Tool Use

For authentication and model list, see Overview.

What is Tool Use

Tool Use (also called function calling) allows Claude to interact with external tools, APIs, and data sources. Claude doesn’t execute tools directly—it generates structured requests that your application executes.

How It Works

Define tools with names, descriptions, and input schemas
Claude identifies when a tool is needed
Claude generates a tool use request (JSON)
Your app executes the tool
Return result to Claude
Claude responds with the final answer

Basic Example

Python
Node.js

import os
import anthropic

client = anthropic.Anthropic(
  api_key=os.getenv('PIPELLM_API_KEY'),
  base_url='https://api.pipellm.ai'
)

# Define tools
tools = [
  {
    "name": "get_weather",
    "description": "Get the current weather in a location",
    "input_schema": {
      "type": "object",
      "properties": {
        "location": {
          "type": "string",
          "description": "City name, e.g., San Francisco"
        }
      },
      "required": ["location"]
    }
  }
]

# First call - Claude decides to use the tool
response = client.messages.create(
  model="claude-sonnet-4-5-20250929",
  max_tokens=1024,
  tools=tools,
  messages=[
    {"role": "user", "content": "What's the weather in Tokyo?"}
  ]
)

# Check if Claude wants to use a tool
if response.stop_reason == "tool_use":
  tool_use = next(b for b in response.content if b.type == "tool_use")
  
  # Execute your tool (example)
  tool_result = {"temperature": "22°C", "condition": "Sunny"}
  
  # Return result to Claude
  final_response = client.messages.create(
    model="claude-sonnet-4-5-20250929",
    max_tokens=1024,
    tools=tools,
    messages=[
      {"role": "user", "content": "What's the weather in Tokyo?"},
      {"role": "assistant", "content": response.content},
      {
        "role": "user",
        "content": [
          {
            "type": "tool_result",
            "tool_use_id": tool_use.id,
            "content": str(tool_result)
          }
        ]
      }
    ]
  )
  
  print(final_response.content[0].text)

import Anthropic from '@anthropic-ai/sdk';

const client = new Anthropic({
  apiKey: process.env.PIPELLM_API_KEY,
  baseURL: 'https://api.pipellm.ai'
});

const tools = [
  {
    name: "get_weather",
    description: "Get the current weather in a location",
    input_schema: {
      type: "object",
      properties: {
        location: {
          type: "string",
          description: "City name, e.g., San Francisco"
        }
      },
      required: ["location"]
    }
  }
];

const response = await client.messages.create({
  model: 'claude-sonnet-4-5-20250929',
  max_tokens: 1024,
  tools: tools,
  messages: [
    { role: 'user', content: "What's the weather in Tokyo?" }
  ]
});

if (response.stop_reason === 'tool_use') {
  const toolUse = response.content.find(b => b.type === 'tool_use');
  
  // Execute your tool
  const toolResult = { temperature: "22°C", condition: "Sunny" };
  
  const finalResponse = await client.messages.create({
    model: 'claude-sonnet-4-5-20250929',
    max_tokens: 1024,
    tools: tools,
    messages: [
      { role: 'user', content: "What's the weather in Tokyo?" },
      { role: 'assistant', content: response.content },
      {
        role: 'user',
        content: [
          {
            type: 'tool_result',
            tool_use_id: toolUse.id,
            content: JSON.stringify(toolResult)
          }
        ]
      }
    ]
  });
  
  console.log(finalResponse.content[0].text);
}

More Resources

Tool Use Documentation

Complete guide on defining tools and handling responses

Overview

Using the API

OpenAI Compatible

Anthropic

Gemini

What is Tool Use

How It Works

Basic Example

More Resources

Tool Use Documentation

​What is Tool Use

​How It Works

​Basic Example

​More Resources

Tool Use Documentation

What is Tool Use

How It Works

Basic Example

More Resources