IDL (Interface Definition Language) for SignalR and Message Broker

Question

Background

We want to use SignalR for communicating bidirectionally between a Web-Client and a server written in C#. There is another client written with C# which also needs to support communicating over SignalR with the server. At the same time we are introducing a message broker (RabbitMQ).

Problem

We have about 35 persons working on the software and need a way for defining the messages formally. For the message broker we thought about using ProtoBuf and had the idea of using the same or similar for SignalR (if possible).

Question

Is there any IDL (Interface Definition Language) we can use for SignalR AND the message broker? If not: Is there any way of formally defining the messages in SignalR?