Although informative, the semantic denition proposed for the most standard agent communication language (FIPA ACL 1997) is complicated and contentious, while published inter-action protocols (IPs) tend to be ambiguous, incomplete, and unveried with respect to message semantics. To clarify and help rectify these problems, this paper proposes an integrated framework based on Propositional Dynamic Logic and Belief and Intention modalities (called the PDL-BI language). Specifically, we provide an axiomatisation of PDL-BI and for an agent’s propositional attitudes (beliefs and intentions) and social attitudes (such as sincerity and trustworthiness). Then, we suggest a revised and simpler core semantics for many of the FIPA ACL speech acts, which, in turn, lead to the specification of the semantics of IPs. As a case study, we specify the semantics of the contract net protocol (CNP) in PDL-BI, which allows to prove that the CNP terminates.