Multiparty Session Types (MPST) are a well-established typing discipline for message-passing processes interacting on sessions involving two or more participants. Session typing can ensure desirable properties: absence of communication errors and deadlocks, and protocol conformance.

We propose a novel MPST theory based on a rely/guarantee typing system, that checks (1) the guaranteed behaviour of the process being typed, and (2) the relied upon behaviour of other processes. Crucially, our theory achieves type safety by enforcing a typing context liveness invariant throughout typing derivations.

Unlike “classic” MPST works, our typing system does not depend on global session types, and does not use syntactic duality checks. As a result, our new theory can prove type safety for processes that implement protocols with complex inter-role dependencies, thus sidestepping an intrinsic limitation of “classic” MPST.