Structured Outputs

Structured outputs let you constrain the model to return valid JSON that conforms to a specific schema. This is essential when the model's response needs to be parsed by code rather than read by a human.

JSON Mode

The simplest way to get JSON output is to set response_format to { "type": "json_object" }. This guarantees the model's response is valid JSON.

{
  "model": "dos-ai",
  "messages": [
    {
      "role": "system",
      "content": "You are a helpful assistant that always responds in JSON format."
    },
    {
      "role": "user",
      "content": "List 3 programming languages with their year of creation."
    }
  ],
  "response_format": { "type": "json_object" }
}

Response:

{
  "choices": [
    {
      "message": {
        "role": "assistant",
        "content": "{\n  \"languages\": [\n    {\"name\": \"Python\", \"year\": 1991},\n    {\"name\": \"JavaScript\", \"year\": 1995},\n    {\"name\": \"Go\", \"year\": 2009}\n  ]\n}"
      },
      "finish_reason": "stop"
    }
  ]
}

The content field is a JSON string. Parse it in your code:

import json

data = json.loads(response.choices[0].message.content)
print(data["languages"][0]["name"])  # "Python"

Important: When using JSON mode, you must include the word "JSON" somewhere in the system or user message. This is a safety measure to ensure the user intends to receive JSON output.

JSON Schema Enforcement

For stronger guarantees, you can provide a JSON Schema that the model's output must conform to. This uses response_format with type: "json_schema":

{
  "model": "dos-ai",
  "messages": [
    {
      "role": "user",
      "content": "Analyze the sentiment of this review: 'The product arrived quickly and works perfectly. Very satisfied with the purchase.'"
    }
  ],
  "response_format": {
    "type": "json_schema",
    "json_schema": {
      "name": "sentiment_analysis",
      "strict": true,
      "schema": {
        "type": "object",
        "properties": {
          "sentiment": {
            "type": "string",
            "enum": ["positive", "negative", "neutral", "mixed"]
          },
          "confidence": {
            "type": "number",
            "description": "Confidence score between 0 and 1."
          },
          "summary": {
            "type": "string",
            "description": "Brief explanation of the sentiment."
          },
          "keywords": {
            "type": "array",
            "items": { "type": "string" },
            "description": "Key phrases that influenced the analysis."
          }
        },
        "required": ["sentiment", "confidence", "summary", "keywords"],
        "additionalProperties": false
      }
    }
  }
}

Response:

{
  "choices": [
    {
      "message": {
        "content": "{\"sentiment\": \"positive\", \"confidence\": 0.95, \"summary\": \"The reviewer expresses strong satisfaction with both shipping speed and product quality.\", \"keywords\": [\"arrived quickly\", \"works perfectly\", \"very satisfied\"]}"
      }
    }
  ]
}

Schema Rules

When using strict: true, the following rules apply:

• All fields must be listed in required.

• additionalProperties must be set to false.

• Supported types: string, number, integer, boolean, array, object, null.

• Use enum to restrict string values.

• Nested objects must also follow these rules.

Code Examples

Python

import json
from openai import OpenAI

client = OpenAI(
    api_key="dos_sk_your_api_key_here",
    base_url="https://api.dos.ai/v1",
)

# Simple JSON mode
response = client.chat.completions.create(
    model="dos-ai",
    messages=[
        {
            "role": "system",
            "content": "Extract the entities from the text. Respond in JSON with keys: persons (array of strings), locations (array of strings), organizations (array of strings).",
        },
        {
            "role": "user",
            "content": "Elon Musk announced that Tesla will open a new factory in Berlin, in partnership with the German government.",
        },
    ],
    response_format={"type": "json_object"},
)

data = json.loads(response.choices[0].message.content)
print(data)
# {
#   "persons": ["Elon Musk"],
#   "locations": ["Berlin"],
#   "organizations": ["Tesla", "German government"]
# }

Python with JSON Schema

import json
from openai import OpenAI

client = OpenAI(
    api_key="dos_sk_your_api_key_here",
    base_url="https://api.dos.ai/v1",
)

response = client.chat.completions.create(
    model="dos-ai",
    messages=[
        {
            "role": "user",
            "content": "Generate a recipe for pho bo (Vietnamese beef noodle soup).",
        }
    ],
    response_format={
        "type": "json_schema",
        "json_schema": {
            "name": "recipe",
            "strict": True,
            "schema": {
                "type": "object",
                "properties": {
                    "name": {"type": "string"},
                    "servings": {"type": "integer"},
                    "prep_time_minutes": {"type": "integer"},
                    "cook_time_minutes": {"type": "integer"},
                    "ingredients": {
                        "type": "array",
                        "items": {
                            "type": "object",
                            "properties": {
                                "item": {"type": "string"},
                                "quantity": {"type": "string"},
                            },
                            "required": ["item", "quantity"],
                            "additionalProperties": False,
                        },
                    },
                    "steps": {
                        "type": "array",
                        "items": {"type": "string"},
                    },
                },
                "required": [
                    "name",
                    "servings",
                    "prep_time_minutes",
                    "cook_time_minutes",
                    "ingredients",
                    "steps",
                ],
                "additionalProperties": False,
            },
        },
    },
)

recipe = json.loads(response.choices[0].message.content)
print(f"Recipe: {recipe['name']}")
print(f"Servings: {recipe['servings']}")
print(f"Prep: {recipe['prep_time_minutes']} min, Cook: {recipe['cook_time_minutes']} min")
for ingredient in recipe["ingredients"]:
    print(f"  - {ingredient['quantity']} {ingredient['item']}")

JavaScript

import OpenAI from "openai";

const client = new OpenAI({
  apiKey: "dos_sk_your_api_key_here",
  baseURL: "https://api.dos.ai/v1",
});

// Simple JSON mode
const response = await client.chat.completions.create({
  model: "dos-ai",
  messages: [
    {
      role: "system",
      content:
        "Extract entities from the text. Respond in JSON with keys: persons, locations, organizations (all arrays of strings).",
    },
    {
      role: "user",
      content:
        "Elon Musk announced that Tesla will open a new factory in Berlin.",
    },
  ],
  response_format: { type: "json_object" },
});

const data = JSON.parse(response.choices[0].message.content);
console.log(data);

JavaScript with JSON Schema

import OpenAI from "openai";

const client = new OpenAI({
  apiKey: "dos_sk_your_api_key_here",
  baseURL: "https://api.dos.ai/v1",
});

const response = await client.chat.completions.create({
  model: "dos-ai",
  messages: [
    {
      role: "user",
      content:
        "Classify this support ticket: 'My order #12345 has not arrived after 2 weeks. I want a refund.'",
    },
  ],
  response_format: {
    type: "json_schema",
    json_schema: {
      name: "ticket_classification",
      strict: true,
      schema: {
        type: "object",
        properties: {
          category: {
            type: "string",
            enum: ["billing", "shipping", "product", "account", "other"],
          },
          priority: {
            type: "string",
            enum: ["low", "medium", "high", "urgent"],
          },
          summary: { type: "string" },
          requires_human: { type: "boolean" },
        },
        required: ["category", "priority", "summary", "requires_human"],
        additionalProperties: false,
      },
    },
  },
});

const ticket = JSON.parse(response.choices[0].message.content);
console.log(ticket);
// {
//   category: "shipping",
//   priority: "high",
//   summary: "Customer reports undelivered order after 2 weeks and requests refund.",
//   requires_human: true
// }

cURL

curl https://api.dos.ai/v1/chat/completions \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer dos_sk_your_api_key_here" \
  -d '{
    "model": "dos-ai",
    "messages": [
      {
        "role": "system",
        "content": "You are a data extraction assistant. Respond in JSON."
      },
      {
        "role": "user",
        "content": "Extract contact info: John Smith, [e-mail], [phone], works at Acme Corp."
      }
    ],
    "response_format": {"type": "json_object"}
  }'

Tips for Reliable JSON Output

1. Always mention JSON in the prompt

When using response_format: { type: "json_object" }, include an explicit instruction about JSON in the system message. Without it, the model may produce an error or unpredictable output.

# Good
messages = [
    {"role": "system", "content": "Respond with a JSON object containing..."},
    {"role": "user", "content": "..."}
]

# Bad -- may fail without JSON mentioned in messages
messages = [
    {"role": "user", "content": "List some countries."}
]

2. Describe the expected shape

Even with JSON mode, the model needs to know what structure you expect. Be explicit about field names and types:

Respond with a JSON object with these fields:
- "name" (string): the item name
- "price" (number): price in USD
- "in_stock" (boolean): availability status
- "tags" (array of strings): relevant categories

3. Use JSON Schema for critical applications

For production systems where invalid JSON would cause failures, prefer json_schema with strict: true over plain json_object mode. The schema enforcement happens at the decoding level, guaranteeing structural correctness.

4. Handle parsing errors gracefully

Even with structured outputs, always wrap JSON parsing in error handling:

import json

try:
    data = json.loads(response.choices[0].message.content)
except json.JSONDecodeError as e:
    print(f"Failed to parse JSON: {e}")
    # Retry or fall back to a default

5. Keep schemas simple

Deeply nested schemas with many optional fields increase the chance of unexpected output. Flatten where possible and use required to ensure critical fields are always present.

Comparison: JSON Mode vs JSON Schema

Feature	JSON Mode	JSON Schema
Setting	{"type": "json_object"}	{"type": "json_schema", "json_schema": {...}}
Guarantee	Valid JSON	Valid JSON conforming to your schema
Field control	Must be described in prompt	Enforced by schema
strict mode	Not applicable	Optional (recommended)
Use case	Simple/flexible JSON responses	Production APIs, data pipelines

Next Steps

• Chat Completions -- learn the basics of the API.

• Function Calling -- let the model call external tools.

• Streaming -- receive tokens in real time.