Force structured output

[WindowsNT]

The model descriptions contain a json field structured_outputs, which is set to true for many models.

                "id": "claude-sonnet-4.6",
                "name": "Claude Sonnet 4.6",
                "capabilities": {
                    "family": "claude-sonnet-4.6",
                    "limits": {
                        "max_context_window_tokens": 200000,
                        "max_non_streaming_output_tokens": 16000,
                        "max_output_tokens": 32000,
                        "max_prompt_tokens": 128000,
                        "vision": {
                            "max_prompt_image_size": 3145728,
                            "max_prompt_images": 5,
                            "supported_media_types": [
                                "image/jpeg",
                                "image/png",
                                "image/webp"
                            ]
                        }
                    },
                    "object": "model_capabilities",
                    "supports": {
                        "adaptive_thinking": true,
                        "max_thinking_budget": 32000,
                        "min_thinking_budget": 1024,
                        "parallel_tool_calls": true,
                        "reasoning_effort": [
                            "low",
                            "medium",
                            "high"
                        ],
                        "streaming": true,
                        "structured_outputs": true,
                        "tool_calls": true,
                        "vision": true,
                        "reasoningEffort": true
                    },
                    "tokenizer": "o200k_base",
                    "type": "chat"

It works when I tell the model in the prompt, for example "return only json in this format ...", but is there a way to enforce it in the session creation or in some other more enforcing way to ensure I will get it exactly as needed?

Like

{"id":"9","jsonrpc":"2.0","method":"session.send","params":{"prompt":"Hello","sessionId":"...","structured_output": 
{....}
}}