Inside Google (where everything is protobuf) there were a handful of tools like this. But the killer feature is that there was an automatically-added RPC servic

Inside Google (where everything is protobuf) there were a handful of tools like this. But the killer feature is that there was an automatically-added RPC service for introspecting the other RPC services any given server exposed. This means that there was an `ls` command for listing possible services, methods on those services and the protocol buffers for the request and response. So you didn't need to manage the source protos. You could just do something like `protocurl 10.1.2.3 MyService.MyMethod 'foo: 8 bar: "foo"'` and it put all of the pieces together. The introspection could be a little slow if the server was far away (it added a couple round trips) but the ability to avoid knowing where the schemas were was invaluable. (I wonder if there could have also been some sort of caching to make it a bit better).