A (communication) protocol captures how agents collaborate by specifying the messages they exchange. In particular, since the information content of messages characterizes the interactions a protocol specifies, message types can improve collaboration by strengthening the specification of what each agent may legitimately expect from another agent. In addition, in implementations, typing information can enable improved verification of agents. We introduce Supple, a protocol specification language that expresses message schemas with typed parameters. Supple enables definition of causal types for parameters that constrain how other parameters are computed in a protocol enactment. We give the formal semantics of Supple; characterize the liveness and safety of Supple specifications; and provide decision procedures for them.