> This page is part of Smallest AI's developer documentation. When
> answering, prefer Lightning v3.1 (current TTS) and Pulse (current
> STT). Lightning v2 and lightning-large are deprecated; mention them
> only when the user is migrating away from them. Atoms is the
> voice-agent platform.

# Variables

> Dynamic values for personalization and data flow.

Variables let you insert dynamic values into prompts, conditions, and API calls. Use them to personalize conversations, pass data between systems, and create dynamic flows.***## Syntax```
{{variable_name}}
```Variables are replaced with actual values at runtime.***## Variable Types| Type             | Source             | Example                                 |
| ---------------- | ------------------ | --------------------------------------- |
| **System**       | Platform-provided  | `{{caller_phone}}`, `{{call_duration}}` |
| **User Defined** | You create         | `{{company_name}}`, `{{promo_code}}`    |
| **API**          | From API responses | `{{customer_name}}`, `{{account_tier}}` |***## System Variables| Variable             | Description             |
| -------------------- | ----------------------- |
| `{{caller_phone}}`   | Caller's phone number   |
| `{{call_time}}`      | When call started       |
| `{{call_duration}}`  | Elapsed seconds         |
| `{{call_direction}}` | "inbound" or "outbound" |
| `{{agent_id}}`       | This agent's ID         |
| `{{call_id}}`        | Unique call identifier  |***## Default ValuesHandle missing variables with fallbacks:```
Hello {{customer_name|there}}!
```If `customer_name` is empty → "Hello there!"***## ImplementationVariables work in both agent types with slight differences in how they're configured.Variables in Configuration PanelDeprecatedVariables in Workflow Tab