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That sounds like an issue with your system prompt. If you're using an LLM to interpret web pages for price information then you'd want to include instructions about what to do if the information simply isn't in the web page to begin with. If you don't tell the AI what to do under those circumstances you can't expect any specific behaviour because it wouldn't know what it's supposed to do.

I suspect from this comment that you haven't actually worked with LLMs much, and are just going off the general "lol they hallucinate" perception they have right now? I've worked with LLMs a fair bit and they very rarely have trouble interpreting what's in their provided context (as would be the case here with web page content). Hallucinations come from relying on their own "trained" information, which they recall imperfectly and often gets a bit jumbled. To continue using a human analogy, it's like asking someone to rely on their own memory rather than reading information from a piece of paper.