> ## Documentation Index
> Fetch the complete documentation index at: https://docs.agentcallback.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Variables

> Define the data your agent needs before a call starts.

Variables hold data your agent needs before a call begins — a customer's name, an account number, an appointment time, or anything else you pass in per call.

## Variable fields

Each variable, configured in the agent's **Variables** tab, has:

| Field             | Description                                                     |
| ----------------- | --------------------------------------------------------------- |
| `name`            | The variable's identifier, used to reference it in your prompt. |
| `type`            | `string`, `number`, `boolean`, or `lookup`.                     |
| `required`        | Whether the call can't start without a value for this variable. |
| `description`     | A note describing what the variable holds.                      |
| `defaultValue`    | The value used when none is supplied.                           |
| `dataset` / `key` | Used only for `lookup`-type variables — see below.              |

## Lookup variables

A `lookup`-type variable resolves its value before the call starts, by looking it up in a reference dataset using another variable as the lookup key. Use this when a call should pull in data — like account details or order status — that isn't passed in directly but can be looked up from a value you do have, such as a phone number or customer ID.

<Note>
  Variables resolve once, before the call starts. If your agent needs to track data that changes *during* the call, use [state variables](/build-voice-agents/state-variables) instead.
</Note>

## Use variables in your prompt

Reference a variable by name with `{{name}}`, and branch your wording with conditionals:

```
{{#if appointment_time}}
I'm calling to confirm your appointment at {{appointment_time}}.
{{else}}
I'm calling to schedule an appointment.
{{/if}}
```

## Add a variable

<Steps>
  <Step title="Open the Variables tab">
    In your agent, select the **Variables** tab.
  </Step>

  <Step title="Add a variable">
    Select the option to add a new variable.
  </Step>

  <Step title="Set its fields">
    Enter a name, type, and description. Mark it required if the call shouldn't start without it, and set a default value if useful.
  </Step>

  <Step title="Configure lookup settings, if needed">
    For a `lookup` variable, select the reference dataset and the key variable to look it up by.
  </Step>

  <Step title="Save">
    Save the agent to apply the new variable.
  </Step>
</Steps>

## Next step

[State variables](/build-voice-agents/state-variables) to track data that changes during the call.