# Structured output

Request parseable JSON or enforce a JSON Schema on capable models.

Use structured output when downstream code needs machine-readable data. Check that the model advertises `json_mode` or `structured_outputs`.

## JSON mode

```json
{
  "model": "qwen/qwen3.8-27b",
  "messages": [{"role": "user", "content": "Return JSON with keys name and purpose for a load balancer."}],
  "response_format": {"type": "json_object"},
  "max_completion_tokens": 240
}
```

JSON mode guarantees parseable JSON, not a particular shape. State the desired fields in the prompt and validate the result.

## JSON Schema

```json
{
  "model": "qwen/qwen3.8-27b",
  "messages": [{"role": "user", "content": "Describe a circuit breaker."}],
  "response_format": {
    "type": "json_schema",
    "json_schema": {
      "name": "pattern",
      "strict": true,
      "schema": {
        "type": "object",
        "properties": {
          "name": {"type": "string"},
          "benefits": {"type": "array", "items": {"type": "string"}}
        },
        "required": ["name", "benefits"],
        "additionalProperties": false
      }
    }
  },
  "max_completion_tokens": 320
}
```

The router validates schemas before forwarding. Unsupported models return a `400` capability error. Parse and validate output in the client even in strict mode; network truncation and terminal stream failures can still make an otherwise valid generation incomplete.

In Responses, place the format under `text.format`. Structured output can disable model reasoning to improve schema adherence. Streaming delivers JSON as fragments, so buffer the complete text before parsing.