp/chessagine-mcp#56