BM25 Outperforms AI Agents in Large-Scale Document Retrieval, Study Finds
A new scaling study has found that the decades-old BM25 lexical retrieval algorithm outperforms sophisticated AI-driven agentic search systems as document collections grow larger. At the largest corpus size tested, BM25 achieved an accuracy score of 50.5, compared to 30.7 for the File-System Agent and 29.9 for DenseRAG. The performance gap widens significantly beyond approximately 10 million corpus tokens, where BM25 consistently dominates all competing methods. Agentic approaches also proved far more expensive, with the File-System Agent consuming 39 times more query tokens than a single BM25 pass. Researchers suggest a hybrid approach — using BM25 first to narrow candidates, then applying lightweight agents for post-ranking refinement — could offer the best balance of cost and accuracy.
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