One of the main issues in the engineering of multi-agent systems is to give a formal semantics to agent communicative acts.
This is generally done by describing how agent interactions with the environment affect and are affected by the agent inner behaviour — e.g., by the agent decisions its perception of the world.
FIPA ACL semantics is one of the best-known examples of this approach.
In this paper, we show how this technique can be seen in terms of providing a <em>grey-box</em> abstract model for agents, where only the agent part responsible for the interaction with the environment is represented, while the hidden part concerning the agent internal behaviour is abstracted away.

We first provide a general framework for describing the semantics of communicative acts in terms of an operational semantics for agents, grounded on the framework of transition systems.
Then, some examples are provided to show its flavours to encode typical aspects of ACL semantics.