MCP Protocol Deep Dive: The Standard Behind Every Server

from mcp.server.fastmcp import FastMCP
 
mcp = FastMCP("weather-demo")
 
 
# A TOOL: model-controlled action. The type hints below become the
# tool's inputSchema; the return value becomes its result content.
@mcp.tool()
def get_forecast(city: str, days: int = 1) -> str:
    """Return a short weather forecast for a city."""
    # Real code would call a weather API here.
    return f"{city}: sunny, 24C for the next {days} day(s)."
 
 
# A RESOURCE: application-driven data, addressed by an RFC 3986 URI.
@mcp.resource("weather://stations")
def list_stations() -> str:
    """The set of stations this server knows about."""
    return "KSFO, KJFK, EGLL"
 
 
# A PROMPT: user-controlled workflow, surfaced as a slash command.
@mcp.prompt()
def trip_briefing(city: str) -> str:
    """Draft a travel weather briefing for a city."""
    return f"Write a concise weather briefing for a trip to {city}."
 
 
if __name__ == "__main__":
    # stdio transport: the host launches this file as a subprocess and
    # speaks newline-delimited JSON-RPC over stdin/stdout.
    mcp.run(transport="stdio")

import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js";
import { z } from "zod";
 
// @modelcontextprotocol/sdk is 1.29.0 on npm as of 2026-06-15.
const server = new McpServer({ name: "weather-demo", version: "1.0.0" });
 
// A tool. The Zod schema is published to clients as the JSON-Schema
// inputSchema, so the model knows exactly what arguments to send.
server.registerTool(
  "get_forecast",
  {
    title: "Get forecast",
    description: "Return a short weather forecast for a city.",
    inputSchema: { city: z.string(), days: z.number().default(1) },
  },
  async ({ city, days }) => ({
    content: [{ type: "text", text: `${city}: sunny, 24C for ${days} day(s).` }],
  }),
);
 
// stdio transport — connect and start serving.
await server.connect(new StdioServerTransport());