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How do you tune the fusion weights between lexical and vector retrieval, RRF k versus a learned alpha?

Hybrid retrieval only beats either method if the fusion is tuned. The signal is knowing why you cannot just add BM25 and cosine scores, how RRF's k constant behaves, and when a learned weight beats rank fusion.

Updated Aug 2026 · Grounded in real Applied AI Engineer interview loops and written to a senior-engineer editorial bar.

Hybrid retrieval only beats either method if the fusion is tuned. The signal is knowing why you cannot just add BM25 and cosine scores, how RRF's k constant behaves, and when a learned weight beats rank fusion.

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