draconisplusplus/subprojects/curl-8.10.1/docs/cmdline-opts/http2-prior-knowledge.md

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Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al. curl http2-prior-knowledge Versions HTTP 7.49.0 http1.1 http1.0 http2 http3 HTTP/2 Use HTTP 2 without HTTP/1.1 Upgrade http boolean
http2
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--http2-prior-knowledge $URL

--http2-prior-knowledge

Issue a non-TLS HTTP requests using HTTP/2 directly without HTTP/1.1 Upgrade. It requires prior knowledge that the server supports HTTP/2 straight away. HTTPS requests still do HTTP/2 the standard way with negotiated protocol version in the TLS handshake.

Since 8.10.0 if this option is set for an HTTPS request then the application layer protocol version (ALPN) offered to the server is only HTTP/2. Prior to that both HTTP/1.1 and HTTP/2 were offered.