A new preprint on arXiv presents a theoretical extension of Aumann's agreement theorem, a classic no-go result in rational decision theory. The theorem states that if two agents share a common prior and their posterior beliefs about an event become common knowledge, they cannot rationally assign different probabilities to that event. Traditional proofs assume an objective underlying state of the world, raising questions about applicability in quantum theory where such ontology may not hold. The authors derive an operational reformulation focused solely on agents' observations rather than any assumed 'real' state. Through mathematical derivation, they demonstrate the theorem remains valid in standard quantum theory, hypothetical postquantum models, and scenarios with indefinite causal order. This work is purely theoretical with no empirical experiments, data collection, or sample sizes. As an arXiv preprint (not peer-reviewed), its conclusions are subject to further scrutiny. The paper reconciles apparently contradictory earlier results and identifies Wigner's friend-type situations, in which observers are quantum systems, as the sole setting where the agreement theorem might fail. Source: https://arxiv.org/abs/2603.23595. Limitations include dependence on abstract definitions of rationality and common knowledge that may prove difficult to realize precisely in laboratory tests.