HTTP/2 & HTTP/3 Checker

The HTTP/2 and HTTP/3 Checker tests which versions of the HTTP protocol a server supports, reporting whether it speaks HTTP/1.1, HTTP/2 (h2), or HTTP/3 (h3 over QUIC). Each generation brought meaningful performance gains: HTTP/2 added multiplexing, header compression, and server push, while HTTP/3 moved the transport from TCP to QUIC over UDP to eliminate head-of-line blocking and speed up connection setup. Knowing what a site supports tells you how efficiently it can deliver modern web traffic.

The tool negotiates a connection and inspects the protocol that gets selected. For HTTP/2 it relies on ALPN (Application-Layer Protocol Negotiation) during the TLS handshake, while HTTP/3 advertisement is typically discovered through an Alt-Svc response header that lists 'h3' along with a port. The result shows which protocols were successfully negotiated and which were merely advertised, so you can tell whether HTTP/3 is actually reachable or only announced.

Site owners use this to confirm that a CDN or load balancer is serving the newest protocols after a configuration change, and to verify that HTTP/3 is enabled end to end rather than only on the edge. Performance engineers check it when diagnosing why a site feels slow despite optimization, since falling back to HTTP/1.1 negates multiplexing benefits. It complements an SSL or TLS checker, because HTTP/2 and HTTP/3 both require a valid TLS configuration to negotiate.

A few practical notes: HTTP/3 runs over UDP port 443, so a firewall that blocks UDP will prevent it even when the server advertises h3. Browsers only upgrade to HTTP/3 after first seeing the Alt-Svc header over an HTTP/2 or HTTP/1.1 connection, so the very first visit usually uses an older protocol. If your origin lacks HTTP/2, putting a modern CDN in front is the fastest way to gain support.