Main Plug implementation for MCP HTTP transport with SSE support.

This module provides a complete MCP server implementation with:

• JSON-RPC 2.0 message handling
• Server-Sent Events (SSE) streaming
• CORS handling and security features
• Session management integration
• Origin validation

Phoenix

defmodule MyAppWeb.Router do
  use MyAppWeb, :router
  # ...

  pipeline :mcp do
    plug :accepts, ["json", "sse"]

    plug Plug.Parsers,
      parsers: [{:json, length: 1_000_000}],
      pass: ["application/json"],
      json_decoder: JSON
  end

  scope "/mcp" do
    pipe_through :mcp

    forward "/", Phantom.Plug,
      router: MyApp.MCPRouter,
      pubsub: MyApp.PubSub
  end
end

Plug.Router

defmodule MyAppWeb.Router do
  use Plug.Router

  plug :match

  plug Plug.Parsers,
      parsers: [{:json, length: 1_000_000}],
      pass: ["application/json"],
      json_decoder: JSON

  plug :dispatch

  forward "/mcp",
      to: Phantom.Plug,
      init_opts: [
        router: MyApp.MCP.Router,
        pubsub: MyApp.PubSub
      ]
end

Here are the defaults:

[
  pubsub: nil,
  origins: ["http://localhost:4000"],
  validate_origin: true,
  session_timeout: 30000,
  max_request_size: 1048576
]

Telemetry

Telemetry is provided with these events:

• [:phantom, :plug, :request, :connect] with meta: ~w[session router conn opts]a
• [:phantom, :plug, :request, :disconnect] with meta: ~w[session router conn]a
• [:phantom, :plug, :request, :terminate] with meta: ~w[session router conn]a
• [:phantom, :plug, :request, :exception] with meta: ~w[session router conn stacktrace request exception]a