# What are custom actions?
URL: https://docs.awellhealth.com/docs/extensions/what-are-custom-actions

> For the complete documentation index, see [llms.txt](https://docs.awellhealth.com/llms.txt).



A care flow is built from **actions**: the smallest building block, defining what needs to happen and
who does it. See [Key terms](/docs/get-started/key-terms) for how actions relate to steps and tracks.

Awell ships **native actions**: forms, messages, checklists, calculations. **Custom actions** are the
ones an extension adds.

## Installing an extension adds its actions [#installing-an-extension-adds-its-actions]

A custom action is tied to an [extension](/docs/extensions/what-are-awell-extensions) enabled for the
care flow. Installing an SMS extension, Twilio for instance, adds a "Send SMS" action. Installing a
scheduling extension adds "Get appointment".

From an author's point of view they behave like any other action: add one to a step, configure it, and
the care flow runs it. Unlike a native action, completing one means talking to a system outside Awell.

## One action at design time, one activity per patient at run time [#one-action-at-design-time-one-activity-per-patient-at-run-time]

Custom actions inherit the same distinction as native ones:

* An action is what sits in a step at design time.
* An activity is what exists at run time when that action activates for a patient.

So a custom action defined once in Studio becomes one activity per patient who reaches it. See
[What happens when a care flow runs](/docs/get-started/what-happens-when-a-care-flow-runs).

## System actions, not user actions [#system-actions-not-user-actions]

Custom actions are almost always **system actions**: Awell completes them automatically rather than
waiting for a person. A custom action calling a slow third-party system is therefore a pause in the care
flow, and if a patient is waiting on Hosted Pages they see it.
See [Care flow design with Hosted Pages](/docs/embed-and-host/care-flow-design-with-hosted-pages).

## Next steps [#next-steps]

**Next:** [Add a custom action to a step](/docs/design-care-flows/add-a-custom-action-to-a-step).