• Description: The framework for building MCP servers and clients in Common Lisp.
• Licence: Unlicense
• Author: Alexander Artemenko svetlyak.40wt@gmail.com
• Homepage: https://40ants.com/mcp/
• Bug tracker: https://github.com/40ants/mcp/issues
• Source control: GIT
• Depends on: alexandria, bordeaux-threads, cl-ppcre, clack, clack-sse, jsonrpc, lack, lack-request, lack-response, local-time, log4cl, log4cl-extras, openrpc-client, openrpc-server, sento, serapeum, sse-server, trivial-gray-streams, uuid, yason

A comprehensive framework for building Model Context Protocol (MCP) servers in Common Lisp. This library provides a complete implementation of the MCP specification with an easy-to-use API for creating servers that can interact with AI assistants like Claude Desktop.

Active development is ongoing and the interface is likely to change.

• ✅ STDIO Transport: Native support for STDIO-based communication
• ✅ StreamableHTTP Transport: Allowing to create remotely hosted MCP servers
• ✅ Tools System: Register and execute custom tools with JSON Schema validation
• ✅ Built on OpenRPC: Leverages the robust 40ants OpenRPC library
• ✅ CLOS-based: Object-oriented design with proper encapsulation
• ✅ Easy Integration: Simple API for adding functionality
• ✅ Error Handling: Comprehensive error management with proper JSON-RPC error codes
• ✅ Interactive Editing: MCP tools can be edited and updated on the fly, using REPL driven approach

• 🔄 Full MCP Specification Support: Complete implementation of MCP protocol version 2025-06-18
• 🔄 Resources System: Serve dynamic and static resources via URI
• 🔄 Prompts System: Provide prompt templates with argument interpolation
• 🔄 MCP Client Protocol: Implement client-side protocol for connecting to MCP servers

You can install this library from Quicklisp, but you want to receive updates quickly, then install it from Ultralisp.org:

(ql-dist:install-dist "http://dist.ultralisp.org/"
                      :prompt nil)
(ql:quickload :40ants-mcp)

Here's a quick example of how to create an MCP server with custom tools:

(defpackage #:my-mcp-server
  (:use #:cl)
  (:import-from #:40ants-mcp/content/text
                #:text-content)
  (:import-from #:openrpc-server))

(in-package #:my-mcp-server)

;; Define your API
(openrpc-server:define-api (my-tools :title "My Custom Tools"))

;; Define a tool that adds two numbers
(40ants-mcp/tools:define-tool (my-tools add) (a b)
  (:summary "Adds two numbers and returns the result.")
  (:param a integer "First number to add.")
  (:param b integer "Second number to add.")
  (:result (soft-list-of text-content))
  (list (make-instance 'text-content
                      :text (format nil "The sum of ~A and ~A is: ~A"
                                  a b (+ a b)))))

;; Start the server
(40ants-mcp/server/definition:start-server my-tools)

For production use, you can create a Roswell script. Create a file my-mcp.ros:

#!/bin/sh
#|-*- mode:lisp -*-|#
#|
exec ros -Q -- $0 \"$@\"
|#

(ql:quickload '(:40ants-mcp :alexandria) :silent t)

;; Your package and tool definitions here...

(defun main (&rest argv)
  (declare (ignore argv))
  (40ants-mcp/server/definition:start-server my-tools))

Build and run the script:

# Build the script
ros build my-mcp.ros

# Run the server
./my-mcp

# With remote debugging support
SLYNK_PORT=4005 ./my-mcp

Each tool you define should return a list of content items. The most common content type is text-content, but you can also return other types defined in the MCP specification.

For more examples, check the examples/ directory in the source code.

Readers

Readers

HTTP transport implementation for MCP (Model Context Protocol) communication. This class handles JSON-RPC message exchange via HTTP POST requests.

Readers

Lack application instance

Function to handle incoming messages

Port number to listen on.

Flag indicating if transport is active

Clack server instance

Accessors

Lack application instance

Function to handle incoming messages

Flag indicating if transport is active

Clack server instance

Readers

Start the MCP server with specified transport. TRANSPORT can be :stdio or :http. PORT is only used when transport is :http.

You should signal this error in case if the tool can't accomplish it's job.

Readers

STDIO transport implementation for MCP (Model Context Protocol) communication. This class handles JSON-RPC message exchange via standard input/output streams. It is designed to work with the MCP protocol specification for AI model communication.

Readers

Input stream for reading JSON-RPC messages. Defaults to standard-input.

Output stream for writing JSON-RPC responses. Defaults to standard-output.

Flag indicating if transport is active and processing messages.

Accessors

Input stream for reading JSON-RPC messages. Defaults to standard-input.

Output stream for writing JSON-RPC responses. Defaults to standard-output.

Flag indicating if transport is active and processing messages.

Receive a JSON-RPC message, returns a message or NIL.

Send a JSON-RPC message, returns no values.

Receive a JSON-RPC message, returns a message or NIL.

Send a JSON-RPC message, returns no values.

Starts message processing using given transport.

Stops message processing using given transport.