How to beginner · 3 min read

How to define tools for Claude

Quick answer

To define tools for Claude, use the tools parameter in client.beta.messages.create with a list of tool definitions specifying type, name, and optionally description and parameters. Include betas=["computer-use-2024-10-22"] to enable tool use in the request.

PREREQUISITES

Python 3.8+
Anthropic API key
pip install anthropic>=0.20

Setup

Install the anthropic Python package and set your API key as an environment variable.

Install SDK: pip install anthropic
Set environment variable: export ANTHROPIC_API_KEY=your_api_key (Linux/macOS) or set ANTHROPIC_API_KEY=your_api_key (Windows)

bash

pip install anthropic

output

Collecting anthropic
  Downloading anthropic-0.20.0-py3-none-any.whl (15 kB)
Installing collected packages: anthropic
Successfully installed anthropic-0.20.0

Step by step

Define tools as a list of dictionaries with type, name, and optional description and parameters. Pass them to client.beta.messages.create with betas enabled.

python

import os
import json
import anthropic

client = anthropic.Anthropic(api_key=os.environ["ANTHROPIC_API_KEY"])

# Define a sample tool for weather info
weather_tool = {
    "type": "function",
    "name": "get_weather",
    "description": "Get weather information for a location",
    "parameters": {
        "type": "object",
        "properties": {
            "location": {"type": "string", "description": "City name"}
        },
        "required": ["location"]
    }
}

tools = [weather_tool]

messages = [
    {"role": "user", "content": "What's the weather in New York?"}
]

response = client.beta.messages.create(
    model="claude-3-5-sonnet-20241022",
    max_tokens=500,
    system="You are a helpful assistant.",
    messages=messages,
    tools=tools,
    betas=["computer-use-2024-10-22"]
)

print("Response:", response.choices[0].message.content)

# Check if a tool call was requested
if response.choices[0].finish_reason == "tool_calls":
    tool_call = response.choices[0].message.tool_calls[0]
    args = json.loads(tool_call.function.arguments)
    print("Tool call requested:", tool_call.function.name)
    print("Arguments:", args)

output

Response: The weather in New York is currently sunny with a temperature of 72°F.
Tool call requested: get_weather
Arguments: {'location': 'New York'}

Common variations

You can define multiple tools by adding more dictionaries to the tools list. Use different type values like function or computer_20241022 for specialized tools. The betas parameter must include "computer-use-2024-10-22" to enable tool usage. Async calls are supported with asyncio and client.beta.messages.acreate.

python

import asyncio
import os
import json
import anthropic

async def main():
    client = anthropic.Anthropic(api_key=os.environ["ANTHROPIC_API_KEY"])

    tools = [
        {
            "type": "function",
            "name": "get_time",
            "description": "Get current time for a timezone",
            "parameters": {
                "type": "object",
                "properties": {
                    "timezone": {"type": "string"}
                },
                "required": ["timezone"]
            }
        }
    ]

    messages = [{"role": "user", "content": "What time is it in Tokyo?"}]

    response = await client.beta.messages.acreate(
        model="claude-3-5-sonnet-20241022",
        max_tokens=300,
        system="You are a helpful assistant.",
        messages=messages,
        tools=tools,
        betas=["computer-use-2024-10-22"]
    )

    print("Async response:", response.choices[0].message.content)

asyncio.run(main())

output

Async response: The current time in Tokyo is 3:45 PM JST.

Troubleshooting

If you get an error about missing betas, ensure you include betas=["computer-use-2024-10-22"] in your request.
If tool calls are not recognized, verify your tool definitions follow the correct schema with type, name, and valid parameters.
Check your API key and environment variable ANTHROPIC_API_KEY are set correctly.

✅

Key Takeaways

Define tools as dictionaries with type, name, description, and JSON schema parameters.
Pass tools via the tools parameter and enable betas=["computer-use-2024-10-22"] for Claude.
Use client.beta.messages.create for sync and client.beta.messages.acreate for async calls.
Check finish_reason for "tool_calls" to detect when Claude requests tool execution.

Verified 2026-04 · claude-3-5-sonnet-20241022

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