Long-context QA stops re-searching every query: a trained builder agent lifts accuracy from 0.42 to 0.75
Long-document question answering usually starts searching only when a query arrives, so the same corpus is reread for every request. Librarian Agents instead trains a builder agent ahead of queries to reorganize raw documents into a multi-level synthetic filesystem with reference links, which a frozen answering agent then navigates. On BrowseComp-Plus with a Qwen3.5-4B answerer, accuracy reaches 0.75 against 0.42 for embedding retrieval and 0.44 for reranking, while an untrained builder reaches 0.28.