***

title: Variables
sidebarTitle: Variables
description: Use dynamic values in your prompts and conversations.
------------------------------------------------------------------

Variables let you personalize conversations with dynamic data. Instead of static text, insert values that change based on the caller, API responses, or information collected during the call.

**Location:** Config Panel (right sidebar) → Variables

***

## Variable Types

<Tabs>
  <Tab title="User Defined">
    <Frame caption="User Defined variables">
      ![User defined variables](https://files.buildwithfern.com/smallest-ai.docs.buildwithfern.com/946e143ee2b83c2205cbcbf9971ff9e7767609914e79cad4c8e3bac483df7012/products/atoms/pages/platform/building-agents/images/variables-user.png)
    </Frame>

    Variables you create with default values.

    | Example             | Use Case          |
    | ------------------- | ----------------- |
    | `{{company_name}}`  | Your company name |
    | `{{support_hours}}` | Operating hours   |
    | `{{promo_code}}`    | Current promotion |
  </Tab>

  <Tab title="System">
    <Frame caption="System variables">
      ![System variables](https://files.buildwithfern.com/smallest-ai.docs.buildwithfern.com/6261428b2d3de7c7f5cee30c0e6fe0003a3b9919884ef55845bfd06358b05691/products/atoms/pages/platform/building-agents/images/variables-system.png)
    </Frame>

    Predefined variables provided by the platform. They are generated at runtime, read-only, and always available.

    | 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  |
  </Tab>

  <Tab title="API">
    <Frame caption="API variables">
      ![API variables](https://files.buildwithfern.com/smallest-ai.docs.buildwithfern.com/3fe760a74e45f5e2a7fa2446284978fd2f63555fa02114e988d73ec917ba6157/products/atoms/pages/platform/building-agents/images/variables-api.png)
    </Frame>

    Variables populated from API responses.

    | Syntax                  | Source             |
    | ----------------------- | ------------------ |
    | `{{api.customer_name}}` | API response field |
    | `{{api.account_tier}}`  | API response field |

    → See [API Calls](/atoms/atoms-platform/single-prompt-agents/configuration-panel/api-calls)
  </Tab>
</Tabs>

***

## Syntax

Use double curly braces:

```
{{variable_name}}
```

***

## Example

**In your prompt:**

```
Hello {{customer_name}}! Thanks for calling {{company_name}}.

I see you're a {{api.tier}} member. Your last order was 
{{api.last_order_status}}.

How can I help you today?
```

**At runtime:**

```
Hello Sarah! Thanks for calling Acme Corp.

I see you're a Premium member. Your last order was 
shipped on Monday.

How can I help you today?
```

***

## Default Values

Handle missing variables gracefully:

```
Hello {{customer_name|there}}!
```

If `customer_name` is empty → "Hello there!"

***

## Creating User Variables

1. Open the Variables panel
2. Go to **User Defined** tab
3. Click **+ Add Variable**
4. Enter name and default value
5. Use in prompts with `{{name}}` syntax

***

## Best Practices

<Accordion title="Use descriptive names">
  | Good                      | Bad      |
  | ------------------------- | -------- |
  | `{{customer_first_name}}` | `{{n}}`  |
  | `{{appointment_date}}`    | `{{d1}}` |
</Accordion>

<Accordion title="Always have fallbacks">
  What if the variable is empty?

  * Use default values: `{{name|there}}`
  * Handle in prompt: "If customer name is unknown, greet generically"
</Accordion>

<Accordion title="Test substitution">
  Verify variables replace correctly. Check Convo Logs to see actual values.
</Accordion>

***

## Related

<CardGroup cols={2}>
  <Card title="API Calls" icon="plug" href="/atoms/atoms-platform/single-prompt-agents/configuration-panel/api-calls">
    Fetch data from external APIs
  </Card>

  <Card title="Writing Prompts" icon="pen" href="/atoms/atoms-platform/single-prompt-agents/prompt-section/writing-prompts">
    Use variables in your prompts
  </Card>
</CardGroup>