What is an Alt-Svc header?
Advertises an alternative way to reach the same service, e.g. over HTTP/3, without a redirect.
Alt-Svc lets a website advertise that the same service is also available over a different protocol - most commonly HTTP/3 over QUIC - so a supporting client can transparently upgrade on a future request without needing a redirect. It's a purely optional performance optimisation, so its absence doesn't indicate anything broken, just that a site hasn't opted into advertising newer protocol support yet.
Common use case
A CDN edge advertises HTTP/3 support via Alt-Svc, letting a supporting browser silently switch to the faster QUIC-based protocol on its next request to the same origin without any redirect.
Example
Alt-Svc: h3=":443"; ma=86400History
Standardised in RFC 7838 (2016), and became especially important with the rollout of HTTP/3 (RFC 9114, 2022).
Did you know?
It's the mechanism that quietly let major sites like Google and Cloudflare roll out HTTP/3 to a huge fraction of the web without visitors or developers needing to change a single URL.