langchain tool - how to create a lang chain tool

There are 3 options to create lang chain tool.  The first option is using 

1. @tool decorator. Let's say we are creating a simple tool and all we need to do is use tool decorator like this

from langchain_core.tools import tool

from pydantic import BaseModel, Field

class CalculatorInput(BaseModel):

    a: int = Field(description="first number")

    b: int = Field(description="second number")

@tool("multiplication tool", args_schema=CalculatorInput)

def multiply(a: int, b: int) -> int:

    """Multiply two numbers."""

    return a * b

Then you can start binding the agent to this tool:

from cal import multiply

tools = [multiply]

model_with_tools = model.bind_tools(tools)

response = model_with_tools.invoke([HumanMessage(content="multiply 2 * 2")])

print(f"ContentString: {response.content}")

print(f"ToolCalls: {response.tool_calls}")

To get the agent to do its work

from langgraph.prebuilt import create_react_agent

agent_executor = create_react_agent(model, tools)

response = agent_executor.invoke(

    {"messages": [HumanMessage(content="multiply 2 with 2")]}

)