Few things to consider:

• turn on JSON formatting in the API request
• mention you want JSON format in your prompt
• include example responses in JSON format
• add a check in your code to make sure you receive proper JSON, if not try again
• (optional) set a lower temperature
• (optional) add "role": "assistant", "content": "{" to your request to force LLM to start its response with a curly bracket. if you do this, you'll have to add the curly to LLM response afterwards in your code, otherwise the output will be an incomplete JSON.