Q. The right chunk is retrieved but ranks twelfth, below the cutoff. What promotes it?
A. A cross-encoder reranker (the Bedrock Rerank API, e.g. Cohere or Amazon Rerank) re-scores the top-N candidates by reading query and document together, far more precisely than the first-stage embedding score. Retrieve wide, rerank narrow.
Why? Reranking fixes ordering, not recall: if the chunk is not in the top-N at all, fix retrieval first.