Docs / Features

Structured outputs

Force a model to return JSON matching a schema you define, instead of free-form text.

How to use it

Pass response_format with a JSON Schema. Models that support structured outputs constrain generation to match the schema; unsupported models ignore the field.

JSON

{
  "model": "openai/gpt-5.5-pro",
  "messages": [{"role": "user", "content": "Extract the invoice total"}],
  "response_format": {
    "type": "json_schema",
    "json_schema": {
      "name": "invoice",
      "schema": {
        "type": "object",
        "properties": { "total": { "type": "number" } },
        "required": ["total"]
      }
    }
  }
}

Tips

Keep schemas shallow, name fields clearly, and add a system message describing the task. Strict schemas with deep nesting reduce reliability on smaller models.