Deleting Body Information from the Action Interface Causes Zero-Shot Transfer Across Robot Arms

Every system that reached zero-shot cross-embodiment manipulation in the first half of 2026 made the same move, deleting body information from the interface between task reasoning and motor control, whether through body-agnostic handheld data, masked end-effectors, language-coded actions, or contact-intent latents. None of these systems tests that the deletion is what causes transfer, characterizes what the interface still retains, or asks whether the interface must be symbolic. This paper supplies all three on a scene-controlled manipulation substrate where appearance confounds cannot operate. A causal interface ladder over five source and five held-out arms shows that a body-blind end-effector interface transfers zero-shot while leaking body channels back into it collapses transfer once the leak passes a threshold, a gap of $0.157$ that every held-out arm reproduces, and that injecting body identity is actively harmful. At matched body-blindness and identical upstream information, a structured symbolic coding of the interface beats a language-token coding by $0.109$ with the margin compounding over task depth, while a low-capacity continuous latent falls below the task's precision floor. On a released vision-language-action model with scene controlled by robot-swap rendering, most apparent body recoverability is scene appearance, yet a modest scene-invariant residue exceeds a raw-pixel control in all three folds, and an in-model test finds the decoded action body-light. Recoverability is not reliance, at the interface and inside the released model alike, which is the mechanism the zero-shot wave depends on and the boundary it must respect.