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.