After ANN retrieval (approximate, optimizes for speed), a cross-encoder reranker scores each retrieved chunk precisely against the query.

After ANN retrieval (approximate, optimizes for speed), a cross-encoder reranker scores each retrieved chunk precisely against the query. Helps when: top-k from ANN includes irrelevant chunks, query is complex/long, or precision matters over recall. Overkill for: simple single-document lookup, latency-critical paths, or when ANN already gives high precision.