ask-rails-harness-mcp

MCP server for Rails app introspection. Exposes all ask-rails-harness tools over the Model Context Protocol.

Coding agents like Claude Code, Cursor, or any MCP-compatible client can connect to inspect your Rails schema, query your database, read models, and more — all through the same tools ask-rails-harness uses internally.

Installation

bundle add ask-rails-harness-mcp

Usage

Option 1: stdio (local development — recommended)

Run from your Rails app root:

cd my-rails-app
ask-rails-harness-mcp

This boots your Rails app and starts an MCP stdio server. Configure in your agent's MCP config:

{
  "mcp": {
    "servers": {
      "ask-rails-harness-mcp": {
        "type": "stdio",
        "command": "ask-rails-harness-mcp",
        "args": []
      }
    }
  }
}

Option 2: HTTP endpoint (remote/production)

Mount in config/routes.rb behind your existing auth:

Rails.application.routes.draw do
  authenticate :user, ->(u) { u.admin? } do
    post "ask/mcp", to: "ask/rails/mcp#handle"
  end
end

Then configure any MCP-compatible agent:

{
  "mcp": {
    "servers": {
      "ask-rails-harness-mcp": {
        "type": "http",
        "url": "https://myapp.com/ask/mcp"
      }
    }
  }
}

The agent discovers 9 tools automatically:

Tool What it does
schema_graph Full schema introspection — all models, tables, columns, associations, validations
query_database Read-only SQL queries with safety guards
read_model Introspect a single ActiveRecord model
route_inspector Parsed route table with filters
read_log Read Rails log files with level/search filtering
search_codebase Full-text grep search
read_file Read any file from Rails.root
run_command Run shell commands in the app root
read_routes Read the raw config/routes.rb

Authentication

The MCP endpoint uses the same Ask::Rails::Harness::Auth system as the chat UI:

Ask::Rails::Harness::Auth.check = -> {
  redirect_to main_app. unless current_user&.admin?
}

Safety

All ask-rails-harness safety features apply automatically:

  • Permissions — access modes (:read_only, :ask_before_changes, :full_access)
  • Command allowlists — allowed_commands / denied_commands for RunCommand
  • Write guards — INSERT/UPDATE/DELETE blocked by QueryDatabase
  • Audit log — every tool call recorded in ask_audit_logs

Development

bundle exec rake test

License

MIT