Tutorial: Build a tool-using agent

A guided walkthrough from a single tool call to a production-ready agentic loop.

This tutorial builds a calendar-management agent in five concentric rings. Each ring is a complete, runnable program that adds exactly one concept to the ring before it. By the end you will have written the agentic loop by hand and then replaced it with the Tool Runner SDK abstraction.

The example tool is create_calendar_event. Its schema uses nested objects, arrays, and optional fields, so you will see how Claude handles realistic input shapes rather than a single flat string.

Ring 1: Single tool, single turn

The smallest possible tool-using program: one tool, one user message, one tool call, one result. The code is heavily commented so you can map each line to the tool use lifecycle.

The request sends a tools array alongside the user message. When Claude determines that a tool call is needed, the response comes back with stop_reason: "tool_use" and a tool_use content block containing the tool name, a unique id, and the structured input. Your code runs the tool, then sends the result back in a tool_result block whose tool_use_id matches the id from the call.

# Ring 1: Single tool, single turn.

import json

import anthropic

# Create a client. It reads ANTHROPIC_API_KEY from the environment.
client = anthropic.Anthropic()

# Define one tool. The input_schema is a JSON Schema object describing
# the arguments Claude should pass when it calls this tool. This schema
# includes nested objects (recurrence), arrays (attendees), and optional
# fields, which is closer to real-world tools than a flat string argument.
tools = [
    {
        "name": "create_calendar_event",
        "description": "Create a calendar event with attendees and optional recurrence.",
        "input_schema": {
            "type": "object",
            "properties": {
                "title": {"type": "string"},
                "start": {"type": "string", "format": "date-time"},
                "end": {"type": "string", "format": "date-time"},
                "attendees": {
                    "type": "array",
                    "items": {"type": "string", "format": "email"},
                },
                "recurrence": {
                    "type": "object",
                    "properties": {
                        "frequency": {"enum": ["daily", "weekly", "monthly"]},
                        "count": {"type": "integer", "minimum": 1},
                    },
                },
            },
            "required": ["title", "start", "end"],
        },
    }
]

# Send the user's request along with the tool definition. Claude decides
# whether to call the tool based on the request and the tool description.
response = client.messages.create(
    model="claude-opus-5",
    max_tokens=1024,
    tools=tools,
    tool_choice={"type": "auto", "disable_parallel_tool_use": True},
    messages=[
        {
            "role": "user",
            "content": "Schedule a 30-minute sync with [email protected] and [email protected] on Monday, March 30, 2026 at 10am.",
        }
    ],
)

# When Claude calls a tool, the response has stop_reason "tool_use"
# and the content array contains a tool_use block alongside any text.
print(f"stop_reason: {response.stop_reason}")

# Find the tool_use block. A response may contain text blocks before the
# tool_use block, so scan the content array rather than assuming position.
tool_use = next(block for block in response.content if block.type == "tool_use")
print(f"Tool: {tool_use.name}")
print(f"Input: {tool_use.input}")

# Execute the tool. In a real system this would call your calendar API.
# Here the result is hardcoded to keep the example self-contained.
result = {"event_id": "evt_123", "status": "created"}

# Send the result back. The tool_result block goes in a user message and
# its tool_use_id must match the id from the tool_use block above. The
# assistant's previous response is included so Claude has the full history.
followup = client.messages.create(
    model="claude-opus-5",
    max_tokens=1024,
    tools=tools,

Ring 1: Single tool, single turn

Ring 1: Single tool, single turn