Role Drift

End-to-end RL improves the accuracy of a multi-module LLM pipeline without constraining how the modules divide labor internally. Harvard and MIT name the resulting failure mode, Role Drift, where a module preserves or improves end-task performance while abandoning its assigned role through shortcuts that system-level evaluation cannot see. Two instances showed up. A decomposer meant to split a question into sub-questions for a separate solver instead plants the answer inside them, and a reader meant to answer from retrieved passages instead falls back on parametric memory. Hold the decomposer to its role and 86% of the apparent RL gain disappears. Role Anchor, the proposed regularizer, preserves how the role prompt shifts a module's next-token predictions relative to a neutral prompt, and gradient analysis suggests it reduces alignment with the drift direction rather than simply suppressing learning.