Under the hood, it's quite similar to function calling. A few differences:

- Structured Outputs is a bit more straightforward. e.g., you don't have to pretend you're writing a function where the second arg could be a two-page report to the user, and then pretend the "function" was called successfully by returning {"success": true}

- Having two interfaces lets us teach the model different default behaviors and styles, depending on which you use

- Another difference is that our current implementation of function calling can return both a text reply plus a function call (e.g., "Let me look up that flight for you"), whereas Structured Outputs will only return the JSON

(I worked on this feature at OpenAI.)