Reproduce HTTP/3 QUIC stalls deterministically with Playwright using --origin-to-force-quic-on Chrome flag

Problem

You suspect HTTP/3 (QUIC) is causing page hangs on a Cloudflare-fronted site, but headless browsers default to HTTP/2 (no cached alt-svc header), so you cannot reproduce the stall programmatically. Manual browser testing with chrome://flags is slow and not automatable.

Solution

Chromium has an --origin-to-force-quic-on flag that forces QUIC from the first request, bypassing alt-svc discovery. Combine it with Playwright (or Puppeteer) to build a deterministic repro harness.

Key flags:

• --enable-quic: enables QUIC support
• --origin-to-force-quic-on=example.com:443: forces QUIC for this origin from the first request
• --quic-version=h3: uses HTTP/3

Playwright example:

import { chromium } from 'playwright';

const browser = await chromium.launch({
  headless: true,
  args: [
    '--enable-quic',
    '--origin-to-force-quic-on=example.com:443',
    '--quic-version=h3',
  ],
});

const page = await (await browser.newContext()).newPage();

for (let i = 0; i < 20; i++) {
  const start = Date.now();
  try {
    await page.goto('https://example.com/page', {
      waitUntil: 'domcontentloaded',
      timeout: 15000,
    });
    console.log(`Run ${i + 1}: ${Date.now() - start}ms - ok`);
  } catch (err) {
    console.log(`Run ${i + 1}: ${Date.now() - start}ms - TIMEOUT`);
  }
}
await browser.close();

Run the same script without the QUIC flags as a control. In our case this produced:

• With QUIC forced: 20/20 timeout at 15s (100% failure)
• Without QUIC flags: 5/5 success in 30-611ms (100% success)

This is useful evidence for hosting provider support tickets (e.g. asking Render to disable HTTP/3 on their managed Cloudflare zone).

Why this works

Normally, browsers discover HTTP/3 support via the alt-svc response header, then cache it and upgrade on subsequent requests. Headless browsers start fresh each launch with no cached alt-svc, so they never upgrade to QUIC. The --origin-to-force-quic-on flag skips the discovery phase and uses QUIC immediately, matching what happens in a real browser that has already cached the alt-svc header.