Strands Agents SDK 実践 — Hooks でエージェントループを制御する

from strands import Agent, tool
from strands.models import BedrockModel
from strands.hooks import BeforeToolCallEvent, AfterToolCallEvent
 
bedrock_model = BedrockModel(
    model_id="us.anthropic.claude-sonnet-4-20250514-v1:0",
    region_name="us-east-1",
)
 
@tool
def get_weather(city: str) -> str:
    """Get the current weather for a city.
 
    Args:
        city: The city name
 
    Returns:
        str: Weather information
    """
    weather_data = {
        "Tokyo": "Sunny, 22°C",
        "London": "Cloudy, 15°C",
        "New York": "Rainy, 18°C",
    }
    return weather_data.get(city, f"No data for {city}")
 
def log_before_tool(event: BeforeToolCallEvent) -> None:
    print(f"[HOOK] Before: {event.tool_use['name']}({event.tool_use['input']})")
 
def log_after_tool(event: AfterToolCallEvent) -> None:
    status = event.result.get("status", "unknown")
    print(f"[HOOK] After: {event.tool_use['name']} -> {status}")
 
agent = Agent(model=bedrock_model, tools=[get_weather], callback_handler=None)
agent.add_hook(log_before_tool)
agent.add_hook(log_after_tool)
 
result = agent("What's the weather in Tokyo and London?")
print(f"\nAnswer: {result.message['content'][0]['text']}")

from strands import Agent, tool
from strands.models import BedrockModel
from strands.hooks import BeforeToolCallEvent, BeforeInvocationEvent
 
bedrock_model = BedrockModel(
    model_id="us.anthropic.claude-sonnet-4-20250514-v1:0",
    region_name="us-east-1",
)
 
@tool
def get_weather(city: str) -> str:
    """Get the current weather for a city.
 
    Args:
        city: The city name
 
    Returns:
        str: Weather information
    """
    weather_data = {
        "Tokyo": "Sunny, 22°C",
        "London": "Cloudy, 15°C",
        "New York": "Rainy, 18°C",
        "Paris": "Windy, 16°C",
        "Sydney": "Clear, 25°C",
    }
    return weather_data.get(city, f"No data for {city}")
 
tool_count = 0
 
def reset_count(event: BeforeInvocationEvent) -> None:
    global tool_count
    tool_count = 0
 
def limit_tool_calls(event: BeforeToolCallEvent) -> None:
    global tool_count
    tool_count += 1
    if tool_count > 2:
        event.cancel_tool = (
            f"Tool '{event.tool_use['name']}' call limit reached (max 2). "
            "DO NOT CALL THIS TOOL ANYMORE."
        )
        print(f"[HOOK] BLOCKED: {event.tool_use['name']}({event.tool_use['input']})")
    else:
        print(f"[HOOK] ALLOWED ({tool_count}/2): {event.tool_use['name']}({event.tool_use['input']})")
 
agent = Agent(model=bedrock_model, tools=[get_weather], callback_handler=None)
agent.add_hook(reset_count)
agent.add_hook(limit_tool_calls)
 
result = agent("What's the weather in Tokyo, London, New York, Paris, and Sydney?")

from strands import Agent, tool
from strands.models import BedrockModel
from strands.hooks import AfterToolCallEvent
 
bedrock_model = BedrockModel(
    model_id="us.anthropic.claude-sonnet-4-20250514-v1:0",
    region_name="us-east-1",
)
 
@tool
def calculate(expression: str) -> str:
    """Evaluate a math expression.
 
    Args:
        expression: A math expression to evaluate (e.g. "2 + 3")
 
    Returns:
        str: The result of the calculation
    """
    result = eval(expression)
    return str(result)
 
def format_result(event: AfterToolCallEvent) -> None:
    if event.tool_use["name"] == "calculate":
        original = event.result["content"][0]["text"]
        expression = event.tool_use["input"]["expression"]
        event.result["content"][0]["text"] = f"[FORMATTED] {expression} = {original}"
        print(f"[HOOK] Modified result: {original} -> {event.result['content'][0]['text']}")
 
agent = Agent(model=bedrock_model, tools=[calculate], callback_handler=None)
agent.add_hook(format_result)
 
result = agent("What is 42 * 58?")
print(f"\nAnswer: {result.message['content'][0]['text']}")