Show HN: Active MCP - A Rails engine that provides MCP capabilities to Rails app(github.com)

4 pointsby moekiorg4 days ago1 comment

rco87863 days ago
Very cool. How does activemcp figure out what tools I have defined? I haven't read the code, does it do a dynamic class lookup based on the tool name like rails does for controllers?
What's the right way to start the server in an existing rails app? Do I just create the server and call start in an initializer or something?
- moekiorg3 days ago
  Executed when a class inheriting from ActiveMcp::Tool is registered with its class name as the tool name and "tools/call" is requested in /mcp.
  Simply mount the engine in route.rb, define the tool and start it.
  - rco87863 days ago
    > start it.
    right I'm wondering where is the right place to start it! In an initializer?
    moekiorg3 days ago
    No. Any file that runs MCP Server is fine anywhere. I am intending a workspace separate from the rails app. The MCP server should be run in a separate process from the rails app. At least until the streamable http transport is ready.
    https://github.com/modelcontextprotocol/specification/pull/2... https://spec.modelcontextprotocol.io/specification/2025-03-2...
    rco87863 days ago
    > The MCP server should be run in a separate process from the rails app.
    Sorry super confused here. How can I do that while also booting up all the Rails stuff like the autoloader, db connections, etc?
    moekiorg3 days ago
    Sorry for the misunderstanding, Active MCP requires two processes.
    1. mount the Engine and then `$ rails s` 2. `server.rb` to start the MCP server. `$ ruby path/to/server.rb`
    `server = ActiveMcp::Server.new(name: "ActiveMcp DEMO", uri: 'https://your-app.example.com/mcp' );server.start`
    Then, with an MCP client such as Claude Desktop, configure the following.
    `{"mcpServers": {"active-mcp-demo": {"command": "/path/to/ruby", "args": ["/path/to/server.rb"]}}}`