Author Elasticsearch relevance regression tests using the Ranking Evaluation API (`POST {index}/_rank_eval`) - judgment lists (query + expected docs at ranks), per-query metrics (Precision@K, Recall@K, MRR, DCG, ERR), reproducible test corpora; pair with Quepid + Splainer for interactive judgment authoring. Use before changing analyzers, synonyms, boosts, or query templates on an Elasticsearch index that serves user-facing search, so the NDCG / MRR baseline is captured first.