15 Engineering Decisions Behind RAG Hybrid Search

Most people think hybrid search in RAG is just "run BM25 and vector search, combine the results." There are actually 15 distinct engineering decisions happening between a user's question and the 6 chunks that reach the LLM. I traced through production source code line by line. Here's every single one, with the math and code. The Pipeline at a Glance Before diving in, here's the full funnel: 100,000 chunks → BM25 + Vector Search → Score Fusion → Cross-Encoder Reranker → 6 chunks → LLM → 1 answer