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arXiv Machine Learning

Robot World Models Are Not Invariant to How the Actions Are Written

A robot policy trained with either absolute joint targets or delta‑relative actions inherits the chosen action parameterization in its world model, leading to catastrophic failures when the model is exposed to the alternate encoding. Experiments on three robot datasets and two morphologies show retrieval performance drops 2.6–13.4×, goal‑conditioned action selection plummets from 53% to 15%, and predictions for the same future become nearly orthogonal. The issue is not a loss of information—both encodings are highly reconstructible—but a lack of invariance in the action channel, which existing visual‑model invariance research does not address. Averaging over the two encodings partially restores performance, yet the worst‑case disagreement remains high, indicating that the defect persists in certain scenarios.

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