It’s something OpenAI should really implement themselves. Implementing it from the client side will mean sending the same request over and over until you get a syntactically correct answer, which is going to be much slower and likely to cost a lot. The server can guide the generation, but the client can (currently) only hint at what it wants. ChatGPT4 is fairly good at following schemas, and that’s what OpenAI currently relies on, but they make no guarantees.

It likely wouldn’t require additional training. It’s a change to the way the server uses the model, not a change to the model itself… but we don’t know ChatGPT4’s true architecture because OpenAI won’t publish anything about it, so it’s hard to say for sure.