mcp-server-builder

MCP Server Builder Skill

MCP Architecture

An MCP server exposes three primitives to AI clients:

Primitive	Purpose	When to Use
Tools	Callable functions the agent invokes (tools/call)	Actions, API calls, computations
Resources	Read-only data the agent fetches (resources/read)	Files, database records, configuration
Prompts	Reusable prompt templates (prompts/get)	Guided workflows, structured interactions

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Option A: Python with FastMCP (Recommended)

pip install fastmcp

Minimal Server

# server.py
from fastmcp import FastMCP

mcp = FastMCP("My Tools Server", version="1.0.0")

@mcp.tool(description="Add two numbers together")
def add(a: float, b: float) -> float:
    """Add two numbers and return the result."""
    return a + b

@mcp.tool(description="Fetch the current weather for a city")
async def get_weather(city: str, unit: str = "celsius") -> dict:
    """Get current weather. unit: 'celsius' or 'fahrenheit'"""
    import httpx
    async with httpx.AsyncClient() as client:
        resp = await client.get(f"https://wttr.in/{city}?format=j1")
        resp.raise_for_status()
        data = resp.json()
    return {
        "city": city,
        "temperature": data["current_condition"][0]["temp_C" if unit == "celsius" else "temp_F"],
        "condition": data["current_condition"][0]["weatherDesc"][0]["value"],
    }

if __name__ == "__main__":
    mcp.run()  # stdio mode (default)

Run Modes

# stdio (for Claude Code, Cursor - recommended for local tools)
python server.py

# SSE (for browser clients and remote access)
MCP_TRANSPORT=sse python server.py           # http://localhost:8000/sse

# Streamable HTTP (modern, bidirectional)
MCP_TRANSPORT=streamable-http python server.py

Resources

@mcp.resource("config://app-settings")
def get_app_settings() -> str:
    """Return current application settings."""
    import json
    return json.dumps({
        "version": "1.0.0",
        "features": ["auth", "analytics"],
        "max_users": 1000,
    })

@mcp.resource("file://{path}")  # template URI with parameter
def read_file(path: str) -> str:
    """Read a file by path."""
    import pathlib
    # IMPORTANT: validate path to prevent traversal
    base = pathlib.Path("./allowed_dir").resolve()
    target = (base / path).resolve()
    if base not in target.parents and base != target:
        raise ValueError("Path outside allowed directory")
    return target.read_text()