--http3

Attempt HTTP/3 to the host in the URL, but fallback to earlier HTTP versions if the HTTP/3 connection establishment fails or is slow. HTTP/3 is only available for HTTPS and not for HTTP URLs.

This option allows a user to avoid using the Alt-Svc method of upgrading to HTTP/3 when you know or suspect that the target speaks HTTP/3 on the given host and port.

When asked to use HTTP/3, curl issues a separate attempt to use older HTTP versions with a slight delay, so if the HTTP/3 transfer fails or is slow, curl still tries to proceed with an older HTTP version. The fallback performs the regular negotiation between HTTP/1 and HTTP/2.

Use --http3-only for similar functionality without a fallback.