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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.

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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. 
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.

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JSON mode versus structured outputs

FeatureJSON modeStructured outputs
Output guaranteeValid JSONValid JSON that matches your schema
Schema enforcementNoneStrict (server rejects non-conforming generations)
SetupSet response_format to json_objectProvide a JSON schema or Pydantic model
Best forLightweight extraction, ad hoc responsesProduction 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.

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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.