Some embodiments of efficient binary protocol marshalling for rule engine sessions have been presented. In one embodiment, a set of marshalling plug-ins is provided to a rule engine. Each of the set of marshalling plug-ins is customized for a type of user objects. In response to encountering a user object, the rule engine selects a marshalling plug-in out of the set of marshalling plug-ins based on a type of the user object to marshall in or to marshall out the user object.