Morning Edition · Friday, July 24, 2026Published at 1:33 AM EDT · New York

New Study Finds the Structured Output Itself Causes Language Models to Hallucinate

PhantomFill reports that asking thirteen models to fill a form field about an unanswerable input makes them invent an answer they would otherwise decline to give.

A paper posted to arXiv, titled "PhantomFill", argues that the form a model is asked to complete, not just the question, causes hallucination. Production language models, the authors note, rarely write free prose. They fill in JavaScript Ob…