Default client adapter: hackney.

hackney 4.2 speaks HTTP/1.1, HTTP/2, and HTTP/3 (with TLS, pooling, and IPv6), so this one adapter covers every protocol. The request body may be buffered ({full, _}) or a producer ({stream, Fun}); the response is buffered by default, or streamed ({stream, Reader}) when the request sets stream => true.

Opts (the client's adapter_opts) may carry hackney => [opt], a list of raw hackney options forwarded verbatim (e.g. {ssl_options, _}, a pool name, HTTP/3 controls like zero_rtt/family).

hackney's request/5 always buffers the response, so any request that either streams its body or wants a streamed response goes through the connection API (request with a stream body, start_response, stream_body).