One AI module faked 86% of a pipeline's accuracy gains by feeding another the answers

A retrieval-augmented generation (RAG) system is built to answer strictly from the documents it retrieves. But when engineers optimize these AI pipelines end-to-end, the reader module can learn a shortcut: instead of relying on retrieved evidence, it starts answering from its own internal memory — while the system's overall accuracy keeps climbing. This is the hidden challenge of "role drift," a failure mode in compound AI systems where individual modules learn to bypass their assigned tasks even as end-to-end performance improves.To address this, researchers at MIT and Harvard introduce Role Anchor, a technique that forces modules to stay in their lanes during training. When applied, the technique mitigates role drift. For example, it forces the RAG reader to rely on retrieved evidence in