Detect which HTTP versions a site supports — HTTP/1.1, HTTP/2 (ALPN), and HTTP/3 (Alt-Svc advertisement)
• Enter a full URL or a bare domain (e.g. example.com); a bare domain starts from http:// and follows redirects to HTTPS
• HTTP/2 is detected by a real TLS ALPN handshake on port 443; HTTP/3 is read from the Alt-Svc response header
• HTTP/3 shows whether the server advertises h3 — not a live QUIC handshake
This tool detects which versions of HTTP a website supports. HTTP/1.1 is the baseline — it is reported as supported whenever any HTTP or HTTPS response can be obtained. HTTP/2 is detected by opening a real TLS connection on port 443 and reading the protocol negotiated through ALPN (Application-Layer Protocol Negotiation): if the server selects h2, HTTP/2 is supported.
HTTP/3 runs over QUIC (UDP) and is discovered through the Alt-Svc response header, which is the standard way browsers learn that a site offers an HTTP/3 endpoint. This tool reports whether the server advertises h3 (or a draft version such as h3-29); it does not perform a live QUIC handshake, so an advertised endpoint is a strong signal but not an end-to-end reachability guarantee.
By performing a TLS handshake on port 443 offering both h2 and http/1.1 via ALPN. If the server negotiates h2, HTTP/2 is supported. The check dials a validated IP with the correct SNI, so it never follows to an untrusted target.
HTTP/3 discovery relies on the Alt-Svc header advertising an h3 endpoint. Verifying it end to end requires a QUIC/UDP handshake, which this tool intentionally does not perform. Advertisement is the standard discovery mechanism and a reliable indicator, but it reflects configuration rather than a completed HTTP/3 request.
Yes. HTTP/1.1 is the universal baseline; any server that returns a response speaks at least HTTP/1.1. HTTP/2 and HTTP/3 are the upgrades worth checking for.