Stop telling your RAG bot not to hallucinate. Make it impossible.

The suggestion every RAG app ignores If you've shipped a retrieval-augmented assistant, you've written some version of this line in your system prompt: "If the answer isn't in the provided context, say you don't know. Do not make things up." And you've watched the model cheerfully ignore it under pressure. A confident-sounding question comes in, retrieval returns something adjacent , and the model stitches together an answer that's plausible, fluent, and wrong. Telling a language model not to hallucinate is a suggestion — and suggestions lose to the model's overwhelming prior toward being helpful. I got tired of fighting this with prompt wording, so I tried a different framing while building MCP SDK Docs Assistant , an assistant for the Model Context Protocol TypeScript SDK. The framing: don't ask the model to refuse — remove its ability to fabricate. Refusal as code, not as a prompt The core idea is that the model can only hallucinate if you hand it material to hallucinate from. So the refusal decision lives in the retrieval tool, before the model ever sees anything. If nothing clears a confidence bar, the tool returns an empty result set, and the model is left with no source text to spin into an answer. In practice, the tool looks roughly like this: const candidates = await hybridSearch ( query , { version , limit : 12 }); if ( ! hasConfidentMatch ( candidates )) { // best cosine sim < 0.45 return { relevant : false , results : [] }; // model has nothing to work with } const results = await rerank ( query , candidates , 6 ); return { relevant : true , results }; The model isn't asked to behave. The system is shaped so that the only coherent next move, when results come back empty, is to say "the docs don't cover this." Refusal stops being a personality trait you're hoping for and becomes a property of the architecture. Why this particular SDK needed it There's a second failure mode this assistant had to solve, and it's specific to fast-moving libraries. The MCP Ty