Structured outputs: two distinct features

Constraining the response body and constraining tool parameters are separate knobs.

5 min read · Lesson 1 of 12 in this domain

There are two separate things you might want to constrain, and the exam expects you to keep them apart. One is the response body — the text Claude sends back — which you shape with output_config.format and a JSON schema. The other is tool parameters — the arguments Claude passes when calling your function — which you lock down with strict: true on the tool definition. They solve different problems and compose freely in the same request. The classic slip is putting strict inside tool_choice; tool_choice only decides which tool runs, never how its inputs are validated.

• JSON outputs — output_config.format with a json_schema constrains the response body.
• Strict tool use — strict: true as a top-level field on the tool definition (beside name and input_schema, not inside tool_choice) guarantees tool parameters validate exactly.
• The two are complementary and can be used in the same request.
• The primary benefit is a stable contract: consumers read known JSON keys directly instead of regex-parsing prose.
• The old top-level output_format parameter is deprecated — use output_config.format.
• A schema must declare additionalProperties: false and list its required fields.

response = client.messages.create(
    model="claude-opus-5", max_tokens=16000,
    output_config={"format": {"type": "json_schema", "schema": {...}}},
    tools=[{
        "name": "search_flights",
        "strict": True,
        "input_schema": {
            "type": "object",
            "properties": {"destination": {"type": "string"}},
            "required": ["destination"],
            "additionalProperties": False
        }
    }],
    messages=[...]
)

Source: Claude Docs — Structured outputs · Independent study aid, not affiliated with or endorsed by Anthropic.