This paper reframes long-horizon world-model failure as a kinematic-versus-dynamic problem rather than generic compounding error. It introduces imagined Kinematic-Consistency Error (iKCE), a per-step diagnostic measuring deviation from a closed-form kinematic null, plus a perturbation protocol for crossing physical regime boundaries. On a released DreamerV3 checkpoint trained on DMC walker-walk, imagined iKCE is reported to be roughly two orders of magnitude higher than matched real-physics rollouts. During a friction sweep crossing the gait-collapse boundary, iKCE remains statistically flat while policy reward collapses, producing the proposed kinematic-not-dynamic signature.
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