Expose a local MCP server

Serve a remote Model Context Protocol (MCP) server from your laptop at https://my-app.hrzn.run/mcp, so a client such as Claude can connect to it.

Horizon streams responses unbuffered, so Server-Sent Events (SSE) responses from your MCP server reach the client as they are written.

Before you beginCopy Anchor Link

• Node.js 18 or later
• A Horizon account and the CLI (see Getting started)
• A reserved subdomain for -s. Reserve one on the Subdomains page.
• Node.js 22.19.0 or later, if you want to test with the MCP Inspector

npm install @modelcontextprotocol/server @modelcontextprotocol/node @modelcontextprotocol/express express zod

import { createMcpExpressApp } from "@modelcontextprotocol/express";
import { toNodeHandler } from "@modelcontextprotocol/node";
import { createMcpHandler, McpServer } from "@modelcontextprotocol/server";
import * as z from "zod/v4";

const handler = createMcpHandler(() => {
  const server = new McpServer({ name: "notes", version: "1.0.0" });
  server.registerTool(
    "add-note",
    { description: "Append a note", inputSchema: z.object({ text: z.string() }) },
    async ({ text }) => ({
      content: [{ type: "text", text: `Saved: ${text}` }],
    }),
  );
  return server;
});

const app = createMcpExpressApp();
const node = toNodeHandler(handler);
app.all("/mcp", (req, res) => void node(req, res, req.body));
app.listen(3000);

npx tsx server.ts

hrzn tunnel http://localhost:3000 -s my-app

HORIZON: Tunnel connected
  URL          https://my-app.hrzn.run (reserved)
  Forwarding   http://localhost:3000
  Request log  https://hrzn.run/dashboard/tunnels/my-app

npx @modelcontextprotocol/inspector --server-url https://my-app.hrzn.run/mcp --transport http

Check it worksCopy Anchor Link

In the Inspector, connect and list the tools. You should see add-note. Call it with text set to hello. The result contains:

Saved: hello

Your Horizon terminal prints one line per call:

  POST    200  /mcp

TroubleshootingCopy Anchor Link

The client can't connectCopy Anchor Link

• Check that the Horizon terminal is still running.
• Check that the URL ends with /mcp, the path in app.all("/mcp", ...).
• Check that npx tsx server.ts is still running on port 3000.

The server rejects requests from the tunnelCopy Anchor Link

createMcpExpressApp validates Host and Origin against localhost values by default. Horizon sends Host: localhost:3000, so the default passes. If you pass allowedHosts, keep localhost in the list.

The URL changed after a restartCopy Anchor Link

You started the tunnel without -s, so Horizon gave you a new random subdomain. Restart with -s my-app and the URL stays the same. -s needs a subdomain you reserved, see Pricing.

Next stepsCopy Anchor Link

• Read the SDK's Express serving guide.
• Read MCP's guide to connecting to remote servers.