HTTP/3 Checker
Verify HTTP/3 for real — we complete a QUIC handshake and make an actual HTTP/3 request, not just read Alt-Svc.
Advertisement and reality
We check how HTTP/3 is advertised — the Alt-Svc header and HTTPS DNS records — and then actually connect over QUIC and make an HTTP/3 request. Both matter: a site can advertise h3 yet fail to serve it.
Our own QUIC stack
The handshake, version negotiation and HTTP/3 request use our own QUIC and HTTP/3 implementation, so we can report the negotiated version, TLS cipher, certificate and timing precisely.
Why real tests matter
An Alt-Svc header only says HTTP/3 may be available. A firewall that blocks UDP 443 makes it fail anyway, so we complete a real QUIC handshake and send an actual request.
Frequently asked questions
What is HTTP/3?
HTTP/3 is the latest version of HTTP, running over QUIC on UDP instead of TCP. It reduces latency and handles packet loss better than HTTP/2.
What is the difference between HTTP/3 and QUIC?
QUIC is the transport protocol built on UDP that provides encryption, multiplexing and fast connection setup. HTTP/3 is HTTP carried over QUIC.
Why does a site advertise HTTP/3 but fail the test?
Typical reasons are UDP 443 blocked by a firewall, a CDN setting that announces HTTP/3 without serving it, or a certificate problem on the QUIC listener.
Do I need HTTP/3 support on my server?
It is optional: browsers fall back to HTTP/2. HTTP/3 helps most on mobile and unreliable networks because it recovers from packet loss faster.