Structured Output Schemas - Phaseo Docs

Good schema design matters more than model choice for reliable structured outputs.

Start simple

Prefer flat objects over deeply nested structures.
Use required for every field you need.
Set additionalProperties: false when you need strict shape control.

Request

{
  "model": "openai/gpt-5-nano",
  "messages": [
    { "role": "user", "content": "Return a purchase summary for order 42." }
  ],
  "response_format": {
    "type": "json_schema",
    "json_schema": {
      "name": "purchase_summary",
      "strict": true,
      "schema": {
        "type": "object",
        "properties": {
          "order_id": { "type": "string" },
          "total_usd": { "type": "number" },
          "status": { "type": "string", "enum": ["pending", "paid", "failed"] }
        },
        "required": ["order_id", "total_usd", "status"],
        "additionalProperties": false
      }
    }
  }
}

Response

{
  "choices": [
    {
      "message": {
        "content": "{\"order_id\":\"42\",\"total_usd\":19.99,\"status\":\"paid\"}"
      }
    }
  ]
}

Common mistakes

Optional fields that should be required.
Overly broad string fields instead of enum constraints.
Schema drift between docs and server-side validator.

Last modified on July 26, 2026