What is RAG Fusion and how does it combine queries?

RAG Fusion generates multiple query variations with a language model, retrieves results for each in parallel, then fuses the results using Reciprocal Rank Fusion before passing to the generator. This combines the recall benefits of multi-query retrieval with the score-agnostic fusion of RRF, often improving retrieval quality without score calibration headaches. RAG Fusion was popularized in 2023 and is implemented in most RAG frameworks. It costs more than single-query retrieval but typically improves recall by 10 to 20 percent on hard queries.