h3x is an HTTP/3 library implemented for dquic. It provides familiar APIs for Request and Response. It is not recommended for direct use; use dhttp instead.
Beneath the hood of a standard QUIC connection, both endpoints have equal ability to concurrently open bidirectional or unidirectional streams. However, the baseline HTTP/3 specification deliberately leaves server-initiated bidirectional streams unexploited. As explicitly stipulated in RFC 9114 - HTTP/3 Section 6.1:
HTTP/3 does not use server-initiated bidirectional streams, though an extension could define a use for these streams. Clients MUST treat receipt of a server-initiated bidirectional stream as a connection error of type H3_STREAM_CREATION_ERROR unless such an extension has been negotiated.
h3x uses exactly these server-initiated bidirectional streams so that the "server" can initiate requests to the "client."