> None of the libraries I looked at actually implemented it.

Can you elaborate on that?

AFAIK gRPC comes with bidirectional streams out of the box. I played around with gRPC on Java and it seemed to work.

We're using them between Python <-> Go with no problems. Can't remember exactly what version it was at when I first tried it but it's worked fine for two years or more.

All core grpc libraries support bidirectional streaming. It's the "routeguide" example.