MCP Core Concepts

Transports

Transports handle the underlying communication between clients and servers using JSON-RPC 2.0 messages. The transport layer is responsible for message serialization, delivery, and connection management.

stdio (Standard Input/Output)

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

// Client
const transport = new StdioClientTransport({
  command: "./mcp-server",
  args: ["--model", "gpt-4"]
});

• Best for: Anything local, such as command-line tools or scripting. Stdio excels when you need simple communication between processes on the same machine without network complexity.

• Benefits: Simple process isolation, easy debugging. Each server runs as a separate process with its own memory space.

• Limitations: Local only, single client. Stdio connections are inherently 1:1 and cannot be accessed remotely, making them unsuitable for web services or multi-user scenarios.

HTTP with SSE (Server-Sent Events)

// Endpoint handles both POST and SSE
app.post("/mcp/v1/session", async (req, res) => {
  const response = await server.handleRequest(req.body);
  
  if (needsStreaming) {
    res.setHeader("Content-Type", "text/event-stream");
    res.setHeader("Cache-Control", "no-cache");
    res.setHeader("Connection", "keep-alive");
    // Stream multiple responses...
  } else {
    res.json(response);
  }
});

• Best for: Anything remote, such as web integrations. HTTP+SSE enables MCP servers to be accessed from web browsers, cloud services, or any HTTP-capable client across networks.

• Benefits: Stateful sessions, resumable connections. Sessions persist across requests using session IDs, and clients can resume interrupted connections using the Last-Event-ID header.

• Security: Requires origin validation, should bind to localhost when local. Always check the Origin header to prevent DNS rebinding attacks, and use 127.0.0.1 instead of 0.0.0.0 for local services.