Edison Flores

Posted on Jul 7

How to build an MCP server with HTTP transport (not just stdio)

#mcp #tutorial #vercel #api

The problem

Most MCP (Model Context Protocol) servers only support stdio transport — you run them as a subprocess:

{
  "mcpServers": {
    "myserver": {
      "command": "npx",
      "args": ["-y", "my-mcp-server"]
    }
  }
}

But what if you want to expose your MCP server over HTTP? So that:

Remote clients can connect without installing anything locally
Multiple agents can share the same server instance
You can deploy it on Vercel/Cloudflare/AWS Lambda

The MCP spec supports HTTP transport (SSE + JSON-RPC), but there's barely any documentation on how to implement it.

This post is the guide I wish I had when building MarketNow's MCP endpoint.

The architecture

Client (Claude Desktop, Cursor, etc.)
    │
    ├── POST /api/mcp  → JSON-RPC 2.0 request/response
    │
    └── GET  /api/mcp  → SSE stream (Server-Sent Events)

POST handles initialize, tools/list, tools/call (JSON-RPC 2.0)
GET opens an SSE stream for server-to-client messages

The code (Vercel serverless function)

// api/mcp.js — Vercel serverless function

export default async function handler(req, res) {
  // CORS
  res.setHeader('Access-Control-Allow-Origin', '*');
  res.setHeader('Access-Control-Allow-Methods', 'GET, POST, OPTIONS');
  res.setHeader('Access-Control-Allow-Headers', 'Content-Type, Authorization');

  if (req.method === 'OPTIONS') {
    res.status(200).end();
    return;
  }

  // GET → SSE stream
  if (req.method === 'GET') {
    res.writeHead(200, {
      'Content-Type': 'text/event-stream',
      'Cache-Control': 'no-cache',
      Connection: 'keep-alive',
    });
    res.write(`data: ${JSON.stringify({ type: 'connected' })}\n\n`);
    return;
  }

  // POST → JSON-RPC
  if (req.method === 'POST') {
    const { jsonrpc, id, method, params } = req.body;
    let result;

    switch (method) {
      case 'initialize':
        result = {
          protocolVersion: '2024-11-05',
          capabilities: { tools: {} },
          serverInfo: { name: 'my-server', version: '1.0.0' },
        };
        break;

      case 'tools/list':
        result = {
          tools: [{
            name: 'hello',
            description: 'Say hello',
            inputSchema: { type: 'object', properties: { name: { type: 'string' } } },
          }],
        };
        break;

      case 'tools/call':
        result = {
          content: [{ type: 'text', text: `Hello, ${params.arguments.name}!` }],
        };
        break;

      default:
        res.status(200).json({
          jsonrpc: '2.0', id,
          error: { code: -32601, message: `Method not found: ${method}` },
        });
        return;
    }

    res.status(200).json({ jsonrpc: '2.0', id, result });
    return;
  }

  res.status(405).json({ error: 'Method not allowed' });
}

The .well-known/mcp.json

Clients discover your MCP server via .well-known/mcp.json:

{
  "name": "my-server",
  "version": "1.0.0",
  "transport": {
    "type": "http+sse",
    "http_url": "https://yourdomain.com/api/mcp",
    "sse_url": "https://yourdomain.com/api/mcp"
  },
  "tools": [
    { "name": "hello", "description": "Say hello" }
  ]
}

Testing it

# Test initialize
curl -X POST https://yourdomain.com/api/mcp \
  -H 'Content-Type: application/json' \
  -d '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{}}}'

# Test tools/list
curl -X POST https://yourdomain.com/api/mcp \
  -H 'Content-Type: application/json' \
  -d '{"jsonrpc":"2.0","id":2,"method":"tools/list"}'

# Test tools/call
curl -X POST https://yourdomain.com/api/mcp \
  -H 'Content-Type: application/json' \
  -d '{"jsonrpc":"2.0","id":3,"method":"tools/call","params":{"name":"hello","arguments":{"name":"World"}}}'

Deploy to Vercel

Create api/mcp.js in your project
Deploy: vercel --prod
Your MCP server is live at https://yourproject.vercel.app/api/mcp

Why this matters

HTTP transport means:

No installation required — clients connect via URL
Multi-tenant — one server serves many agents
Scalable — runs on serverless (Vercel, Cloudflare Workers, AWS Lambda)
Stateless — each request is independent

This is how MarketNow works — one Vercel serverless function serves all MCP clients.

Try it

MarketNow's MCP endpoint is live:

curl -X POST https://marketnow.site/api/mcp \
  -H 'Content-Type: application/json' \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/list"}'

Returns 4 tools: search_skills, get_skill, list_categories, health.

This post was written while building MarketNow — the trust layer for agent commerce. 8,764 MCP servers, each security-audited.

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