An AI agent can return a syntactically perfect JSON object with the wrong number inside it. That is the exact failure mode identified by the Structured Output Benchmark, or SOB (arXiv:2604.25359, April 2026), which tested 21 models for both schema compliance and factual accuracy. Across all 21 models, the best value accuracy reached was 83.0% for text tasks and 67.2% for image tasks — despite near-perfect schema compliance.
That gap is the whole problem with treating structured output as a proxy for correct output. Schema validation confirms an agent filled in the right fields in the right format. It says nothing about whether the number, the recommendation, or the claim inside that format is actually true. A system that only checks for valid JSON will pass its own tests while being confidently wrong.
For a buyer evaluating an AI vendor, that distinction is exactly what a demo will not show. A live demo runs a handful of friendly inputs through the happy path. It does not run the edge cases that surface in the first month of production traffic, and it does not report an accuracy number at all — only a schema that happened to validate.