1) I forgot to mention - the backend is written in Go. Even if both were TS, all of the interoperability and other advantages will apply.

2) The TS classes are auto-generated and the protobuf definitions are the only authoritative definition.

3) We're using JWT for authentication (in particular, short-lived tokens issues by Keycloak, using their JS library). gRPC has request metadata and it was straight-forward to use it with the Keycloak library.

We generate client libraries for customers to use and give them the option of using the REST API if they don't want to deal with gRPC. We consider the gRPC API to be a competitive advantage - our customers are enterprises and like well-defined and typed interfaces.