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 is a comprehensive Elixir SDK for the Model Context Protocol, aiming to provide complete client and server implementations. The library leverages Elixir's exceptional concurrency model and fault tolerance capabilities to deliver a robust MCP experience.

Currently, Hermes MCP offers a feature-complete client implementation conforming to the MCP 2024-11-05 specification. Server-side components are planned for future releases.

Roadmap

Current Status

- [x] Complete client implementation (MCP 2024-11-05)
- [x] Multiple transport options (STDIO, HTTP/SSE, and WebSocket)
- [x] Built-in connection supervision and automatic recovery
- [x] Comprehensive capability negotiation
- [x] Progress tracking and cancellation support
- [x] Structured logging system

Upcoming

- [ ] Client support for MCP 2025-03-26 specification
- Authorization framework (OAuth 2.1)
- Streamable HTTP transport
- JSON-RPC batch operations
- Enhanced tool annotations

- [ ] Server Implementation
- STDIO transport
- HTTP/SSE transport
- Streamable HTTP transport
- Support for resources, tools, and prompts

- [ ] Sample Implementations
- Reference servers
- Integration examples with popular Elixir libraries

For a more detailed roadmap, see ROADMAP.md.

Installation

Library Usage

Add Hermes MCP to your dependencies in mix.exs:

def deps do
  [
    {:hermes_mcp, "~> 0.5"}  # x-release-please-version
  ]
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 WebSocket server
hermes_cli --transport websocket --base-url="http://localhost:8000" --base-path="/mcp" --ws-path="/ws"
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 WebSocket server
mix hermes.websocket.interactive --base-url="http://localhost:8000" --base-path="/mcp" --ws-path="/ws"
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

> [!IMPORTANT]
> Process Startup Order: Always start the client process before the transport process. The client hibernates waiting for the transport's :initialize message, so this order prevents race conditions during initialization.

defmodule MyApp.Application do
  use Application
def start(_type, _args) do

    children = [

      # IMPORTANT: Start the client first - it will hibernate waiting for transport's :initialize message

      {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" => %{}

        }

      ]},

      

      # Start the transport after the client

      {Hermes.Transport.STDIO, [

        name: MyApp.MCPTransport,

        client: MyApp.MCPClient, 

        command: "mcp",

        args: ["run", "path/to/server.py"]

      ]}

    ]

    

    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.