fix: native WebSocket over H2 server after undici import

lib/dispatcher/dispatcher1-wrapper.js

@@ -86,6 +86,12 @@ class Dispatcher1Wrapper extends Dispatcher {
   }
 
   dispatch (opts, handler) {
+    // Legacy (v1) consumers do not support HTTP/2, so force HTTP/1.1.
+    // See https://github.com/nodejs/undici/issues/4989
+    if (opts.allowH2 !== false) {
+      opts = { ...opts, allowH2: false }
+    }
+
     return this.#dispatcher.dispatch(opts, Dispatcher1Wrapper.wrapHandler(handler))
   }
 

test/websocket/issue-4989.js

@@ -0,0 +1,80 @@
+'use strict'
+
+// Regression test for https://github.com/nodejs/undici/issues/4989
+//
+// Importing undici v8 sets a global dispatcher (including the legacy
+// Symbol.for('undici.globalDispatcher.1') used by Node.js's bundled undici).
+// The new Agent defaults allowH2 → true, so TLS ALPN negotiates h2.
+// Undici v8's own fetch has a dispatchWithProtocolPreference fallback that
+// retries with allowH2: false when Extended CONNECT is unavailable, but
+// Node.js's bundled undici fetch does NOT have this fallback.
+// As a result, globalThis.WebSocket (backed by the bundled undici) breaks
+// when connecting to servers that advertise h2 but don't support RFC 8441.
+
+const { test } = require('node:test')
+const { once } = require('node:events')
+const { createSecureServer } = require('node:http2')
+
+const { tspl } = require('@matteo.collina/tspl')
+const { WebSocketServer } = require('ws')
+const { key, cert } = require('@metcoder95/https-pem')
+
+// Self-signed certs require this since native WebSocket uses the
+// bundled dispatcher which has no rejectUnauthorized override.
+process.env.NODE_TLS_REJECT_UNAUTHORIZED = '0'
+
+// Importing undici sets the global dispatcher — this is what triggers the bug.
+require('../..')
+
+test('globalThis.WebSocket connects to h2+http1.1 server after undici import', async (t) => {
+  const planner = tspl(t, { plan: 2 })
+
+  // HTTP/2 server with HTTP/1.1 fallback.
+  // Advertises h2 in ALPN but does NOT enable Extended CONNECT (RFC 8441).
+  // WebSocket must fall back to HTTP/1.1 upgrade.
+  const server = createSecureServer({ cert, key, allowHTTP1: true })
+  const wsServer = new WebSocketServer({ noServer: true })
+
+  server.on('upgrade', (req, socket, head) => {
+    wsServer.handleUpgrade(req, socket, head, (ws) => {
+      wsServer.emit('connection', ws, req)
+    })
+  })
+
+  wsServer.on('connection', (ws) => {
+    ws.send('hello')
+  })
+
+  server.listen(0)
+  await once(server, 'listening')
+
+  t.after(async () => {
+    await new Promise((resolve) => wsServer.close(resolve))
+    await new Promise((resolve) => server.close(resolve))
+  })
+
+  // globalThis.WebSocket is Node.js's native WebSocket (backed by bundled undici).
+  // It reads the global dispatcher set by the undici v8 import above.
+  const ws = new globalThis.WebSocket(`wss://localhost:${server.address().port}`)
+
+  await Promise.race([
+    new Promise((resolve, reject) => {
+      ws.addEventListener('open', () => {
+        planner.ok(true, 'connection opened')
+      })
+      ws.addEventListener('message', (evt) => {
+        planner.strictEqual(evt.data, 'hello')
+        ws.close()
+        resolve()
+      })
+      ws.addEventListener('error', () => {
+        reject(new Error('native WebSocket failed — global dispatcher h2 not falling back'))
+      })
+    }),
+    new Promise((_resolve, reject) =>
+      setTimeout(() => reject(new Error('Timeout after 5s')), 5000)
+    )
+  ])
+
+  await planner.completed
+})