HTTP/2 & HTTP/3 Check

Check whether a server supports HTTP/2 (ALPN) and HTTP/3 (Alt-Svc).

HTTP/2 & HTTP/3 Check tests in real time whether a domain's web server offers HTTP/2 and HTTP/3. HTTP/2 is determined precisely from the ALPN result (h2) negotiated during the TLS handshake, while HTTP/3 is inferred from whether the response advertises h3 in its Alt-Svc header.

Modern protocols speed up page loads through multiplexing, header compression and reduced connection latency. HTTP/3 in particular runs over QUIC (UDP), which helps on lossy networks. Just enter a domain — no protocol or path needed.

Domaine.g. example.com (domain only, no protocol or path)

ALPN and the HTTP/2 verdict

ALPN (Application-Layer Protocol Negotiation) is a TLS extension where client and server pick the upper-layer protocol during the handshake. This tool offers h2 and http/1.1; if the server selects h2, HTTP/2 is reported as supported.

Alt-Svc and the HTTP/3 inference

HTTP/3 usually starts as TLS over TCP, then the server advertises a QUIC endpoint via the Alt-Svc response header, after which the browser upgrades subsequent requests to HTTP/3. For example:

alt-svc: h3=":443"; ma=86400 — advertises HTTP/3 (h3)
Draft tokens such as h3-29 are also treated as HTTP/3

QUIC and the limits of inference

QUIC is a UDP-based transport that underpins HTTP/3. The HTTP/3 verdict here is an inference from the Alt-Svc advertisement, not an actual QUIC handshake. A server may advertise h3 while real HTTP/3 fails because UDP 443 is blocked by a firewall.

To read the raw Alt-Svc header and the rest of the response yourself, open the HTTP header checker.

Frequently asked questions

How is HTTP/2 determined?

During the TLS handshake we offer h2 / http/1.1 via ALPN and read which protocol the server selects. If it picks h2, HTTP/2 is reported as supported.

Why is the HTTP/3 result an inference?

HTTP/3 is typically advertised via the Alt-Svc header. We check whether h3 appears there, so the advertisement may exist while a real connection fails (e.g. UDP 443 blocked).

Does an empty Alt-Svc mean no HTTP/3?

It is reported as unsupported, but some servers omit Alt-Svc on the first response. An unsupported result is not an absolute conclusion.

Can I paste a URL instead of a domain?

Yes. Addresses with http:// or https:// or a path are accepted — only the host is extracted and checked. Port 443 (HTTPS) is used.

What do I gain by switching to HTTP/2 or HTTP/3?

HTTP/2 multiplexes many requests over a single connection and compresses headers, removing HTTP/1.1's per-connection serialization limits. HTTP/3 runs over QUIC (UDP), so connections set up faster and a lost packet no longer blocks the other streams, improving perceived speed on mobile and high-latency networks.

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