Does your server support HTTP/3 for faster connections on lossy networks?
HTTP/3 runs over QUIC, eliminating connection stalls on flaky mobile and edge connections.
What this signal tests
We check whether your server supports HTTP/3, the latest version of the HTTP protocol. Unlike HTTP/2 (which runs over TCP), HTTP/3 runs over QUIC, a UDP-based transport that recovers faster from packet loss and handles network changes (e.g., switching from Wi-Fi to mobile) without dropping the connection. We detect support either by reading an Alt-Svc header advertising h3 or by attempting a direct QUIC handshake on UDP port 443.
Why it matters for your visibility in AI
HTTP/3 matters most on unreliable networks, where TCP head-of-line blocking causes long stalls. AI agent browsers operating from mobile devices, edge regions, or congested networks see meaningful improvements in fetch consistency, which reduces crawler timeouts on pages with many resources. This signal is weighted lower than HTTP/2 because adoption is still rolling out and HTTP/2 already covers the bulk of the performance benefit. Treat HTTP/3 as a forward-looking upgrade: low effort to enable via a CDN, modest benefit today, growing benefit as more agents and mobile clients standardise on QUIC. Sites on a modern CDN often have HTTP/3 enabled without realising it.
Pass criteria at a glance
| Criterion | Passes when |
|---|---|
| Alt-Svc contains 'h3=' OR direct QUIC ALPN h3 succeeds. |
How we test it
We send a HEAD request to your domain over HTTPS and inspect the Alt-Svc response header for an h3 token, which is the standard advertising mechanism. If we find it, the signal passes. As a secondary check, we attempt a direct QUIC handshake on UDP port 443 with ALPN h3 - some servers accept HTTP/3 connections without advertising via Alt-Svc, and a successful handshake also passes the check.
Show technical detection method
HEAD request and inspect Alt-Svc for h3 token; or attempt QUIC handshake on UDP/443 with ALPN h3.
If your site fails: how to fix it
- If you use Cloudflare, Vercel, Fastly, or AWS CloudFront, HTTP/3 is available with a single toggle in the dashboard and is often on by default. This is by far the easiest enablement path.
- On nginx 1.25 or newer, enable the experimental HTTP/3 module: listen 443 quic reuseport; add_header Alt-Svc 'h3=":443"; ma=86400'; and ensure UDP/443 is open on your firewall.
- On Caddy, HTTP/3 is enabled by default with TLS. No configuration change required - confirm with a header check.
- Open UDP port 443 on your firewall and any upstream load balancer. HTTP/3 uses UDP, not TCP, so a firewall that allows only TCP/443 will block HTTP/3 traffic.
- Verify with curl --http3 https://yourdomain.com/ -I (requires a curl build with HTTP/3 support) or by inspecting the Network panel in Chrome DevTools - the Protocol column should show h3.
Quick facts
| Maturity | EMERGING |
|---|---|
| Weight | low |
| Category | Performance |
Primary sources
Related signals
Frequently asked questions
Do I need a CDN?
Practically, yes - CDNs are the simplest path to HTTP/3 because most origins still require manual QUIC configuration. Cloudflare, Vercel, and Fastly all support HTTP/3 out of the box and handle the UDP firewall and TLS plumbing for you.
Is HTTP/3 widely supported?
All major browsers (Chrome, Firefox, Safari, Edge) support HTTP/3, as do AI agent browsers built on Chromium. Adoption among server-side crawlers is growing but still uneven, so HTTP/2 remains essential as a fallback.
How fast does my site need to be?
HTTP/3 itself has no specific speed threshold; the benefit shows up indirectly through Time to First Byte and Core Web Vitals on lossy or mobile networks. If you already pass those, HTTP/3 is a small further improvement, not a fix for a broken site.
Should I disable HTTP/2 if I enable HTTP/3?
No. Always keep HTTP/2 enabled. Clients that cannot use HTTP/3 (or where UDP is blocked) fall back to HTTP/2 automatically. Removing HTTP/2 would break a meaningful portion of traffic for no real gain.
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