Where vs what: structural and content failures in structured outputs
When a model fills a JSON schema you usually check one thing: did it parse and match the shape. This paper grades it twice instead — did the value you asked for come back anywhere, and did it land at the path it belonged in — and the two numbers come apart badly as nesting deepens. At four levels the values are nearly all present (91% to 96% for the stronger models) while 24% to 74% of them sit at the wrong position, so a single accuracy score reads an addressing problem as a content problem. Repeated key names and opaque field names each add about eight points to the misplacement rate, making a schema rename the cheapest fix available; training a small model on a reward that pays for correct placement took it from 26% to 63%.