Plan Existence Problem Proven Undecidable for Epistemic Actions

A new proof establishes that the plan existence problem in epistemic logic is undecidable, even under severely restricted conditions. The problem asks whether a sequence of epistemic actions exists to achieve a given goal from an initial epistemic state. The proof shows undecidability when action preconditions have modal depth at most 1 and there are no postconditions. This result resolves a previously open question in the field of logic in computer science. The work is published on arXiv under the identifier 2604.22736.