Tool Use in Claude: Official Course · Lesson 2

Your First Simple Tool

Build a calculator tool, define it with JSON Schema, pass it to Claude, and handle the ToolUseBlock response to get accurate arithmetic results.

25 min read3 questions in quizReady prompt includedIn progress

Practical exercise

What to do after this lesson

Write a temperature_converter tool with parameters value (number) and unit ('celsius'|'fahrenheit'). Implement the conversion function and a complete call cycle via Claude.

Task grader

Write a temperature_converter tool with parameters value (number) and unit ('celsius'|'fahrenheit'). Implement the conversion function and a complete call cycle via Claude.

Your answer

Ready-to-use prompt

Template for this lesson

Copy and adapt to your context. Text in angle brackets should be replaced.

I want to give Claude a tool for <task>.
The function is already written:

def my_tool(<params>):
    # ...

Write the correct tool definition in JSON Schema format
with name, description, and input_schema fields.
Explain each parameter and whether it's required or optional.

Prompt sandbox

Prompt

Common mistakes

What people get wrong

Forgetting enum for string parameters with a fixed set of valid values.
Confusing response.content[0].text (text) with response.content[0].input (tool inputs).

response = client.messages.create( model="claude-3-haiku-20240307", messages=[{"role": "user", "content": "Multiply 1984135 by 9343116. Only respond with the result"}], max_tokens=400 ) # Claude replied: 18593367726060 # Correct answer: 18538003464660 # Off by 55,364,261,400!

Solution: Give Claude a Calculator

def calculator(operation, operand1, operand2): if operation == "add": return operand1 + operand2 elif operation == "subtract": return operand1 - operand2 elif operation == "multiply": return operand1 * operand2 elif operation == "divide": if operand2 == 0: raise ValueError("Cannot divide by zero.") return operand1 / operand2 calculator_tool = { "name": "calculator", "description": "A simple calculator that performs basic arithmetic operations.", "input_schema": { "type": "object", "properties": { "operation": { "type": "string", "enum": ["add", "subtract", "multiply", "divide"], "description": "The arithmetic operation to perform." }, "operand1": {"type": "number", "description": "The first operand."}, "operand2": {"type": "number", "description": "The second operand."} }, "required": ["operation", "operand1", "operand2"] } }

Full Cycle: Extract Inputs and Call the Function

response = client.messages.create( model="claude-3-haiku-20240307", messages=[{"role": "user", "content": "Multiply 1984135 by 9343116. Only respond with the result"}], max_tokens=300, tools=[calculator_tool] ) # response.stop_reason == "tool_use" tool_name = response.content[0].name # "calculator" tool_inputs = response.content[0].input # {"operation": "multiply", "operand1": 1984135, "operand2": 9343116} result = calculator(tool_inputs["operation"], tool_inputs["operand1"], tool_inputs["operand2"]) # RESULT: 18538003464660 — correct!

Report a bug

Your First Simple Tool

Task grader

Prompt sandbox

Quiz — 3 questions

Discussion

The Problem: Claude Gets Math Wrong

Solution: Give Claude a Calculator

Full Cycle: Extract Inputs and Call the Function

System Prompt Against Tool-Hungry Claude