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When you need the model to return JSON that exactly matches a schema, use response_format. Cogito enforces the schema with grammar-constrained decoding — the model literally cannot emit tokens that would break the schema, so you don’t need to ask nicely or add “respond only in JSON” to the system prompt.

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JSON schema mode

response = client.chat.completions.create(
    model="gpt-oss-120b",
    messages=[
        {"role": "user", "content": "Extract entities from: 'Apple acquired Beats in 2014 for $3B.'"}
    ],
    response_format={
        "type": "json_schema",
        "json_schema": {
            "name": "entity_extraction",
            "strict": True,
            "schema": {
                "type": "object",
                "properties": {
                    "company": {"type": "string"},
                    "acquired": {"type": "string"},
                    "year": {"type": "integer"},
                    "amount_usd": {"type": "string"}
                },
                "required": ["company", "acquired", "year", "amount_usd"],
                "additionalProperties": False
            }
        }
    }
)

import json
data = json.loads(response.choices[0].message.content)
# {"company": "Apple", "acquired": "Beats", "year": 2014, "amount_usd": "$3B"}

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What “strict” guarantees

With strict: true, every property in required is present, every value matches its declared type, and no extra fields are emitted. You can json.loads() without a try/except in production.

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json_object mode

For “give me any valid JSON” without a schema: 
response = client.chat.completions.create(
    model="gpt-oss-120b",
    messages=[...],
    response_format={"type": "json_object"},
)
Less strict — the structure is up to the model — but the result is still guaranteed valid JSON.

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Streaming structured outputs

Both modes work with stream: true. Chunks arrive as token-level deltas and assemble into valid JSON once the stream completes. Don’t JSON.parse partial chunks; only the assembled string is guaranteed valid.

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Errors

If the schema is malformed (recursive without a depth limit, mutually exclusive constraints), Cogito returns a 400 with code: "invalid_response_format" before generation starts. We don’t waste your tokens on impossible schemas.