WebSockets (4)

Last updated Jan 28, 2026 - WebSockets Reference - Using WebSockets

Server

Miniflare will always upgrade Web Socket connections. The Worker must respond with a status 101 Switching Protocols response including a webSocket. For example, the Worker below implements an echo WebSocket server:

export default {
    fetch(request) {
        const [client, server] = Object.values(new WebSocketPair());

        server.accept();
        server.addEventListener("message", (event) => {
            server.send(event.data);
        });

        return new Response(null, {
            status: 101,
            webSocket: client,
        });
    },
};

When using dispatchFetch, you are responsible for handling WebSockets by using the webSocket property on Response. As an example, if the above worker script was stored in echo.mjs:

import { Miniflare } from "miniflare";

const mf = new Miniflare({
    modules: true,
    scriptPath: "echo.mjs",
});

const res = await mf.dispatchFetch("https://example.com", {
    headers: {
        Upgrade: "websocket",
    },
});
const webSocket = res.webSocket;
webSocket.accept();
webSocket.addEventListener("message", (event) => {
    console.log(event.data);
});

webSocket.send("Hello!"); // Above listener logs "Hello!"

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description: Attach a Node.js debugger to Miniflare for setting breakpoints and inspecting Cloudflare Workers code. title: Attaching a Debugger image: https://developers.cloudflare.com/workers/testing/miniflare/developing/debugger/og.png?v=7249a4a711859c61

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