Vancouver makes it easy to add Model Context Protocol (MCP) functionality to your Phoenix/Bandit server. Vancouver handles initialization, request validation, and offers helper functions to simplify the creation of MCP tools and prompts.

The goal is to let you create MCP servers with working tools/prompts in minutes, without needing to know much about the MCP protocol.

How it works

You create tools like this:

defmodule MyApp.Tools.CalculateSum do
  use Vancouver.Tool

  def name, do: "calculate_sum"
  def description, do: "Add two numbers together"

  def input_schema do
    %{
      "type" => "object",
      "properties" => %{
        "a" => %{"type" => "number"},
        "b" => %{"type" => "number"}
      },
      "required" => ["a", "b"]
    }
  end

  def run(conn, %{"a" => a, "b" => b}) do
    send_text(conn, "#{a + b}")
  end
end

create prompts like this:

defmodule MyApp.Prompts.CodeReview do
  use Vancouver.Prompt

  def name, do: "code_review"
  def description, do: "Asks the LLM to analyze code quality and suggest improvements"

  def arguments do
    [
      %{
        "name" => "code",
        "description" => "The code to review",
        "required" => true
      }
    ]
  end

  def run(conn, %{"code" => code}) do
    send_text(conn, "Please review this code: #{code}")
  end
end

and configure via your (phoenix) router like this:

forward "/mcp", Vancouver.Router, 
  tools: [MyApp.Tools.CalculateSum],
  prompts: [MyApp.Prompts.CodeReview]

To use your tools locally with e.g. Claude Desktop, you can update your claude_desktop_config.json file (see below), run your server, and refresh Claude Desktop.

{
    "mcpServers": {
        "MyApp": {
            "command": "npx",
            "args": [
                "mcp-remote",
                "http://localhost:4000/mcp"
            ]
        }
    }
}

More info

• Model Context Protocol
• Vancouver (GitHub)
• Vancouver (Hex)