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