What a tool actually is

A tool is a Python function with a docstring. The decorator reads your function and writes the description a model needs.

from langchain_core.tools import tool

@tool
def lookup_order(order_id: str) -> str:
    """Look up the status of an order by its id."""
    return f"Order {order_id} shipped on 3 March."

print("name:       ", lookup_order.name)
print("description:", lookup_order.description)
print("args:       ", lookup_order.args)
print("calling it: ", lookup_order.invoke({"order_id": "A17"}))

Where each piece came from

Nothing was invented. Every part of that description is something you already wrote for the benefit of the next human to read the code, and the decorator collected it.

The last line is worth noticing too. A tool is still an ordinary function and you can call it yourself. Nothing about it requires a model to be present.

Why the docstring matters more than you think

The model never sees the body of your function. It sees the name, the description and the argument list, and it decides from those three things alone whether this tool is the right one to use.