Structured Output

Structured output is constraining the model’s answer to a declared schema, typically JSON with named and typed fields, so the next step of a program can parse it instead of guessing at prose. The modern APIs enforce the schema during generation, which makes the guarantee exact: the shape is certain even when the content is wrong.

A schema does not make the model correct; it makes the model parseable, and those are different promises. The invoice total will always arrive as a number in a field called total. Whether it is the right number is a separate question.

What the schema buys is somewhere to put a check. A date field can be tested as a date, an enum against its allowed values, a required field found missing, loudly, at the boundary. Free text offers no purchase for any of that, so a pipeline that passes prose between steps fails silently, and one built on schemas fails where it broke.