0% vs 50%: Making a RAG Agent Refuse to Hallucinate

0 % vs 50 %: making a RAG agent refuse to hallucinate 2026-05-31 · LLM / RAG A retrieval-augmented agent is only as trustworthy as its behaviour on questions whose answer isn't in the corpus . The failure mode is quiet: instead of saying "I don't know," the model invents a confident, well-formed, wrong answer. This post shows a single guardrail that takes that from common to never — and, crucially, measures it. Reference architecture: nim-agent-blueprint — agentic RAG on the NVIDIA NIM stack with a built-in eval harness. The ablation The agent loop is plan → retrieve → generate → validate . The interesting variable is the generation prompt's contract with the retrieved context: Configuration Out-of-corpus hallucination rate Generate freely from context ~50 % Guarded prompt (answer only from context; otherwise abstain) 0 % Same model, same retriever, same questions. The only change is a prompt that makes "I can't answer that from the provided sources" a first-class, rewarded output — plus a validate step that checks the answer is grounded in retrieved spans before returning it. On in-corpus questions, retrieval recall@3 stayed at 94–100 % , so the guardrail buys safety without costing coverage. Why "just prompt better" isn't the lesson The lesson isn't the prompt — it's that the difference between 50 % and 0 % is invisible without an eval harness . A demo that only asks in-corpus questions looks perfect in both configurations. You only see the 50 % when you deliberately ask things the corpus can't answer and score groundedness . So the blueprint ships with: retrieval hit-rate (is the answer even retrievable?), answer groundedness via LLM-as-judge (is the answer supported by what was retrieved?), latency , and OpenTelemetry traces per agent step. That's the difference between "it works on my five questions" and "here is the number a partner can hold me to." Takeaway For enterprise RAG, abstention is a feature, not a failure. Make "I don't know" a rewarded output, vali