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Merge pull request #444 from koic/add_rails_example
Add Rails example app
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‎README.md‎

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the [MCP Streamable HTTP transport spec](https://modelcontextprotocol.io/specification/latest/basic/transports#streamable-http),
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so no additional route configuration is needed.
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A complete runnable application using this approach is available in [`examples/rails`](examples/rails).
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##### Rails (controller)
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While the mount approach creates a single server at boot time, the controller approach creates a new server per request.

‎examples/README.md‎

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- Provide an interactive menu to trigger notifications
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- Display all received SSE events in real-time
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### 7. Rails Server (`rails/`)
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A minimal Rails application that mounts `StreamableHTTPTransport` in its routes, following the "Rails (mount)" pattern from the top-level README.
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It demonstrates class-based tools in `app/tools/` and a resource with a read handler.
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**Usage:**
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```console
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$ cd examples/rails
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$ bundle install
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$ bundle exec puma --port 9292
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```
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The MCP endpoint is available at `http://localhost:9292/mcp`. See [`rails/README.md`](rails/README.md) for a full curl-based walkthrough.
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### Testing with MCP Inspector
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[MCP Inspector](https://modelcontextprotocol.io/docs/tools/inspector) is a browser-based tool for testing and debugging MCP servers.

‎examples/rails/.gitignore‎

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/Gemfile.lock
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/log/
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/tmp/

‎examples/rails/Gemfile‎

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# frozen_string_literal: true
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source "https://rubygems.org"
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gem "mcp", path: "../.."
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gem "puma"
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gem "rails", "~> 8.0"

‎examples/rails/README.md‎

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# Rails Example MCP Server
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A minimal Rails application that serves an MCP server over the Streamable HTTP transport,
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following the "Rails (mount)" pattern from the [top-level README](../../README.md).
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The application provides:
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- `add_tool` tool (`app/tools/add_tool.rb`) - adds two numbers, demonstrates `input_schema`
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- `greeting_tool` tool (`app/tools/greeting_tool.rb`) - greets a name, demonstrates `annotations` and `server_context`
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- `example://rails/readme` resource - a text resource served via `resources_read_handler`
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The MCP server and transport are built once at boot in `config/routes.rb`, where the transport is mounted at `/mcp`.
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## Requirements
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- Ruby >= 3.2 (required by Rails 8; the `mcp` gem itself supports older Rubies)
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- curl >= 7.82 (for the `--json` flag used below)
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## Running
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```console
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$ cd examples/rails
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$ bundle install
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$ bundle exec puma --port 9292
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```
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The MCP endpoint is now available at `http://localhost:9292/mcp`.
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## Testing with cURL
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POST requests must include an `Accept` header that allows both `application/json` and `text/event-stream`,
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per the MCP Streamable HTTP transport spec. Responses arrive as SSE `data:` lines.
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1. Initialize a session and capture the session ID:
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```console
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SESSION_ID=$(curl -s -D - -o /dev/null http://localhost:9292/mcp \
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-H "Accept: application/json, text/event-stream" \
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--json '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{},"clientInfo":{"name":"curl","version":"1.0"}}}' \
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| grep -i "^mcp-session-id:" | cut -d' ' -f2 | tr -d '\r')
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```
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2. Complete the handshake (expect `202 Accepted`):
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```console
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curl -i http://localhost:9292/mcp \
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-H "Accept: application/json, text/event-stream" \
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-H "Mcp-Session-Id: $SESSION_ID" \
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--json '{"jsonrpc":"2.0","method":"notifications/initialized"}'
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```
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3. List and call tools:
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```console
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curl http://localhost:9292/mcp \
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-H "Accept: application/json, text/event-stream" \
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-H "Mcp-Session-Id: $SESSION_ID" \
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--json '{"jsonrpc":"2.0","id":2,"method":"tools/list"}'
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curl http://localhost:9292/mcp \
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-H "Accept: application/json, text/event-stream" \
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-H "Mcp-Session-Id: $SESSION_ID" \
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--json '{"jsonrpc":"2.0","id":3,"method":"tools/call","params":{"name":"add_tool","arguments":{"a":5,"b":3}}}'
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curl http://localhost:9292/mcp \
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-H "Accept: application/json, text/event-stream" \
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-H "Mcp-Session-Id: $SESSION_ID" \
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--json '{"jsonrpc":"2.0","id":4,"method":"tools/call","params":{"name":"greeting_tool","arguments":{"name":"Rails"}}}'
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```
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4. List and read the resource:
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```console
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curl http://localhost:9292/mcp \
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-H "Accept: application/json, text/event-stream" \
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-H "Mcp-Session-Id: $SESSION_ID" \
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--json '{"jsonrpc":"2.0","id":5,"method":"resources/list"}'
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curl http://localhost:9292/mcp \
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-H "Accept: application/json, text/event-stream" \
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-H "Mcp-Session-Id: $SESSION_ID" \
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--json '{"jsonrpc":"2.0","id":6,"method":"resources/read","params":{"uri":"example://rails/readme"}}'
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```
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5. Optionally, open the standalone SSE stream for server-to-client messages
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(in another terminal):
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```console
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curl -N http://localhost:9292/mcp \
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-H "Accept: text/event-stream" \
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-H "Mcp-Session-Id: $SESSION_ID"
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```
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6. End the session:
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```console
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curl -i -X DELETE http://localhost:9292/mcp -H "Mcp-Session-Id: $SESSION_ID"
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```
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## Testing with MCP Inspector
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Start [MCP Inspector](https://modelcontextprotocol.io/docs/tools/inspector) with `npx @modelcontextprotocol/inspector`,
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set Transport Type to "Streamable HTTP", and connect to `http://localhost:9292/mcp`.
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## Notes
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- `StreamableHTTPTransport` keeps session and SSE state in memory, so it must run in a single process.
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Puma runs in single mode (`workers 0`) by default; do not enable clustered mode for this app.
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When running multiple instances behind a load balancer, use sticky sessions keyed on the `Mcp-Session-Id` header,
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or pass `stateless: true` to the transport.
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- The server is built once at boot in `config/routes.rb`, so code reloading is disabled
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(`config.enable_reloading = false` in `config/application.rb`).
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Restart the server after changing tool or resource code. Note that tool classes cannot be referenced from
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`config/initializers` because Zeitwerk has not set up autoloading at that point; routes load late enough that they can.
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- For a per-request server with request-specific tools or context, see the "Rails (controller)" section in the top-level README,
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which uses `stateless: true` and `transport.handle_request(request)` inside a controller action.

‎examples/rails/app/tools/add_tool.rb‎

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# frozen_string_literal: true
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class AddTool < MCP::Tool
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title "Add Tool"
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description "A simple example tool that adds two numbers"
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input_schema(
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properties: {
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a: { type: "number" },
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b: { type: "number" },
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},
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required: ["a", "b"],
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)
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class << self
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def call(a:, b:)
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MCP::Tool::Response.new([{
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type: "text",
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text: "The sum of #{a} and #{b} is #{a + b}",
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}])
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end
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end
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end

‎examples/rails/app/tools/greeting_tool.rb‎

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# frozen_string_literal: true
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class GreetingTool < MCP::Tool
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title "Greeting Tool"
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description "Greets the given name and reports which app served the request"
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input_schema(
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properties: {
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name: { type: "string" },
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},
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required: ["name"],
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)
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annotations(
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destructive_hint: false,
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idempotent_hint: true,
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open_world_hint: false,
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read_only_hint: true,
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)
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class << self
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def call(name:, server_context:)
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MCP::Tool::Response.new([{
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type: "text",
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text: "Hello, #{name}! (served by #{server_context[:app_name]})",
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}])
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end
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end
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end

‎examples/rails/config.ru‎

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# frozen_string_literal: true
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require_relative "config/environment"
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run(Rails.application)

‎examples/rails/config/application.rb‎

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# frozen_string_literal: true
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ENV["BUNDLE_GEMFILE"] ||= File.expand_path("../Gemfile", __dir__)
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require "bundler/setup"
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require "rails"
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require "action_controller/railtie"
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require "mcp"
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module McpRailsExample
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class Application < Rails::Application
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config.load_defaults(8.0)
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# This example has no views, assets, or database.
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config.api_only = true
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# The MCP server is built once at boot (config/routes.rb) and holds references to the tool classes in app/tools,
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# so code reloading is disabled; restart the server after changing tool or resource code.
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# A reloading app would need `config.to_prepare` and a way to swap the transport's server instead.
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config.enable_reloading = false
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config.eager_load = true
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config.logger = ActiveSupport::Logger.new($stdout)
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end
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end

‎examples/rails/config/environment.rb‎

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# frozen_string_literal: true
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require_relative "application"
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Rails.application.initialize!

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