Anthropic Tool Use (Function Calling)

Overview

Tool use lets Claude call functions you define — query databases, hit APIs, read files, do math. Claude decides when to call a tool, you execute it, and feed the result back. This is how you build Claude-powered agents.

Note: Anthropic does not offer an embeddings API. For embeddings + vector search, pair Claude with a dedicated embedding model (OpenAI, Cohere, or Voyage).

Prerequisites

• Completed clade-model-inference
• Understanding of JSON Schema for tool definitions

Instructions

Step 1: Define Tools

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

const client = new Anthropic();

const tools: Anthropic.Tool[] = [
  {
    name: 'get_weather',
    description: 'Get current weather for a city. Call this when the user asks about weather.',
    input_schema: {
      type: 'object',
      properties: {
        city: { type: 'string', description: 'City name, e.g. "San Francisco"' },
        unit: { type: 'string', enum: ['celsius', 'fahrenheit'], description: 'Temperature unit' },
      },
      required: ['city'],
    },
  },
];

Step 2: Send Message with Tools

const response = await client.messages.create({
  model: 'claude-sonnet-4-20250514',
  max_tokens: 1024,
  tools,
  messages: [{ role: 'user', content: "What's the weather in San Francisco?" }],
});

// Claude responds with stop_reason: 'tool_use'
// response.content includes a tool_use block:
// { type: 'tool_use', id: 'toolu_01...', name: 'get_weather', input: { city: 'San Francisco' } }

Step 3: Execute Tool and Return Result

// Find the tool use block
const toolUse = response.content.find(block => block.type === 'tool_use');

// Execute your function
const weatherData = await fetchWeather(toolUse.input.city);

// Send result back to Claude
const finalResponse = await client.messages.create({
  model: 'claude-sonnet-4-20250514',
  max_tokens: 1024,
  tools,
  messages: [
    { role: 'user', content: "What's the weather in San Francisco?" },
    { role: 'assistant', content: response.content },
    {
      role: 'user',
      content: [{
        type: 'tool_result',
        tool_use_id: toolUse.id,
        content: JSON.stringify(weatherData),
      }],
    },
  ],
});

console.log(finalResponse.content[0].text);
// "The weather in San Francisco is currently 65°F and partly cloudy."