Structured Outputs

FastRouter supports structured JSON outputs, allowing you to enforce a specific schema in LLM responses. This feature ensures that responses are machine-readable and conform to a predefined structure.

Overview

Using the response_format parameter, you can define a custom JSON schema that the model must follow. FastRouter will guide compatible models to format their outputs accordingly.

Supported Format

• Type: json_schema

• Schema: Defined using standard JSON Schema format

• Strict Mode: If strict: true, compatible models will respond with a JSON object that strictly follows your schema.

Example: Enforcing Weather Data Schema

This example instructs the model to return weather data in a strict JSON format:

curl --location 'https://api.fastrouter.ai/api/v1/chat/completions' \
--header 'Content-Type: application/json' \
--header 'Authorization: Bearer API-KEY' \
--data '{
    "model": "openai/gpt-4o",
    "messages": [
      {
        "role": "system",
        "content": "You are a helpful assistant that responds in JSON format."
      },
      {
        "role": "user",
        "content": "What is 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
        }
      }
    }
}' 

Response Example

{
  "location": "London",
  "temperature": 17.5,
  "conditions": "Partly cloudy"
}

Schema Validation Options

Parameter	Description
type	Must be set to "json_schema"
name	Optional name for the schema block (useful for logging/debugging)
strict	Informs the provider to validate the output against the schema (true/false)
schema	A valid JSON Schema definition
additionalProperties	Set to false to block unexpected fields in the response

Best Practices

• Always define a system prompt that clearly instructs the model to respond in JSON format.

• Use strict: true for use cases that demand guaranteed structure.

• Combine with stream: false for easier schema validation (as streaming responses may break structure).

• Validate responses client-side to catch schema mismatches early.