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OpenRouter supports structured outputs for compatible models, ensuring responses follow a specific JSON Schema format. This feature is particularly useful when you need consistent, well-formatted responses that can be reliably parsed by your application.

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Overview

Structured outputs allow you to:
  • Enforce specific JSON Schema validation on model responses
  • Get consistent, type-safe outputs
  • Avoid parsing errors and hallucinated fields
  • Simplify response handling in your application

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Using Structured Outputs

To use structured outputs, include a response_format parameter in your request, with type set to json_schema and the json_schema object containing your schema: 
{
  "messages": [
    { "role": "user", "content": "What's the weather like in London?" }
  ],
  "response_format": {
    "type": "json_schema",
    "json_schema": {
      "name": "weather",
      "strict": true,
      "schema": {
        "type": "object",
        "properties": {
          "location": {
            "type": "string",
            "description": "City or location name"
          },
          "temperature": {
            "type": "number",
            "description": "Temperature in Celsius"
          },
          "conditions": {
            "type": "string",
            "description": "Weather conditions description"
          }
        },
        "required": ["location", "temperature", "conditions"],
        "additionalProperties": false
      }
    }
  }
}
The model will respond with a JSON object that strictly follows your schema: 
{
  "location": "London",
  "temperature": 18,
  "conditions": "Partly cloudy with light drizzle"
}

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Model Support

Structured outputs are supported by select models. You can find a list of models that support structured outputs on the models page.
  • OpenAI models (GPT-4o and later versions) Docs
  • Google Gemini models Docs
  • Anthropic models (Sonnet 4.5, Opus 4.1+) Docs
  • Most open-source models
  • All Fireworks provided models Docs
To ensure your chosen model supports structured outputs:
  1. Check the model’s supported parameters on the models page
  2. Set require_parameters: true in your provider preferences (see Provider Routing)
  3. Include response_format and set type: json_schema in the required parameters

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Best Practices

  1. Include descriptions: Add clear descriptions to your schema properties to guide the model
  2. Use strict mode: Always set strict: true to ensure the model follows your schema exactly

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Example Implementation

Here’s a complete example using the Fetch API:

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Streaming with Structured Outputs

Structured outputs are also supported with streaming responses. The model will stream valid partial JSON that, when complete, forms a valid response matching your schema. To enable streaming with structured outputs, simply add stream: true to your request: 
{
  "stream": true,
  "response_format": {
    "type": "json_schema",
    // ... rest of your schema
  }
}

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Error Handling

When using structured outputs, you may encounter these scenarios:
  1. Model doesn’t support structured outputs: The request will fail with an error indicating lack of support
  2. Invalid schema: The model will return an error if your JSON Schema is invalid

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Response Healing

For non-streaming requests using response_format with type: "json_schema", you can enable the Response Healing plugin to reduce the risk of invalid JSON when models return imperfect formatting. Learn more in the Response Healing documentation.