Building MCP Servers.

{
  "name": "acme-time-mcp",
  "type": "module",
  "bin": { "acme-time-mcp": "./dist/index.js" },
  "dependencies": {
    "@modelcontextprotocol/sdk": "^1.0.0",
    "zod": "^3.23.0"
  }
}

#!/usr/bin/env node
import { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js';
import { StdioServerTransport } from '@modelcontextprotocol/sdk/server/stdio.js';
import { z } from 'zod';

const server = new McpServer({ name: 'acme-time', version: '1.0.0' });

server.registerTool(
  'currentTime',
  {
    title: 'Current time',
    description: 'Returns the current time in a given IANA timezone.',
    inputSchema: { zone: z.string().describe('e.g. America/New_York') }
  },
  async ({ zone }) => {
    const now = new Date().toLocaleString('en-US', { timeZone: zone });
    return { content: [{ type: 'text', text: now }] };
  }
);

const transport = new StdioServerTransport();
await server.connect(transport);

{
  "mcpServers": {
    "acme-time": {
      "command": "npx",
      "args": ["-y", "acme-time-mcp"]
    }
  }
}

import express from 'express';
import { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js';
import { StreamableHTTPServerTransport } from '@modelcontextprotocol/sdk/server/streamableHttp.js';
import { z } from 'zod';

function build() {
  const server = new McpServer({ name: 'acme-search', version: '1.0.0' });

  server.registerTool(
    'searchDocs',
    {
      title: 'Search internal docs',
      description: 'Full-text search over the company docs corpus.',
      inputSchema: {
        q:     z.string().min(2).describe('query'),
        limit: z.number().int().min(1).max(20).default(5)
      }
    },
    async ({ q, limit }) => {
      const hits = await searchCorpus(q, limit); // your code
      return { content: [{ type: 'text', text: JSON.stringify(hits) }] };
    }
  );
  return server;
}

const app = express();
app.use(express.json());

app.post('/mcp', async (req, res) => {
  const server = build();
  const transport = new StreamableHTTPServerTransport({
    sessionIdGenerator: undefined // stateless: simplest, scales horizontally
  });
  res.on('close', () => { transport.close(); server.close(); });
  await server.connect(transport);
  await transport.handleRequest(req, res, req.body);
});

app.listen(3000);

app.post('/mcp', async (req, res) => {
  const user = await authenticate(req); // throws 401 on failure
  if (!user) return res.status(401).end();

  const server = buildFor(user); // tools close over user.id, user.scope
  const transport = new StreamableHTTPServerTransport({ sessionIdGenerator: undefined });
  res.on('close', () => { transport.close(); server.close(); });
  await server.connect(transport);
  await transport.handleRequest(req, res, req.body);
});