Tool Use at the API Level: The Primitive

import anthropic

client = anthropic.Anthropic()

tools = [
    {
        "name": "get_weather",
        "description": "Get current weather for a city.",
        "input_schema": {
            "type": "object",
            "properties": {
                "city": {"type": "string", "description": "City name"},
                "unit": {"type": "string", "enum": ["C", "F"]}
            },
            "required": ["city"]
        }
    }
]

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

while True:
    resp = client.messages.create(
        model="claude-sonnet-4-6",
        max_tokens=1024,
        tools=tools,
        messages=messages,
    )
    if resp.stop_reason == "end_turn":
        print(resp.content[0].text)
        break
    if resp.stop_reason == "tool_use":
        tool_use = next(b for b in resp.content if b.type == "tool_use")
        result = run_tool(tool_use.name, tool_use.input)  # your code
        messages.append({"role": "assistant", "content": resp.content})
        messages.append({
            "role": "user",
            "content": [{
                "type": "tool_result",
                "tool_use_id": tool_use.id,
                "content": str(result)
            }]
        })

# Parallel execution pattern
import asyncio

async def handle_tools(tool_uses):
    results = await asyncio.gather(*[
        run_tool_async(t.name, t.input) for t in tool_uses
    ])
    return [
        {"type": "tool_result", "tool_use_id": t.id, "content": str(r)}
        for t, r in zip(tool_uses, results)
    ]