subelsky/bundler_mcp

A Model Context Protocol (MCP) server enabling AI agents to query information about dependencies in a Ruby project's Gemfile. Built with fast-mcp.

CI Gem Version

Installation

Install the gem and add to the application's Gemfile by executing:

bundle add bundler_mcp --group=development

Usage

1. Generate the binstub:

bundle binstubs bundler_mcp

2. Configure your client to execute the binstub. Here are examples that work for Claude and Cursor:

Basic Example (mcp.json)

{
  "mcpServers": {
    "bundler-mcp": {
      "command": "/Users/mike/my_project/bin/bundler_mcp"
    }
  }
}

Example with logging and explicit Gemfile

{
  "mcpServers": {
    "bundler-mcp": {
      "command": "/Users/mike/my_project/bin/bundler_mcp",

      "env": {
        "BUNDLER_MCP_LOG_FILE": "/Users/mike/my_project/log/mcp.log",
        "BUNDLE_GEMFILE": "/Users/mike/my_project/subdir/Gemfile"
      }
    }
  }
}

Documentation

Available on RubyDoc

Available Tools

The server provides two tools for AI agents:

list_project_gems

Lists all bundled Ruby gems with their:

• Versions
• Descriptions
• Installation paths
• Top-level documentation locations (e.g. README and CHANGELOG)

get_gem_details

Retrieves detailed information about a specific gem, including:

• Version
• Description
• Installation path
• Top-level documentation locations
• Source code file locations

Environment Variables

• BUNDLE_GEMFILE: Used by Bundler to locate your Gemfile. If you use the binstub method described in the Usage section, this is usually not needed.
• BUNDLER_MCP_LOG_FILE: Path to log file. Useful for troubleshooting (defaults to no logging)

Development

After checking out the repo, run bin/setup to install dependencies and bundle exec rspec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

Testing with the MCP Inspector

You can test the server directly using the MCP inspector:

# Basic usage
npx @modelcontextprotocol/inspector ./bin/bundler_mcp

# With logging enabled
BUNDLER_MCP_LOG_FILE=/tmp/log/mcp.log npx @modelcontextprotocol/inspector ./bin/bundler_mcp

# With custom Gemfile
BUNDLE_GEMFILE=./other/Gemfile npx @modelcontextprotocol/inspector ./bin/bundler_mcp

Release Process

To install this gem onto your local machine, run bundle exec rake install. To release a new version:

1. Update the version number in version.rb
2. Run bundle exec rake release

This will:

• Create a git tag for the version
• Push git commits and the created tag
• Push the .gem file to rubygems.org

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/subelsky/bundler_mcp.

License

Open source under the terms of the MIT License.

Author

Mike Subelsky