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# Variables

> Learn about the variable types available in Synthflow, including system variables, custom variables, and action results.

Variables are placeholders that let your agent work with dynamic data. Use them in prompts, Custom Actions, call transfers, SIP headers, and webhook payloads by wrapping the variable name in curly braces, for example `{call_id}`.

Synthflow supports three types of variables:

| Type                                                | When available             | How it's created                          |
| :-------------------------------------------------- | :------------------------- | :---------------------------------------- |
| [System variables](#system-variables)               | Every call, automatically  | Provided by Synthflow                     |
| [Custom variables](#custom-variables)               | Before the call starts     | Passed via the API when initiating a call |
| [Action result variables](#action-result-variables) | After a Custom Action runs | Returned from an external API response    |

Flow Designer agents also support collecting variables during conversations and managing them through the variable drawer. See [Variables in Flow Designer](/collect-variables) for details.

## System variables

System variables are built-in values that Synthflow populates automatically for every call. You do not need to define them.

| Variable            | Description                                                                         |
| :------------------ | :---------------------------------------------------------------------------------- |
| `call_id`           | Unique identifier for the current call session.                                     |
| `user_phone_number` | For inbound calls, the caller's number. For outbound calls, the recipient's number. |
| `from_phone_number` | For outbound calls, the number your agent used to place the call.                   |
| `to_phone_number`   | For inbound calls, the number that was dialed (the number you own).                 |

### Usage example

Pass system variables in a Custom Action request body to identify the call in an external system:

```json
{
  "id": "{call_id}",
  "caller": "{user_phone_number}",
  "dialed_number": "{to_phone_number}",
  "agent_number": "{from_phone_number}"
}
```

You can also reference system variables in prompts:

```text
The caller's phone number is {user_phone_number}.
```

## Custom variables

Custom variables are key-value pairs passed through the API when you start a call. They inject external data into your agent's prompt without requiring the agent to collect it during the conversation.

### How to use custom variables

1. Identify the data you want to inject (for example, a company name or user role).
2. Include the data in the `custom_variables` field of your API request.
3. Reference the values in your agent's prompt.

```json
{
  "custom_variables": {
    "Company": "Synthflow",
    "Role": "Sales Specialist"
  }
}
```

For more details, see [Custom variables](/custom-variables) and [Dynamic prompt injection](/dynamic-prompt-injection).

## Action result variables

Action result variables store data returned by [Custom Action](/about-custom-actions) API calls. When a Custom Action executes, the response is parsed and stored in variables you can [reference later.](#reference-action-results-in-your-prompt)

![](https://files.buildwithfern.com/synthflow.docs.buildwithfern.com/a0acdc413d2b7b99d8dc8a0d253d200c31324d9bcd1c73978e05930e6f1ffc6d/docs/assets/screenshots/custom_actions_action_results.png)

### How they work

1. A Custom Action sends an API request (GET, POST, PUT, etc.).
2. The external API returns a response.
3. The response data is stored in action result variables.
4. Reference the results in prompts or pass them to other actions.

For example, if a Custom Action calls an order-status API and returns `{"status": "shipped"}`, you can reference `{status}` in your prompt or in subsequent actions.

For more details on configuring Custom Actions and using their responses, see [Custom Actions](/about-custom-actions).

### Reference action results in your prompt

In the agent **Prompt** editor, type `<` and Synthflow will list the available data of the response of your initialized custom action. You can also use the **Action results** control in the prompt toolbar.

![](https://files.buildwithfern.com/synthflow.docs.buildwithfern.com/0601c8c4069344cac54fa419f67c1b1bd5d7ff30c978896cc28a238e7495b9eb/docs/assets/screenshots/custom_action_action_results_attach.png)

### In-call messaging replies

[In-Call Messaging](/in-call-messaging) actions populate a dedicated reply variable when the customer responds by text:

| Variable         | Populated by                           |
| :--------------- | :------------------------------------- |
| `sms_reply`      | A reply to an in-call SMS action.      |
| `whatsapp_reply` | A reply to an in-call WhatsApp action. |

The variable is set as soon as the reply arrives — whether the action was configured to block the agent until then, or to let the call continue in the background. If no reply arrives before the call ends, the variable remains empty.

## Action placeholders and mapping \[#action-placeholders-and-mapping]

Several actions let you embed **placeholders** with angle-bracket names—for example `<customer_name>` or `<queue_ref>`—in specific configuration fields (such as the **SMS message** body, **call transfer** targets or SIP headers, or the **`lead_email`** input on **real-time booking**). Each distinct name becomes an **input variable** on that action.

After you **attach** the action to an agent, you **map** each input to its source: values **collected during the call**, **system** fields, **custom** data from the API, **pre-call** payloads, **action results** from another step, or a fixed value you enter—the same families summarized in the overview table at the top of this page.

Field-by-field setup lives in each action guide:

* [Custom Actions](/about-custom-actions#variables)
* [Call transfers](/call-transfers#variables)
* [Real-time booking](/create-a-real-time-booking-action#variables)
* [Send SMS](/how-to-create-a-send-sms-action#variables)

## Where to use variables \[#where-to-use-variables]

Angle-bracket placeholders on supported actions are covered in [Action placeholders and mapping](#action-placeholders-and-mapping). Variables can also be referenced in several other places across your agent configuration:

* **Prompts**: Personalize your agent's behavior, for example `"Hi {lead_name}, thanks for calling {company_name}."`.
* **Custom Actions**: Pass variables in API parameters or body fields on [Custom Actions](/about-custom-actions#variables), shape JSON with placeholders, and reuse response fields in later steps.
* **Real-Time Booking**: When you attach [real-time booking](/create-a-real-time-booking-action#variables), define variables so details the caller gives you—like `<user_name>` or `<user_email>`—flow into the booking request.
* **Call Transfers**: On [call transfers](/call-transfers#variables), define variables to capture context like `<caller_name>` or `<issue_type>` for whispers, summaries, or SIP headers.
* **Send SMS**: On [Send SMS](/how-to-create-a-send-sms-action#variables), define variables to personalize templates, for example `"Hi <customer_name>, your appointment is confirmed for <appointment_date>."`.
* **Webhooks**: Reference variables when configuring webhook payloads.
* **Flow Designer nodes**: Use variables in Conversation, Message, Branch, and Custom Action nodes in the [Flow Designer](/collect-variables).
* **SIP headers**: Pass system variables in SIP X-headers for telephony integrations, as described in [Use SIP X-Headers in Prompts, Actions & Transfers](/sip-x-headers-in-prompts).