WebSocket connections

This guide shows you how to work with WebSocket servers running in your sandboxes.

Choose your approach

Expose via preview URL - Get a public URL for external clients to connect to. Best for public chat rooms, multiplayer games, or real-time dashboards.

Connect with wsConnect() - Your Worker establishes the WebSocket connection. Best for custom routing logic, authentication gates, or when your Worker needs real-time data from sandbox services.

Connect to WebSocket echo server

Create the echo server:

echo-server.tstypescript

Bun.serve({
	port: 8080,
	hostname: "0.0.0.0",
	fetch(req, server) {
		if (server.upgrade(req)) {
			return;
		}
		return new Response("WebSocket echo server");
	},
	websocket: {
		message(ws, message) {
			ws.send(`Echo: ${message}`);
		},
		open(ws) {
			console.log("Client connected");
		},
		close(ws) {
			console.log("Client disconnected");
		},
	},
});

console.log("WebSocket server listening on port 8080");

Extend the Dockerfile:

Dockerfiledockerfile

FROM docker.io/cloudflare/sandbox:0.3.3

# Copy echo server into the container
COPY echo-server.ts /workspace/echo-server.ts

# Create custom startup script
COPY startup.sh /container-server/startup.sh
RUN chmod +x /container-server/startup.sh

Create startup script:

startup.shbash

#!/bin/bash
# Start your WebSocket server in the background
bun /workspace/echo-server.ts &
# Start SDK's control plane (needed for the SDK to work)
exec bun dist/index.js

Connect from your Worker:

import { getSandbox } from "@cloudflare/sandbox";

export { Sandbox } from "@cloudflare/sandbox";

export default {
	async fetch(request, env) {
		if (request.headers.get("Upgrade")?.toLowerCase() === "websocket") {
			const sandbox = getSandbox(env.Sandbox, "echo-service");
			return await sandbox.wsConnect(request, 8080);
		}

		return new Response("WebSocket endpoint");
	},
};

import { getSandbox } from '@cloudflare/sandbox';

export { Sandbox } from "@cloudflare/sandbox";

export default {
  async fetch(request: Request, env: Env): Promise<Response> {
    if (request.headers.get('Upgrade')?.toLowerCase() === 'websocket') {
      const sandbox = getSandbox(env.Sandbox, 'echo-service');
      return await sandbox.wsConnect(request, 8080);
    }

    return new Response('WebSocket endpoint');

}
};

Client connects:

const ws = new WebSocket('wss://your-worker.com');
ws.onmessage = (event) => console.log(event.data);
ws.send('Hello!'); // Receives: "Echo: Hello!"

Expose WebSocket service via preview URL

Get a public URL for your WebSocket server:

import { getSandbox, proxyToSandbox } from "@cloudflare/sandbox";

export { Sandbox } from "@cloudflare/sandbox";

export default {
	async fetch(request, env) {
		// Auto-route all requests via proxyToSandbox first
		const proxyResponse = await proxyToSandbox(request, env);
		if (proxyResponse) return proxyResponse;

		// Extract hostname from request
		const { hostname } = new URL(request.url);
		const sandbox = getSandbox(env.Sandbox, "echo-service");

		// Expose the port to get preview URL
		const { url } = await sandbox.exposePort(8080, { hostname });

		// Return URL to clients
		if (request.url.includes("/ws-url")) {
			return Response.json({ url: url.replace("https", "wss") });
		}

		return new Response("Not found", { status: 404 });
	},
};

import { getSandbox, proxyToSandbox } from '@cloudflare/sandbox';

export { Sandbox } from '@cloudflare/sandbox';

export default {
  async fetch(request: Request, env: Env): Promise<Response> {
    // Auto-route all requests via proxyToSandbox first
    const proxyResponse = await proxyToSandbox(request, env);
    if (proxyResponse) return proxyResponse;

    // Extract hostname from request
    const { hostname } = new URL(request.url);
    const sandbox = getSandbox(env.Sandbox, 'echo-service');

    // Expose the port to get preview URL
    const { url } = await sandbox.exposePort(8080, { hostname });

    // Return URL to clients
    if (request.url.includes('/ws-url')) {
      return Response.json({ url: url.replace('https', 'wss') });
    }

    return new Response('Not found', { status: 404 });

}
};

Client connects to preview URL:

// Get the preview URL
const response = await fetch('https://your-worker.com/ws-url');
const { url } = await response.json();

// Connect
const ws = new WebSocket(url);
ws.onmessage = (event) => console.log(event.data);
ws.send('Hello!'); // Receives: "Echo: Hello!"

Connect from Worker to get real-time data

Your Worker can connect to a WebSocket service to get real-time data, even when the incoming request isn't a WebSocket:

import { getSandbox } from "@cloudflare/sandbox";

export { Sandbox } from "@cloudflare/sandbox";

let initialized = false;

export default {
	async fetch(request, env) {
		// Get or create a sandbox instance
		const sandbox = getSandbox(env.Sandbox, "data-processor");

		// Check for WebSocket upgrade
		const upgrade = request.headers.get("Upgrade")?.toLowerCase();

		if (upgrade === "websocket") {
			// Initialize server on first connection
			if (!initialized) {
				await sandbox.writeFile(
					"/workspace/server.js",
					`Bun.serve({
            port: 8080,
            fetch(req, server) {
              server.upgrade(req);
            },
            websocket: {
              message(ws, msg) {
                ws.send(\`Echo: \${msg}\`);
              }
            }
          });`,
				);
				await sandbox.startProcess("bun /workspace/server.js");
				initialized = true;
			}
			// Connect to WebSocket server
			return await sandbox.wsConnect(request, 8080);
		}

		return new Response("Processed real-time data");
	},
};

import { getSandbox } from '@cloudflare/sandbox';

export { Sandbox } from '@cloudflare/sandbox';

let initialized = false;

export default {
  async fetch(request: Request, env: Env): Promise<Response> {

     // Get or create a sandbox instance
    const sandbox = getSandbox(env.Sandbox, 'data-processor');


    // Check for WebSocket upgrade
    const upgrade = request.headers.get('Upgrade')?.toLowerCase();

    if (upgrade === 'websocket') {
      // Initialize server on first connection
      if (!initialized) {
        await sandbox.writeFile(
          '/workspace/server.js',
          `Bun.serve({
            port: 8080,
            fetch(req, server) {
              server.upgrade(req);
            },
            websocket: {
              message(ws, msg) {
                ws.send(\`Echo: \${msg}\`);
              }
            }
          });`
        );
        await sandbox.startProcess(
          'bun /workspace/server.js'
        );
        initialized = true;
      }
      // Connect to WebSocket server
      return await sandbox.wsConnect(request, 8080);
    }

    return new Response('Processed real-time data');

}
};

This pattern is useful when you need streaming data from sandbox services but want to return HTTP responses to clients.

Troubleshooting

Upgrade failed

Verify request has WebSocket headers:

console.log(request.headers.get("Upgrade")); // 'websocket'
console.log(request.headers.get("Connection")); // 'Upgrade'

console.log(request.headers.get('Upgrade'));    // 'websocket'
console.log(request.headers.get('Connection')); // 'Upgrade'

Local development

Expose ports in Dockerfile for wrangler dev: