JSON mode

JSON mode forces a model to emit syntactically valid JSON. It’s a feature of the OpenAI Chat Completions API, enabled by setting response_format to {"type": "json_object"}. Use JSON mode when you need parseable JSON but don’t need to enforce a specific schema. For most production use cases, prefer structured outputs. Structured outputs guarantee that the response matches a JSON schema you provide, which is stricter, type-safe, and removes the need to retry or validate after the fact.

How it works

JSON mode tells the server to constrain the output to valid JSON. You still describe the fields you want in the prompt; the server only enforces well-formedness, not shape.

json_mode.py

import os
from openai import OpenAI

client = OpenAI(
    base_url="https://inference.baseten.co/v1",
    api_key=os.environ["BASETEN_API_KEY"],
)

response = client.chat.completions.create(
    model="deepseek-ai/DeepSeek-V4-Pro",
    messages=[
        {"role": "system", "content": "You are a helpful assistant that responds only in JSON."},
        {"role": "user", "content": "List three planets with their distance from the sun in km. Respond as a JSON object with a 'planets' array."},
    ],
    response_format={"type": "json_object"},
)

print(response.choices[0].message.content)

Ask for JSON in the prompt so the model produces a useful shape. The server constrains output to valid JSON but doesn’t infer the schema from the request.

JSON mode versus structured outputs

Feature	JSON mode	Structured outputs
Output guarantee	Valid JSON	Valid JSON that matches your schema
Schema enforcement	None	Strict (server rejects non-conforming generations)
Setup	Set `response_format` to `json_object`	Provide a JSON schema or Pydantic model
Best for	Lightweight extraction, ad hoc responses	Production data extraction, typed pipelines

Reach for JSON mode when you don’t want to define a schema and the downstream consumer can tolerate flexible field sets. Otherwise, use structured outputs.

Model support

JSON mode and structured outputs are supported on a per-model basis. See the feature support table on the Model APIs overview for which models support each.

Structured outputs: Schema-enforced JSON output.
Model APIs overview: Supported models and feature matrix.
Chat Completions reference: Full request and response schema.

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How it works

JSON mode versus structured outputs

Model support

​How it works

​JSON mode versus structured outputs

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How it works

JSON mode versus structured outputs

Model support

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