Minimal vanilla mcp server in python

minimal_mcp_server.py

import sys, json, inspect
from typing import get_type_hints

## MCP Server "Framework" ##

TOOLS, TOOL_DEFINITIONS = {}, []

def tool(f):
    TOOLS[f.__name__] = f
    TOOL_DEFINITIONS.append(tool_definition(f))

    return f

def write(data):
    data["jsonrpc"] = "2.0"
    print(json.dumps(data), flush=True)

def tool_definition(func):
    type_hints = get_type_hints(func)
    properties, required = {}, []

    type_map = {int: "integer", str: "string", float: "number", bool: "boolean", list: "array", dict: "object"}

    for name, param in inspect.signature(func).parameters.items():
        if param.default is inspect.Parameter.empty:
            required.append(name)

        properties[name] = {"title": name, "type": type_map[type_hints[name]]}

    return {
        "name": func.__name__,
        "description": (inspect.getdoc(func) or "").strip(),
        "inputSchema": {
            "properties": properties,
            "required": required,
            "type": "object"
        }
    }

def initialize(title, data):
    return {
        "id": data["id"],
        "result": {
            "protocolVersion": "2024-11-05",
            "capabilities": {
                "tools": {}
            },
            "serverInfo": {
                "name": title,
                "version": "1.0"
            }
        }
    }

def tools_list(data):
    return {"id": data["id"], "result": {"tools": TOOL_DEFINITIONS}}

def tools_call(data):
    try:
        tool_func = TOOLS[data["params"]["name"]]
        result, is_error = tool_func(**data["params"]["arguments"]), False
    except Exception as e:
        result, is_error = e, True

    return {
        "id": data["id"],
        "result": {
            "content": [{
                "type": "text",
                "text": str(result),
            }],
            "isError": is_error
        }
    }

def run(title):
    for line in sys.stdin:
        try:
            data = json.loads(line)
        except json.JSONDecodeError:
            continue

        method = data["method"]

        if method == "initialize":
            write(initialize(title, data))
        elif method == "tools/list":
            write(tools_list(data))
        elif method == "tools/call":
            write(tools_call(data))

## Example MCP server ##

@tool
def addition(num1: int, num2: int):
    """Addition of two numbers."""
    return num1 + num2

@tool
def subtraction(num1: int, num2: int):
    """Subtraction of two numbers."""
    return num1 - num2

@tool
def multiplication(num1: int, num2: int):
    """Multiplication of two numbers."""
    return num1 * num2

@tool
def division(num1: float, num2: float):
    """Division of two numbers."""
    if num2 == 0:
        raise ValueError("Cannot divide by zero")
    return num1 / num2

run("Basic math")