HTTP/2 and HTTP/3: confirmed support versus a timeout
Understand what a successful protocol connection actually proves.
check.uk.app technical team · Reviewed 26 September 2026
What we test
We send two bounded requests to the exact HTTPS URL. The HTTP/2 request may negotiate an earlier version, which is reported as fallback. The HTTP/3 request uses QUIC on UDP 443 with no fallback. TLS verification stays enabled, and this test does not follow redirects.
What counts as confirmation
A response received using the requested protocol confirms support at the tested address. Even a 301 or 403 can confirm the protocol while indicating a redirect or access restriction. Alt-Svc advertises an alternative; it is not proof that the alternative can be reached.
What to do with an unconfirmed result
Check your server or CDN configuration and UDP 443 filtering. Repeat from another network before concluding that HTTP/3 is unsupported. Missing HTTP/3 capability on the probe is reported separately. A successful test does not guarantee lower latency for every visitor.
Reproduce with curl
Use a curl build that lists HTTP2 and HTTP3 in curl -V. The HTTP/3-only option prevents an HTTP/2 response being mistaken for HTTP/3.
curl -V
curl --http2 --max-time 10 -o /dev/null -w '%{http_version}\n' https://example.com/
curl --http3-only --max-time 10 -o /dev/null -w '%{http_version}\n' https://example.com/