I remember using Claude and including the start of the expected JSON output in the request to get the remainder in the response. I couldn't believe that was an actual recommendation from the company to get structured responses.
Like, you'd end your prompt like this: 'Provide the response in JSON: {"data":'
That's what I thought when starting and it functions so poorly that I think they should remove it from their docs. You can enforce a schema by creating a tool definition with json in the exact shape you want the output, then set "tool_choice" to "any". They have a picture that helps.
Unfortunately it doesn't support the full JSON schema. You can't union or do other things you would expect. It's manageable since you can just create another tool for it to chose from that fits another case.
Like, you'd end your prompt like this: 'Provide the response in JSON: {"data":'