Intuitionistic K is a Bisimulation-Invariant Fragment of Intuitionistic First-Order Logic

We define the notion of IK-bisimulation between the relational semantics for the intuitionistic modal logic IK, and prove that IK arises as the IK-bisimulation-invariant fragment of intuitionistic first-order logic. En route, we provide an intrinsic characterisation result of this logic by way of a Hennessy-Milner-style theorem and develop some intuitionistic first-order model theory, including intuitionistic analogues of Łoś's Theorem, elementary embeddings and countable saturation.