fix: support WebSocket upgrades in Vite dev server

WebSocket upgrade requests never reached Fresh handlers in dev mode
because Vite's Connect-based server fires 'upgrade' events on the
underlying http.Server rather than routing them through middleware.

Fix by starting a Deno HTTP server on a random port alongside Vite
and proxying upgrade requests to it via raw TCP socket forwarding.
This allows Deno.upgradeWebSocket() to work natively.

Fixes #3350

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: bartlomieju

packages/plugin-vite/demo/routes/tests/ws.tsx

@@ -0,0 +1,17 @@
+import type { FreshContext } from "fresh";
+
+export const handler = {
+  GET(ctx: FreshContext) {
+    if (ctx.req.headers.get("upgrade") !== "websocket") {
+      return new Response("Not a WebSocket request", { status: 400 });
+    }
+
+    const { socket, response } = Deno.upgradeWebSocket(ctx.req);
+
+    socket.addEventListener("message", (event) => {
+      socket.send(`echo: ${event.data}`);
+    });
+
+    return response;
+  },
+};

packages/plugin-vite/src/plugins/dev_server.ts

@@ -1,5 +1,6 @@
 import type { DevEnvironment, Plugin } from "vite";
 import * as path from "@std/path";
+import { connect } from "node:net";
 import { ASSET_CACHE_BUST_KEY } from "fresh/internal";
 import { createRequest, sendResponse } from "@remix-run/node-fetch-server";
 import { hashCode } from "../shared.ts";
@@ -16,6 +17,70 @@ export function devServer(): Plugin[] {
       configureServer(server) {
         const IGNORE_URLS = /^\/(@(vite|fs|id)|\.vite)\//;
 
+        // Start a Deno HTTP server on a random port to handle WebSocket
+        // upgrades. Vite's Connect-based server fires 'upgrade' events
+        // on the underlying http.Server, but those never reach Connect
+        // middleware. We proxy upgrade requests to this Deno server
+        // where Deno.upgradeWebSocket() works natively.
+        let wsPort = 0;
+        const wsServer = Deno.serve(
+          { port: 0, onListen: ({ port }) => wsPort = port },
+          async (req) => {
+            try {
+              const mod = await server.ssrLoadModule("fresh:server_entry");
+              return (await mod.default.fetch(req)) as Response;
+            } catch {
+              return new Response("Internal Server Error", { status: 500 });
+            }
+          },
+        );
+
+        const originalClose = server.close;
+        server.close = async () => {
+          await wsServer.shutdown();
+          return originalClose.call(server);
+        };
+
+        server.httpServer?.on(
+          "upgrade",
+          (
+            req: {
+              url?: string;
+              method: string;
+              httpVersion: string;
+              rawHeaders: string[];
+            },
+            clientSocket: import("node:net").Socket,
+            head: Buffer,
+          ) => {
+            // Let Vite handle its own HMR WebSocket upgrades
+            if (
+              req.url === "/__vite_hmr" ||
+              req.url === "/__vite_ping"
+            ) {
+              return;
+            }
+
+            const proxySocket = connect(wsPort, "127.0.0.1", () => {
+              // Rebuild the HTTP upgrade request for the Deno server
+              let raw = `${req.method} ${req.url} HTTP/${req.httpVersion}\r\n`;
+              for (let i = 0; i < req.rawHeaders.length; i += 2) {
+                raw += `${req.rawHeaders[i]}: ${req.rawHeaders[i + 1]}\r\n`;
+              }
+              raw += "\r\n";
+
+              proxySocket.write(raw);
+              if (head.length > 0) proxySocket.write(head);
+
+              clientSocket.pipe(proxySocket);
+              proxySocket.pipe(clientSocket);
+            });
+
+            proxySocket.on("error", () => clientSocket.destroy());
+            clientSocket.on("error", () => proxySocket.destroy());
+          },
+        );
+
         server.middlewares.use(async (nodeReq, nodeRes, next) => {
           const serverCfg = server.config.server;
 

packages/plugin-vite/tests/dev_server_test.ts

@@ -536,6 +536,39 @@ Deno.test({
   sanitizeResources: false,
 });
 
+// issue: https://github.com/denoland/fresh/issues/3350
+Deno.test({
+  name: "vite dev - websocket upgrade",
+  fn: async () => {
+    const wsUrl = demoServer.address().replace("http", "ws");
+    const ws = new WebSocket(`${wsUrl}/tests/ws`);
+
+    const result = await new Promise<string>((resolve, reject) => {
+      const timeout = setTimeout(() => {
+        ws.close();
+        reject(new Error("WebSocket timed out"));
+      }, 5000);
+
+      ws.onopen = () => {
+        ws.send("hello");
+      };
+      ws.onmessage = (e) => {
+        clearTimeout(timeout);
+        resolve(e.data);
+        ws.close();
+      };
+      ws.onerror = (e) => {
+        clearTimeout(timeout);
+        reject(e);
+      };
+    });
+
+    expect(result).toEqual("echo: hello");
+  },
+  sanitizeResources: false,
+  sanitizeOps: false,
+});
+
 Deno.test({
   name: "vite dev - source mapped stack traces",
   fn: async () => {