Strict tool use

Setting strict: true on a tool definition guarantees Claude's tool inputs match your JSON Schema by constraining the model's token sampling to schema-valid outputs (a technique called grammar-constrained sampling). This page covers why strict mode matters for agents, how to enable it, and common use cases. For the supported JSON Schema subset, see JSON Schema limitations. For non-strict schema guidance, see Define tools.

Strict tool use validates tool parameters, ensuring Claude calls your functions with correctly-typed arguments. Use strict tool use when you need to:

• Validate tool parameters
• Build agentic workflows
• Ensure type-safe function calls
• Handle complex tools with nested properties

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Why strict tool use matters for agents

Building reliable agentic systems requires guaranteed schema conformance. Without strict mode, Claude might return incompatible types ("2" instead of 2) or omit required fields, breaking your functions and causing runtime errors.

Strict tool use guarantees type-safe parameters:

• Functions receive correctly-typed arguments every time
• No need to validate and retry tool calls
• Production-ready agents that work consistently at scale

For example, suppose a booking system needs passengers: int. Without strict mode, Claude might provide passengers: "two" or passengers: "2". With strict: true, the response always contains passengers: 2.

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Quick start

client = anthropic.Anthropic()

response = client.messages.create(
    model="claude-opus-4-8",
    max_tokens=1024,
    messages=[{"role": "user", "content": "What's the weather like in San Francisco?"}],
    tools=[
        {
            "name": "get_weather",
            "description": "Get the current weather in a given location",
            "strict": True,  # Enable strict mode
            "input_schema": {
                "type": "object",
                "properties": {
                    "location": {
                        "type": "string",
                        "description": "The city and state, e.g. San Francisco, CA",
                    },
                    "unit": {
                        "type": "string",
                        "enum": ["celsius", "fahrenheit"],
                        "description": "The unit of temperature, either 'celsius' or 'fahrenheit'",
                    },
                },
                "required": ["location"],
                "additionalProperties": False,
            },
        }
    ],
)
print(response.content)

Response format: Tool use blocks with validated inputs in response.content[x].input

{
  "type": "tool_use",
  "name": "get_weather",
  "input": {
    "location": "San Francisco, CA"
  }
}

Guarantees:

• Tool input strictly follows the input_schema
• Tool name is always valid (from provided tools or server tools)

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How it works

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Common use cases

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Data retention

Strict tool use compiles tool input_schema definitions into grammars using the same pipeline as structured outputs. Tool schemas are temporarily cached for up to 24 hours since last use. Prompts and responses are not retained beyond the API response.

Strict tool use is HIPAA eligible, but PHI must not be included in tool schema definitions. The API caches compiled schemas separately from message content, and these cached schemas do not receive the same PHI protections as prompts and responses. Do not include PHI in input_schema property names, enum values, const values, or pattern regular expressions. PHI should only appear in message content (prompts and responses), where it is protected under HIPAA safeguards.

For ZDR and HIPAA eligibility across all features, see API and data retention.

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Next steps