h2check

HTTP/2 and HTTP/3 test

Connects to a host and reports which HTTP versions it actually negotiates, which is a different question from what its documentation claims.

Checker offline. Follow the manual steps below, they give the same answer.

What it checks

  • The protocol negotiated on a normal request.
  • Whether HTTP/2 is offered over ALPN.
  • Whether HTTP/3 completes a QUIC handshake.
  • The alt-svc header, which is how a browser learns HTTP/3 is available at all.
  • The alt-svc max age, which decides how long that knowledge survives.

What is not stored

This checker runs on a server, and the server is not deployed yet, so the form above is disabled. The manual steps on the procedures below return the same answer from your own machine, and they are what the checker automates.

Limits

  • A negative result can mean the network path blocks UDP rather than that the server lacks HTTP/3.
  • The first request to any host is over TCP. HTTP/3 is used from the second one, after alt-svc is read.
  • Only the URL you give is tested, not every host the page loads from.

Procedures that explain the result

Put it on your own page

Link to it with the value filled in, and the checker runs as soon as the page opens.

<a href="https://h2check.org/tools/http2-http3/">HTTP/2 and HTTP/3 test</a>