What is multi-query retrieval and when should you use it?

Multi-query retrieval generates several paraphrased versions of the user's query using a language model, runs retrieval for each, and merges the results. It improves recall when the original query phrasing differs from document phrasing or when the question has multiple valid interpretations. The cost is additional language model calls and retrieval operations, typically three to five times the baseline. Use multi-query when retrieval misses relevant documents that exist in the corpus, verified by inspection of failed cases on an evaluation set.