That's a main tool yea.
You can turn any function into a tool using FunctionTool tho (it's somewhere in the docs)
from llama_index.tools import FunctionTool
def add(a: int, b: int) -> int:
"""Add two numbers."""
return a+ b
tool = FunctionTool.from_defaults(fn=add)
Here, the tool name is the function name, the description is the docstring. The type hints are also important
We also have community tools on llama hub (just set the filter to agent tools)
https://llamahub.ai/