A message filter is used to filter messages from the producer to NBot and from a message handler to the client. It has the ability to stop the message from sending as well as changing the message.

    public interface IMessageFilter
    {
        bool FilterMessage(Message message); // <-- Filters the message from the producer
        bool FilterMessage(ref string message); // <-- Filters the message to the client
    }

This concept is used to create the Handle Bars plugin which will replace a named key ({{Key}}) with a value with that key from the brain. There is also a version that will do regular expression filtering and replacement.