arXiv:2608.24885v2 Announce Type: replace-cross Abstract: Action-conditioned world models increasingly serve as learned simulators for robotic policy improvement, yet their reliability rests on an unverified assumption: generated futures faithfully reflect the consequences of arbitrary valid actions. Systematic evaluation against action-specific ground truth remains limited beyond expert demonstrations. To address this gap, we introduce WorldEcho, which evaluates action following over a broader action distribution through visual integrity and SE(3) end-effector trajectory alignment. Our diagnosis shows that expert-trained models perform better on demonstrated actions but struggle with off-expert trajectories, producing motion inconsistent with the commanded actions or visually invalid rollouts. To mitigate these failures, we propose WorldSync, which broadens action coverage, grounds intermediate video representations in action-induced robot dynamics through an Action-Forcing Expert, and aligns predicted and ground-truth changes under action interventions. Experiments show that WorldSync reduces integrity-gated error on RoboTwin and, when used for iterative policy improvement, yields higher success rates in simulation and on real robots.