rails-routes-mcp

A two-tool MCP server in Ruby for a Rails route table. Stdio JSON-RPC. Built so RubyMine 2025.2+ can attach it as an MCP client (Claude / Junie talking to this server, alongside RubyMine's own IDE server).

JetBrains' February 2026 post Building LLM-Friendly MCP Tools in RubyMine is the design spec: paginate, filter, return file+line for the next editor jump, do not dump bin/rails routes.

Portfolio piece. Ships a 15-route shop catalog so CI does not need a Rails app. Point it at a real routes.rb later.

Tools

Two tools. Not a Rails SDK.

Related MCP server: ACE-MCP

RubyMine

RubyMine already is an MCP server (call hierarchy, get_rails_routes, run configurations). This repo is the other direction: a small server the IDE's AI Assistant consumes.

Settings → Tools → MCP Server → manual stdio config:

{
  "mcpServers": {
    "rails-routes": {
      "command": "ruby",
      "args": ["/absolute/path/to/rails-routes-mcp/bin/rails-routes-mcp"]
    }
  }
}

Or paste Copy Stdio Config from that same settings page and swap in this command.

Eval

ruby evals/run.rb

Committed report: naive rails routes dump is 4.83× a typical shaped page; 8/8 queries fill show_route from verb/path/file/line. See evals/REPORT.md.

Tradeoffs

• No mcp gem. The protocol surface we need is initialize / tools/list / tools/call. Owning the loop means ruby test/ runs on stock MRI 3.1 with zero bundler. The official Ruby SDK is the right move for Streamable HTTP + Rails mount; this is the stdio slice.

• Catalog, not bin/rails routes. Parsing a live app is a follow-up. The eval is about payload shape, which does not need Bootsnap.

• No writes. A generate_controller tool would be a different repo and would need RubyMine's own apply_patch, not a second writer.

Streamlit demo

Ruby stdio is the server. Streamlit is the click-through of search_routes / show_route.

pip install -r requirements.txt
streamlit run app.py

Streamlit Cloud: app file app.py. No secrets.

Test

ruby -Itest test/catalog_test.rb test/server_test.rb
ruby evals/run.rb

What I would do differently

• Shell out to bin/rails routes --json when RAILS_ROOT is set, with the same pagination wrapper.

• Add a resource routes://table for clients that prefer resources over tools.

License

MIT. Ashley Gross, 2026.