How do you tune the fusion weights between lexical and vector retrieval, RRF k versus a learned alpha?
Hybrid retrieval only beats either method when the fusion is tuned. The signal is knowing why you cannot simply add BM25 and cosine scores, how RRF's k constant behaves, and when a learned weight beats rank fusion.
Updated Sep 2026 · Grounded in real GenAI, LLM, and AI/ML engineering interview loops and written to a senior-engineer editorial bar.
Hybrid retrieval only beats either method when the fusion is tuned. The signal is knowing why you cannot simply add BM25 and cosine scores, how RRF's k constant behaves, and when a learned weight beats rank fusion.
Lead with where the obvious approach breaks, because that is the judgment they are screening for — most candidates jump straight to the happy path and lose the room.
Then walk the failure back through the pipeline in order, naming the one metric the customer's exec sponsor actually cares about before you propose the fix.
