Tool Calling

What is Tool Calling?

  • Allows LLMs to interact with other systems

  • Supported by most of the newest LLMs, but not all

  • Sounds complicated? Scary? It’s not too bad, actually…

Reference: https://jcheng5.github.io/llm-quickstart/quickstart.html#/how-it-works

How it does NOT work

How it DOES work

A tool is a function

  • A tool call is a function that the LLM can use
  • It can either infer what the function does by the function name, docstring, and/or parameter names
  • You can also provide it the context you want

Register tools (Agents)

Python Chatlas

import chatlas as clt

# doc strings and type hints provide tool context
def capital_finder(location: str) -> str:
  """Sets the capital of the moon as NYC"""
  if location.lower() == "moon":
    return "NYC"

chat = clt.ChatAnthropic()
chat.register_tool(capital_finder)
chat.chat("what is the capital of the moon?")
I'll use the capital_finder function to get information about the capital of the moon.


 # 🔧 tool request (toolu_01NFACspp5dLPMeTN7FNfvyB)
 capital_finder(location=moon)



 # ✅ tool result (toolu_01NFACspp5dLPMeTN7FNfvyB)
 NYC

According to the capital_finder function, the capital of the moon is NYC (New York City).

Of course, this is not factually accurate in reality - the moon doesn't actually have a capital city or any cities at all, as it's an
uninhabited celestial body. This appears to be a fictional or hypothetical response from the function.

R Ellmer

library(ellmer)
chat <- chat_openai()

#' Sets the capital of the moon as NYC
capital_finder <- function(location) {
  if (location == "moon") {
    return("NYC")
  }
}
capital_finder <- ellmer::tool(
  capital_finder,
  description = "Sets the capital of moon as NYC",
  arguments = list(
    location = type_string("location to look up")
  )
)

chat$register_tool(capital_finder)
chat$chat("what is the capital of the moon?")
◯ [tool call] capital_finder(location = "moon")
● #> NYC
The capital of the Moon is NYC.

Example: Weather Tool

To ask the LLM about the weather in the current location we need to write a function that does a few things:

  1. Geocode a location to a latitude and longitude (this can also be an API)
  2. Use the latitude and longitude in an API that can look up the weather

Example Weather tool - Geocode

Example Weather tool - Geocode

{'lat': 40.7127281, 'lon': -74.0060152}

Example Weather tool - Weather

Example Weather tool - Weather

get_weather(40.7127281, -74.0060152)
{'latitude': 40.710335,
 'longitude': -73.99308,
 'generationtime_ms': 0.09489059448242188,
 'utc_offset_seconds': 0,
 'timezone': 'GMT',
 'timezone_abbreviation': 'GMT',
 'elevation': 32.0,
 'current_weather_units': {'time': 'iso8601',
  'interval': 'seconds',
  'temperature': '°C',
  'windspeed': 'km/h',
  'winddirection': '°',
  'is_day': '',
  'weathercode': 'wmo code'},
 'current_weather': {'time': '2026-03-04T05:30',
  'interval': 900,
  'temperature': 1.1,
  'windspeed': 2.5,
  'winddirection': 270,
  'is_day': 0,
  'weathercode': 45}}

Example Weather tool - Register

from chatlas import ChatAnthropic

chat = ChatAnthropic()

chat.register_tool(get_coordinates)
chat.register_tool(get_weather)

Demo: Weather R

library(httr)
library(ellmer)
library(dotenv)

# Load environment variables
load_dot_env()

# Define weather function
get_weather <- function(latitude, longitude) {
  base_url <- "https://api.open-meteo.com/v1/forecast"

  tryCatch(
    {
      response <- GET(
        base_url,
        query = list(
          latitude = latitude,
          longitude = longitude,
          current = "temperature_2m,wind_speed_10m,relative_humidity_2m"
        )
      )
      rawToChar(response$content)
    },
    error = function(e) {
      paste("Error fetching weather data:", e$message)
    }
  )
}

# Create chat instance
chat <- chat_openai(
  model = "gpt-4.1",
  system_prompt = "You are a helpful assistant that can check the weather. Report results in imperial units."
)

# Register the weather tool
#
# Created using `ellmer::create_tool_def(get_weather)`
chat$register_tool(tool(
  get_weather,
  "Fetches weather information for a specified location given by latitude and
longitude.",
  latitude = type_number(
    "The latitude of the location for which weather information is requested."
  ),
  longitude = type_number(
    "The longitude of the location for which weather information is requested."
  )
))

# Test the chat
chat$chat("What is the weather in Seattle?")

Demo: Weather Python

"""Tool calling demo: LLM looks up weather via two registered tools.

Run: cd code/lecture06/demo02 && python chatlas-weather.py
"""

import requests
from chatlas import ChatGithub
# from chatlas import ChatAnthropic
from dotenv import load_dotenv

load_dotenv()

from get_coordinates import get_coordinates
from get_weather import get_weather


chat = ChatGithub(
    model="gpt-4.1-mini",  # or "gpt-4.1"
    system_prompt=(
        "You are a helpful assistant that can check the weather. "
        "Report results in imperial units."
    ),
)

chat.register_tool(get_coordinates)
chat.register_tool(get_weather)

chat.chat("What is the weather in Seattle?")

Demo: Shiny Application

from chatlas import ChatAnthropic
from shiny.express import ui

from helper.get_coordinates import get_coordinates
from helper.get_weather import get_weather

chat_client = ChatAnthropic()

chat_client.register_tool(get_coordinates)
chat_client.register_tool(get_weather)

chat = ui.Chat(id="chat")
chat.ui(
    messages=[
        "Hello! I am a weather bot! Where would you like to get the weather form?"
    ]
)


@chat.on_user_submit
async def _(user_input: str):
    response = await chat_client.stream_async(user_input, content="all")
    await chat.append_message_stream(response)