Cannot Connect to WebSocket via Http/2

[adiwajshing]

Bug Description

On some domains, over TLS, http/2 web socket connects fail without a good explanation. Consistently reproducing w the domain in the example.

Reproduction

Standalone reproduction script:

import { Agent, WebSocket } from 'undici'
// turning this flag off, prints success
const dispatcher = new Agent({ allowH2: true })

const url = 'wss://web.whatsapp.com/ws/chat'
const ws = new WebSocket(url, { dispatcher })

ws.onopen = () => {
	console.log('Success!')
	ws.close()
}

ws.onerror = (err) => {
	console.log('error', err)
}

ws.onclose = ev => {
	console.log('close code', ev.code)
}

Steps to reproduce (if not using the script above):

Run the script above.

Expected Behavior

Successful connect

Actual Behavior

Disconnects w 1006

Environment

• OS: MacOS 26
• Node.js version: 26.1.0
• undici version: 8.x.x

[metcoder95]

The problem seems to root to the way the server responds to the H2 extended CONNECT request;
it is replying with empty headers when WS requires the Sec-WebSocket-Accept header, which is present in H1 but missed in H2.

It will be worth it to verify the endpoint works with H2 connections

[adiwajshing]

Ah too many non-compliant servers in the wild. I verified that Chrome also is using http/1.1 for this WS connection.
If you believe this is strictly a non-compliant server issue, please do close the issue.

The workaround has been to use an explicitly allowH2: false dispatcher