Tool Calling Examples

Tool calling (also known as function calling) allows LLMs to decide which functions to call and extract the appropriate parameters. BAML treats tool calling as structured output, making it type-safe and transparent.

Understanding Tool Calling

In BAML, tools are represented as classes. The LLM chooses which tool to call and provides the parameters:

Define tools as BAML classes
LLM selects which tool to call
LLM extracts parameters for the tool
Your code executes the function with those parameters

Single Tool Selection

Weather API Example

Let’s start with a simple weather API tool:

weather_tool.baml

class WeatherAPI {
  api_name "weather_request"
  city string @description("The user's city")
  timeOfDay string @description("As an ISO8601 timestamp")
}

function UseTool(user_message: string) -> WeatherAPI {
  client "openai/gpt-4o-mini"
  prompt #"
    Given a message, extract info.
    
    {{ ctx.output_format }}

    {{ _.role('user') }}
    {{ user_message }}
  "#
}

The api_name field uses a string literal to identify the tool.

Calling the Tool

Python
TypeScript

from baml_client import b
from baml_client.types import WeatherAPI
import datetime

def get_weather(city: str, time_of_day: str):
    # Your actual weather API implementation
    return f"Weather in {city} at {time_of_day}: Sunny, 72°F"

def main():
    # Extract tool parameters
    weather_info = b.UseTool("What's the weather like in San Francisco?")
    
    print(f"Tool: {weather_info.api_name}")
    print(f"City: {weather_info.city}")
    print(f"Time: {weather_info.timeOfDay}")
    
    # Call your actual function
    result = get_weather(
        city=weather_info.city,
        time_of_day=weather_info.timeOfDay
    )
    print(f"Result: {result}")

if __name__ == '__main__':
    main()

import { b } from './baml_client'
import { WeatherAPI } from './baml_client/types'

function getWeather(city: string, timeOfDay: string): string {
  // Your actual weather API implementation
  return `Weather in ${city} at ${timeOfDay}: Sunny, 72°F`
}

async function main() {
  // Extract tool parameters
  const weatherInfo = await b.UseTool("What's the weather like in San Francisco?")
  
  console.log(`Tool: ${weatherInfo.api_name}`)
  console.log(`City: ${weatherInfo.city}`)
  console.log(`Time: ${weatherInfo.timeOfDay}`)
  
  // Call your actual function
  const result = getWeather(weatherInfo.city, weatherInfo.timeOfDay)
  console.log(`Result: ${result}`)
}

main()

Multiple Tool Selection

Use unions to let the LLM choose from multiple tools:

multi_tool.baml

class WeatherAPI {
  tool_name "get_weather" @description("Get current weather forecast")
  city string @description("The city for which to get weather")
}

class CalculatorAPI {
  tool_name "basic_calculator" @description("Perform basic calculations")
  operation "add" | "subtract" | "multiply" | "divide"
  numbers float[]
}

function SelectTool(message: string) -> WeatherAPI | CalculatorAPI {
  client "openai/gpt-4o"
  prompt #"
    Given a message, select the appropriate tool and extract parameters.

    {{ ctx.output_format }}

    {{ _.role("user") }}
    {{ message }}
  "#
}

Handling Multiple Tools

Python
TypeScript

from baml_client import b
from baml_client.types import WeatherAPI, CalculatorAPI

def handle_weather(weather: WeatherAPI) -> str:
    return f"The weather in {weather.city} is sunny."

def handle_calculator(calc: CalculatorAPI) -> str:
    numbers = calc.numbers
    if calc.operation == "add":
        result = sum(numbers)
    elif calc.operation == "subtract":
        result = numbers[0] - sum(numbers[1:])
    elif calc.operation == "multiply":
        result = 1
        for n in numbers:
            result *= n
    elif calc.operation == "divide":
        result = numbers[0]
        for n in numbers[1:]:
            result /= n
    return f"The result is {result}"

def main():
    user_input = input("What would you like to do? ")
    
    # Get tool selection from LLM
    tool_response = b.SelectTool(user_input)

    # Handle based on tool type
    if isinstance(tool_response, WeatherAPI):
        result = handle_weather(tool_response)
        print(f"Weather: {result}")
    elif isinstance(tool_response, CalculatorAPI):
        result = handle_calculator(tool_response)
        print(f"Calculator: {result}")

if __name__ == "__main__":
    main()

import { b } from './baml_client'
import { WeatherAPI, CalculatorAPI } from './baml_client/types'

function handleWeather(weather: WeatherAPI): string {
  return `The weather in ${weather.city} is sunny.`
}

function handleCalculator(calc: CalculatorAPI): string {
  const numbers = calc.numbers
  let result: number

  switch (calc.operation) {
    case "add":
      result = numbers.reduce((a, b) => a + b, 0)
      break
    case "subtract":
      result = numbers.slice(1).reduce((a, b) => a - b, numbers[0])
      break
    case "multiply":
      result = numbers.reduce((a, b) => a * b, 1)
      break
    case "divide":
      result = numbers.slice(1).reduce((a, b) => a / b, numbers[0])
      break
    default:
      return "Unknown operation"
  }

  return `The result is ${result}`
}

async function main() {
  const readline = await import('readline')
  const rl = readline.createInterface({
    input: process.stdin,
    output: process.stdout
  })

  rl.question("What would you like to do? ", async (userInput) => {
    // Get tool selection from LLM
    const toolResponse = await b.SelectTool(userInput)

    // Handle based on tool type (check for discriminator field)
    if ('city' in toolResponse) {
      const result = handleWeather(toolResponse as WeatherAPI)
      console.log(`Weather: ${result}`)
    } else if ('operation' in toolResponse) {
      const result = handleCalculator(toolResponse as CalculatorAPI)
      console.log(`Calculator: ${result}`)
    }

    rl.close()
  })
}

main()

Multiple Tool Calls

To allow the LLM to call multiple tools in a single response:

multi_call.baml

function UseMultipleTools(message: string) -> (WeatherAPI | CalculatorAPI)[] {
  client "openai/gpt-4o-mini"
  prompt #"
    Given a message, extract all tool calls needed.
    
    {{ ctx.output_format }}

    {{ _.role('user') }}
    {{ message }}
  "#
}

Python
TypeScript

from baml_client import b
from baml_client.types import WeatherAPI, CalculatorAPI

def main():
    message = "What's the weather in SF and NY? Also calculate 5 + 3."
    
    tools = b.UseMultipleTools(message)
    
    for tool in tools:
        if isinstance(tool, WeatherAPI):
            result = handle_weather(tool)
            print(f"Weather: {result}")
        elif isinstance(tool, CalculatorAPI):
            result = handle_calculator(tool)
            print(f"Calculator: {result}")

if __name__ == "__main__":
    main()

import { b } from './baml_client'
import { WeatherAPI, CalculatorAPI } from './baml_client/types'

async function main() {
  const message = "What's the weather in SF and NY? Also calculate 5 + 3."
  
  const tools = await b.UseMultipleTools(message)
  
  tools.forEach(tool => {
    if ('city' in tool) {
      const result = handleWeather(tool as WeatherAPI)
      console.log(`Weather: ${result}`)
    } else if ('operation' in tool) {
      const result = handleCalculator(tool as CalculatorAPI)
      console.log(`Calculator: ${result}`)
    }
  })
}

main()

Disambiguating Similar Tools

When tools have overlapping parameters, use descriptive fields:

disambiguate.baml

class GetWeather {
  tool_name "get_weather" @description("Get current weather forecast for a city")
  city string @description("The city for which to get weather")
}

class GetTimezone {
  tool_name "get_timezone" @description("Find the current timezone of a city")
  city string @description("The city for which to find timezone")
}

function ChooseTool(query: string) -> GetWeather | GetTimezone {
  client "openai/gpt-4o"
  prompt #"
    Determine the primary intent and select the appropriate tool.

    {{ ctx.output_format }}

    {{ _.role('user') }}
    {{ query }}
  "#
}

The LLM will use the descriptions to disambiguate between “What’s the time in London?” (timezone) vs “What’s the weather in London?” (weather).

Building an Agent

Create an agentic loop that continuously uses tools:

agent.baml

class WeatherAPI {
  intent "weather_request"
  city string
  time string @description("Current time in ISO8601 format")
}

class CalculatorAPI {
  intent "basic_calculator"
  operation "add" | "subtract" | "multiply" | "divide"
  numbers float[]
}

function SelectTool(message: string) -> WeatherAPI | CalculatorAPI {
  client "openai/gpt-4o"
  prompt #"
    Given a message, extract the appropriate tool info.

    {{ ctx.output_format }}

    {{ _.role("user") }}
    {{ message }}
  "#
}

Python
TypeScript

from baml_client import b
from baml_client.types import WeatherAPI, CalculatorAPI

def handle_weather(weather: WeatherAPI):
    return f"The weather in {weather.city} at {weather.time} is sunny."

def handle_calculator(calc: CalculatorAPI):
    numbers = calc.numbers
    if calc.operation == "add":
        result = sum(numbers)
    elif calc.operation == "subtract":
        result = numbers[0] - sum(numbers[1:])
    elif calc.operation == "multiply":
        result = 1
        for n in numbers:
            result *= n
    elif calc.operation == "divide":
        result = numbers[0]
        for n in numbers[1:]:
            result /= n
    return f"The result is {result}"

def main():
    print("Agent started! Type 'exit' to quit.\n")
    
    while True:
        user_input = input("You: ")
        if user_input.lower() == 'exit':
            break

        # Call BAML to select tool
        tool_response = b.SelectTool(user_input)

        # Handle the tool response
        if isinstance(tool_response, WeatherAPI):
            result = handle_weather(tool_response)
            print(f"Agent (Weather): {result}\n")
        elif isinstance(tool_response, CalculatorAPI):
            result = handle_calculator(tool_response)
            print(f"Agent (Calculator): {result}\n")

if __name__ == "__main__":
    main()

import { b } from './baml_client'
import { WeatherAPI, CalculatorAPI } from './baml_client/types'
import * as readline from 'readline'

function handleWeather(weather: WeatherAPI): string {
  return `The weather in ${weather.city} at ${weather.time} is sunny.`
}

function handleCalculator(calc: CalculatorAPI): string {
  const numbers = calc.numbers
  let result: number

  switch (calc.operation) {
    case "add":
      result = numbers.reduce((a, b) => a + b, 0)
      break
    case "subtract":
      result = numbers.slice(1).reduce((a, b) => a - b, numbers[0])
      break
    case "multiply":
      result = numbers.reduce((a, b) => a * b, 1)
      break
    case "divide":
      result = numbers.slice(1).reduce((a, b) => a / b, numbers[0])
      break
    default:
      return "Unknown operation."
  }

  return `The result is ${result}`
}

async function main() {
  console.log("Agent started! Type 'exit' to quit.\n")

  const rl = readline.createInterface({
    input: process.stdin,
    output: process.stdout
  })

  rl.on('line', async (input) => {
    if (input.toLowerCase() === 'exit') {
      rl.close()
      return
    }

    const toolResponse = await b.SelectTool(input)

    if (toolResponse.intent === "weather_request") {
      const result = handleWeather(toolResponse as WeatherAPI)
      console.log(`Agent (Weather): ${result}\n`)
    } else if (toolResponse.intent === "basic_calculator") {
      const result = handleCalculator(toolResponse as CalculatorAPI)
      console.log(`Agent (Calculator): ${result}\n`)
    }
  })
}

main()

Example Output

Agent started! Type 'exit' to quit.

You: What's the weather in Seattle?
Agent (Weather): The weather in Seattle at 2024-03-15T12:00:00Z is sunny.

You: What's 5+2?
Agent (Calculator): The result is 7.0

You: exit

Dynamic Tool Schemas

You can define tool schemas dynamically from your Python/TypeScript code:

dynamic.baml

class WeatherAPI {
  @@dynamic  // params defined from code
}

function UseTool(user_message: string) -> WeatherAPI {
  client "openai/gpt-4o-mini"
  prompt #"
    Given a message, extract info.
    {{ ctx.output_format }}

    {{ _.role('user') }}
    {{ user_message }}
  "#
}

Python

import inspect
from baml_client import b
from baml_client.type_builder import TypeBuilder

async def get_weather(city: str, time_of_day: str):
    print(f"Getting weather for {city} at {time_of_day}")
    return {"temp": 72, "condition": "sunny"}

def main():
    # Build schema from function signature
    tb = TypeBuilder()
    type_map = {int: tb.int(), float: tb.float(), str: tb.string()}
    
    signature = inspect.signature(get_weather)
    for param_name, param in signature.parameters.items():
        tb.WeatherAPI.add_property(param_name, type_map[param.annotation])
    
    # Use the dynamic schema
    tool = b.UseTool(
        "What's the weather in San Francisco this afternoon?",
        {"tb": tb}
    )
    
    print(tool)
    # Call the actual function
    weather = get_weather(**tool.model_dump())
    print(weather)

if __name__ == '__main__':
    main()

Advanced: Todo List Agent

A more complex example with multiple tool types:

todo.baml

class AddTodoItem {
  type "add_todo_item"
  item string
  time string
  description string @description("20 word description")
}

class TodoMessageToUser {
  type "todo_message_to_user"
  message string @description("A message to the user, about 50 words")
}

type TodoTool = AddTodoItem | TodoMessageToUser

function ChooseTodoTools(query: string) -> TodoTool[] {
  client "openai/gpt-4o"
  prompt #"
    Choose tools to satisfy the user query.
    For example, if they ask for "5 todo items for learning chess",
    return a list of 5 "add_todo_item" objects and a single 
    "todo_message_to_user" object.
    
    All requests should end with a "todo_message_to_user" object.

    {{ ctx.output_format }}
    
    {{ _.role('user') }}
    {{ query }}
  "#
}

Python
TypeScript

from baml_client import b
from baml_client.types import AddTodoItem, TodoMessageToUser

def main():
    query = "Give me 5 todo items for learning chess"
    tools = b.ChooseTodoTools(query)
    
    for tool in tools:
        if isinstance(tool, AddTodoItem):
            print(f"📝 Add: {tool.item}")
            print(f"   Time: {tool.time}")
            print(f"   Description: {tool.description}")
            # Add to your todo database
        elif isinstance(tool, TodoMessageToUser):
            print(f"\n💬 {tool.message}")

if __name__ == "__main__":
    main()

import { b } from './baml_client'
import { AddTodoItem, TodoMessageToUser } from './baml_client/types'

async function main() {
  const query = "Give me 5 todo items for learning chess"
  const tools = await b.ChooseTodoTools(query)
  
  tools.forEach(tool => {
    if (tool.type === "add_todo_item") {
      const item = tool as AddTodoItem
      console.log(`📝 Add: ${item.item}`)
      console.log(`   Time: ${item.time}`)
      console.log(`   Description: ${item.description}`)
      // Add to your todo database
    } else if (tool.type === "todo_message_to_user") {
      const msg = tool as TodoMessageToUser
      console.log(`\n💬 ${msg.message}`)
    }
  })
}

main()

Best Practices

1. Use Clear Tool Names

class GetWeather {
  tool_name "get_weather"  // Clear and descriptive
  // ...
}

2. Add Descriptions

class SearchDatabase {
  tool_name "search_database" @description("Search the product database")
  query string @description("The search query")
  limit int @description("Maximum number of results (1-100)")
}

3. Validate Tool Parameters

def handle_search(search: SearchDatabase):
    # Validate limit
    if not 1 <= search.limit <= 100:
        raise ValueError("Limit must be between 1 and 100")
    
    # Validate query
    if len(search.query.strip()) == 0:
        raise ValueError("Query cannot be empty")
    
    # Proceed with search
    return perform_search(search.query, search.limit)

4. Handle Tool Errors Gracefully

try:
    tool = b.SelectTool(user_input)
    result = execute_tool(tool)
except ValueError as e:
    print(f"Invalid parameters: {e}")
except Exception as e:
    print(f"Tool execution failed: {e}")

Why BAML for Tool Calling?

Type Safety: Full type checking for tool parameters
Transparency: See exactly what gets sent to the LLM (use VS Code Playground)
Better Performance: Prompting outperforms native function-calling APIs
No Token Waste: More efficient than OpenAI’s function calling format
Flexibility: Works with any LLM, not just those with function-calling APIs

Next Steps

Learn about Dynamic Types for runtime schema generation
Explore RAG to combine tool calling with knowledge retrieval
Check out Streaming for real-time tool selection

Use Cases

Tool Calling Examples

Understanding Tool Calling

Single Tool Selection

Weather API Example

Calling the Tool

Multiple Tool Selection

Handling Multiple Tools

Multiple Tool Calls

Disambiguating Similar Tools

Building an Agent

Example Output

Dynamic Tool Schemas

Advanced: Todo List Agent

Best Practices

1. Use Clear Tool Names

2. Add Descriptions

3. Validate Tool Parameters

4. Handle Tool Errors Gracefully

Why BAML for Tool Calling?

Next Steps

Build docs developers (and LLMs) love

Use Cases

​Understanding Tool Calling

​Single Tool Selection

​Weather API Example

​Calling the Tool

​Multiple Tool Selection

​Handling Multiple Tools

​Multiple Tool Calls

​Disambiguating Similar Tools

​Building an Agent

​Example Output

​Dynamic Tool Schemas

​Advanced: Todo List Agent

​Best Practices

​1. Use Clear Tool Names

​2. Add Descriptions

​3. Validate Tool Parameters

​4. Handle Tool Errors Gracefully

​Why BAML for Tool Calling?

​Next Steps

Build docs developers (and LLMs) love

Understanding Tool Calling

Single Tool Selection

Weather API Example

Calling the Tool

Multiple Tool Selection

Handling Multiple Tools

Multiple Tool Calls

Disambiguating Similar Tools

Building an Agent

Example Output

Dynamic Tool Schemas

Advanced: Todo List Agent

Best Practices

1. Use Clear Tool Names

2. Add Descriptions

3. Validate Tool Parameters

4. Handle Tool Errors Gracefully

Why BAML for Tool Calling?

Next Steps