Book: RAG Pocket Guide My project: Hermes IDE | GitHub — an IDE for developers who ship with Claude Code and other AI coding tools Me: xgabriel.com | GitHub Picture this: a user types KB-2024-7831 into your support bot. The dense retriever returns three articles about ticket-tracking systems, none of which contain the string KB-2024-7831 . The BM25 retriever next to it returns the exact article in position one. You have been A/B testing the dense retriever for weeks and were about to make it default. A screenshot of that one query, dropped into team Slack, kills the rollout. Hybrid search is the phrase you will hear at every RAG talk in 2026, and the reason is boring: pure dense retrieval is bad at proper nouns and exact identifiers, pure BM25 is bad at paraphrase, and stitching them with Reciprocal Rank Fusion recovers both signals at the cost of about 6ms per query. There is no version of this trade where pure retrieval wins. Why dense alone misses Embeddings collapse meaning. That is the feature.…