Rewriting, routing and fusing queries

Users do not type clean, self-contained, single-hop queries in the corpus's language. They type follow-ups ("and for Brava?"), Portuguese questions over English contracts, questions that need two lookups joined together, and questions that belong in a database rather than a document index. Query-side techniques transform the question before retrieval: condense it into a standalone query, rewrite or translate it, generate several variants and fuse the results, generate a hypothetical answer to search with (HyDE), decompose it into sub-questions, or step back to a more general question. A router decides which back end should answer at all. Each technique adds an LLM call and some risk, so add them one at a time, behind an eval, and skip them when a deterministic route already works.

• Route first. An exact ID or a numeric tariff question should never go through paraphrasing and vector search.
• Use rules for what is certain and a schema-constrained LLM for what is not; validate every LLM output against an allowed set and fail safe.
• Filters extracted from the question can only narrow results. Permissions come from identity, never from the text.
• Multi-query plus RRF (RAG-Fusion) mostly helps recall on vocabulary mismatch and cross-lingual queries.
• Log the original query, every rewrite, the route and the filters. You will need them for evals, audits and incident review.