curl-w32/docs/cmdline-opts/http3.md

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Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al. curl http3 Versions HTTP 7.66.0 http1.1 http1.0 http2 http2-prior-knowledge http3-only HTTP/3 Use HTTP v3 http mutex
http1.1
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--http3 $URL

--http3

Tells curl to try HTTP/3 to the host in the URL, but fallback to earlier HTTP versions if the HTTP/3 connection establishment fails. 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 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.

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