Hermes MCP

[!WARNING]

This library is under active development, may expect breaking changes

A high-performance Model Context Protocol (MCP) implementation in Elixir.

Overview

Hermes MCP provides a robust client implementation for the Model Context Protocol, leveraging Elixir's exceptional concurrency model and fault tolerance capabilities.

Features

• Complete client implementation with protocol lifecycle management
• Multiple transport options (STDIO and HTTP/SSE)
• Built-in connection supervision and automatic recovery
• Comprehensive capability negotiation
• Progress notification support for tracking long-running operations
• Structured logging system with log level control

Installation

Library Usage

Add Hermes MCP to your dependencies in mix.exs:

def deps do
  [
    {:hermes_mcp, "~> 0.3"}
  ]
end

Standalone CLI Installation

Download the appropriate binary for your platform from the GitHub releases page.

# Make it executable (Linux/macOS)
# or hermes_cli-macos-intel, hermes_cli-macos-arm
mv hermes_mcp_linux hermes-mcp && chmod +x hermes-mcp

# Run it
./hermes-mcp --transport sse --base-url="http://localhost:8000"

Quick Start

Interactive Testing

Hermes MCP provides interactive tools for testing MCP servers with a user-friendly CLI.

Using the CLI Binary:

# Test an SSE server
hermes_cli --transport sse --base-url="http://localhost:8000" --base-path="/mcp"

# Test a local process via STDIO
hermes_cli --transport stdio --command="mcp" --args="run,path/to/server.py"

Using Mix Tasks (For Elixir Developers):

# Test an SSE server
mix hermes.sse.interactive --base-url="http://localhost:8000" --base-path="/mcp"

# Test a local process via STDIO
mix hermes.stdio.interactive --command="mcp" --args="run,path/to/server.py"

These interactive shells provide commands for listing and calling tools, exploring prompts, and accessing resources.

Setting up a Client

defmodule MyApp.Application do
  use Application

  def start(_type, _args) do
    children = [
      # Start the MCP transport
      {Hermes.Transport.STDIO, [
        name: MyApp.MCPTransport,
        client: MyApp.MCPClient, 
        command: "mcp",
        args: ["run", "path/to/server.py"]
      ]},
      
      # Start the MCP client using the transport
      {Hermes.Client, [
        name: MyApp.MCPClient,
        transport: [layer: Hermes.Transport.STDIO, name: MyApp.MCPTransport],
        client_info: %{
          "name" => "MyApp",
          "version" => "1.0.0"
        },
        capabilities: %{
          "roots" => %{"listChanged" => true},
          "sampling" => %{}
        }
      ]}
    ]
    
    opts = [strategy: :one_for_all, name: MyApp.Supervisor]
    Supervisor.start_link(children, opts)
  end
end

Making Client Requests

# Call a tool
{:ok, result} = Hermes.Client.call_tool(MyApp.MCPClient, "example_tool", %{"param" => "value"})

# Handle errors properly
case Hermes.Client.call_tool(MyApp.MCPClient, "unavailable_tool", %{}) do
  {:ok, %Hermes.MCP.Response{}} ->
    # Handle successful result
    
  {:error, %Hermes.MCP.Error{} = err} ->
    # Handle error response
    IO.puts(inspect(err, pretty: true))
end

Documentation

For detailed guides and examples, visit the official documentation

Why Hermes?

The library is named after Hermes, the Greek god of boundaries, communication, and commerce. This namesake reflects the core purpose of the Model Context Protocol: to establish standardized communication between AI applications and external tools.

Like Hermes who served as a messenger between gods and mortals, this library facilitates seamless interaction between Large Language Models and various data sources or tools.

Local Development

# Setup the project
mix setup

# Run tests
mix test

# Code quality
mix lint

# Start development MCP servers
# Echo server (Python)
just echo-server
# Calculator server (Go)
just calculator-server

The MCP servers in priv/dev require:

• Python 3.11+ with uv (for echo server)
• Go 1.21+ (for calculator server)

See CONTRIBUTING.md for detailed contribution guidelines.

License

Hermes MCP is released under the MIT License. See LICENSE for details.