// Tool
HTTP/3 Detector
Probe whether a site speaks HTTP/3 (QUIC). The browser fetches a tiny resource from the target host and reads the negotiated protocol from the Performance API. Caveat: browsers learn HTTP/3 endpoints opportunistically via the Alt-Svc header. The first connection is usually H2; H3 is used on subsequent connections after the browser has cached the advertisement.
Tries 3 probe fetches with a small delay between them — gives the browser a chance to upgrade to H3 after seeing Alt-Svc on the first response.
// Detection method
This page uses fetch() in no-cors mode against the target's origin and reads nextHopProtocol from performance.getEntriesByType('resource'). The browser exposes that field if the response includes a Timing-Allow-Origin header (most CDNs do). Without it, the entry exists but the protocol field is empty.
For a fully reliable check, use the command line:
curl -sIv --http3-only https://example.comor
nghttp -nv --http3 https://example.com