anecdotally i found gpt3.5 was decently reliable at returning structured data but it took some prompt tuning and gpt4 has never returned invalid JSON when asked

anecdotally i found gpt3.5 was decently reliable at returning structured data but it took some prompt tuning and gpt4 has never returned invalid JSON when asked for it, though it doesn't always return the same fields or field names. it performs better when the schema is explicit and included in the prompt, but that can add a non trivial number of tokens to each request.