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

# set_default_device

> Set the persistent default device for calls that do not specify one.

Sets the persistent default device. Tools that take an optional `device_id` - such as [`phone_run_task`](/api-reference/tools/phone-run-task), [`phone_session_start`](/api-reference/tools/phone-session-start), and [`phone_get_state`](/api-reference/tools/phone-get-state) - fall back to this device when the parameter is omitted.

## Parameters

<ParamField body="device_id" type="string" required>
  The device to make the default. Get IDs from
  [`list_devices`](/api-reference/tools/list-devices).
</ParamField>

## Behavior

* The default is **persistent** - it survives across sessions and connections
  until you change it again.
* Changing the default does **not** reroute existing work: sessions and tasks
  stay bound to the device they started on. After a session starts, its
  `session_id` determines the target phone for `phone_run_task` and
  `phone_session_end`, and task controls use the device that owns the
  `task_id`.

<Tip>
  With multiple devices, set the default once at the start of a workflow rather
  than passing `device_id` on every call - it keeps prompts simpler and avoids
  targeting the wrong phone mid-workflow.
</Tip>

## Related

* [`list_devices`](/api-reference/tools/list-devices) - see which device is currently the default
* [`phone_device_status`](/api-reference/tools/phone-device-status) - confirm the device is online before starting work