Agentic / Reasoning RAG: From Search-R1's RL Multi-Turn Search to Deep Research and MCP's Reasoning × Retrieval Paradigm
In 2025 RAG stopped being 'retrieve once, generate once.' Search-R1 trains models to search autonomously in multiple turns with RL, REX-RAG/AlignRAG add policy and alignment branches, OpenAI Deep Research productizes the loop, and MCP generalizes retrieval into unified tool invocation. This post unpacks the design philosophy, trade-offs against ten generations, and when to adopt the new paradigm.