Strict tool use

Setting strict: true on a tool definition uses grammar-constrained sampling to guarantee Claude's tool inputs match your JSON Schema. 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

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 missing 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 will always contain passengers: 2.

Quick start

curl https://api.anthropic.com/v1/messages \
  -H "content-type: application/json" \
  -H "x-api-key: $ANTHROPIC_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -d '{
    "model": "claude-opus-4-6",
    "max_tokens": 1024,
    "messages": [
      {"role": "user", "content": "What is the weather in San Francisco?"}
    ],
    "tools": [{
      "name": "get_weather",
      "description": "Get the current weather in a given location",
      "strict": true,
      "input_schema": {
        "type": "object",
        "properties": {
          "location": {
            "type": "string",
            "description": "The city and state, e.g. San Francisco, CA"
          },
          "unit": {
            "type": "string",
            "enum": ["celsius", "fahrenheit"]
          }
        },
        "required": ["location"],
        "additionalProperties": false
      }
    }]
  }'

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)

How it works

Common use cases