The client SDK describes the entire interface, though. Low level protocols like HTTP are arguably different for different roles, which is where I think this co

The client SDK describes the entire interface, though. Low level protocols like HTTP are arguably different for different roles, which is where I think this confusion comes from. For example, a socket server has to listen, while a socket client has to connect, and those are different processes. But an application's interface is describing a set of, broadly speaking, function call signatures. Specifically, it's describing the names of the function calls, the parameter types and return types. (And it describes those types as well.) Those must be identical for the API to work. There's no difference between the "client API" and "server API" by the definition of API, clients and servers are different in role and implementation.