--http3-only

Instruct curl to use HTTP/3 to the host in the URL, with no fallback to earlier HTTP versions. HTTP/3 can only be used for HTTPS and not for HTTP URLs. For HTTP, this option triggers an error.

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

This option makes curl fail if a QUIC connection cannot be established, it does not attempt any other HTTP versions on its own. Use --http3 for similar functionality with a fallback.