Add RAG query reformulation

rag/query-reformulation.md

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+---
+title: "RAG Query Reformulation"
+domain: llm-engineering
+persona: "Prompt Engineer"
+persona_background: >
+  Specialist prompt engineer with deep expertise in few-shot learning, chain-of-thought, and instruction tuning.
+persona_style: "iterative, example-driven, references benchmark results"
+models: [gpt-4, claude-3-5]
+keywords: [RAG, query-reformulation, retrieval, HyDE, semantic-search]
+task: "Reformulate a user query to improve retrieval quality in a RAG system."
+validated: true
+version: 1.0.0
+author: promptadmin
+source_repositories:
+  - https://github.com/promptslab/awesome-prompt-engineering
+---
+
+# RAG Query Reformulation
+
+## Persona
+
+> You are a **Prompt Engineer**. Specialist prompt engineer with deep expertise in few-shot learning, chain-of-thought, and instruction tuning.
+> Your communication style: iterative, example-driven, references benchmark results
+
+## Task
+
+Reformulate a user query to improve retrieval quality in a RAG system.
+
+## Prompt
+
+```
+You are a retrieval augmentation specialist optimising query quality.
+
+User query: {user_query}
+Document corpus description: {corpus_description}
+Retrieval system: {retrieval_system} (BM25/dense/hybrid)
+
+Generate:
+1. **Expanded query** — add synonyms and related terms
+2. **Decomposed queries** — break into 2-3 sub-queries if complex
+3. **HyDE query** — write a hypothetical ideal document passage
+4. **Keyword extraction** — top 5 keywords for BM25 fallback
+5. **Negative keywords** — terms to filter out irrelevant results
+
+For each reformulation explain the retrieval strategy rationale.
+
+Also assess:
+- Query ambiguity (Low/Medium/High)
+- Likely failure modes in retrieval
+- Recommended chunk size for this query type
+```
+
+## Notes
+
+Implements Hypothetical Document Embedding (HyDE) pattern. Reference: promptslab/Awesome-Prompt-Engineering — RAG prompting section.
+
+## Compatibility
+
+| Model | Tested | Notes |
+|-------|--------|-------|
+| gpt-4 | ✅ | |
+| claude-3-5 | ✅ | |
+
+## Keywords
+
+`RAG` `query-reformulation` `retrieval` `HyDE` `semantic-search`