# Voice agent tools for live API calls · Vaakyo

> How to design tools a voice agent calls during a call (checking availability, looking up orders, creating leads), with timeouts, errors and security.

Source: https://vaakyo.com/blog/voice-agent-tools-mid-call

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# Give your agent tools: check slots, look up orders and create leads mid-call

An agent that can only talk is a phone menu with better manners. Tools let it look things up and get things done while the caller is on the line.

The Vaakyo team 29 September 2026 2 min read

-   Tools
-   Integrations
-   Engineering

Without tools, a voice agent can only answer from its prompt and knowledge base. With tools, it can check a doctor's real availability, find an order by its number, create a lead in your CRM or book a slot, all while the caller waits a second or two.

## What a tool is

A tool is an HTTP endpoint on your side that the agent calls with arguments it collects from the conversation. You describe it to the agent with a name, a description and the parameters it takes:

Name

Description

Parameters

`find_order`

Look up an order by its number or the caller's phone

`order_id` (optional), `phone`

`check_availability`

Open appointment slots for a doctor on a date

`doctor`, `date`

`create_lead`

Save a qualified lead to the CRM

`name`, `budget`, `timeline`

The agent decides when to call a tool from the conversation and your descriptions, so write descriptions the way you'd explain the tool to a new colleague.

## Design tools for a phone call

-   **Return little, return it fast.** The agent has to turn the result into one or two sentences. Return the three slots it should offer, not a calendar of the month. Aim for responses well under a second.
-   **Return text the agent can say.** "Tuesday 11:30 AM with Dr. Mehra" is easier to speak than an ISO timestamp.
-   **Make errors explicit.** Return `{"found": false, "reason": "no order with that number"}` rather than a 500, so the agent can say something useful.
-   **Make writes idempotent.** If a call drops and the caller calls back, booking twice should not create two appointments. Accept a key such as the call id.

## Say something while you wait

For anything that might take more than a moment, have the agent say a short filler ("Ek second, main check karti hoon") before the tool runs. Silence on a phone line reads as a dropped call.

## Security

-   Tools must be on the public internet over HTTPS; never expose internal admin APIs to an agent.
-   Authenticate the agent's requests to your endpoint (a secret header) and validate every argument: they come from a conversation, so treat them as user input.
-   Give each tool the narrowest permission it needs. A tool that books appointments should not be able to delete them.

## Test them before callers do

Use the tool tester in the console to send sample arguments and check the response, then talk to the agent and watch the call timeline: it shows each tool call, its arguments, the result and how long it took.

[Read next Webhooks for call results: signatures, retries and idempotency](https://vaakyo.com/blog/reliable-call-webhooks)

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