The assumption has become almost automatic: if you need to extract structured data from web pages, you reach for an LLM. Feed it the HTML, write a prompt, get JSON back. It works in a demo. It works on ten pages. So teams build pipelines around it and move on. The problem shows up later, quietly, in production. When extraction fails without telling you The most dangerous failure mode in any data pipeline is not a crash. It is a wrong value that looks correct. LLM-based extrac