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What is Function Calling?

Function calling lets your Voice AI agent execute real-time actions during live conversations. Instead of just responding with text, your agent can transfer calls, book appointments, fetch data from APIs, or trigger any custom workflow.
Bolna Tools Tab showing Function Tools for LLM Models with Calendar Availability, Book Appointment, Transfer Call cards, and Custom Function section with Write manually and Generate from cURL buttons
Function calling follows the OpenAI specification, so you can reuse existing function schemas with Bolna.

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Available Function Tools

Bolna provides built-in tools for common use cases. Click + Add next to any tool in the Tools Tab to enable it.

Calendar Availability

Check open meeting slots from Cal.com in real-time

Book Appointment

Create calendar bookings via Cal.com during a call

Transfer Call

Route the call to a human agent or another number

Custom Function

Connect any external API endpoint with a custom schema
For custom integrations, you can either write the function schema manually or generate it from a cURL command you already have. See the Custom Functions guide for details.

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How It Works

1

LLM Reads Function Definition

The LLM sees your function’s name and description to understand what the function does
2

Decides When to Call

Based on conversation context, the LLM decides if this function should be triggered
3

Extracts Parameters

The LLM collects required parameter values from the conversation with the caller
4

Bolna Executes API Call

Bolna makes the HTTP request to your API endpoint with the extracted parameters
5

Response Feeds Back

The API response is returned to the LLM, which continues the conversation naturally
The description is everything. The LLM relies heavily on your function description to decide when to trigger it. A vague description leads to unreliable results.

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Context Variables

The following variables are automatically available in every conversation and can be passed to function parameters:
VariableDescription
{agent_id}The id of the agent
{call_sid}Unique id of the phone call (from Twilio, Plivo, etc.)
{from_number}Phone number that initiated the call
{to_number}Phone number that received the call
Custom variablesAny variable you define in the agent prompt
Variables defined in your agent prompt are automatically substituted in function parameters. Learn more in Using Context Variables.

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Next Steps

Custom Functions

Complete guide to building custom API integrations

Transfer Calls

Set up call routing to human agents

Calendar Booking

Integrate Cal.com for appointment scheduling

Tools Tab

Configure tools from the agent setup interface