Extend Shopify Flow with a custom trigger and a custom action by building a Flow app extension

In-depth guide

Steps

1. Add a Flow extension to your Shopify app using `shopify app generate extension --type flow_action` (or `flow_trigger`) and configure the extension TOML with `type`, `title`, `description`, and the payload schema
2. For a custom trigger: your app calls the `flowTriggerReceive` REST endpoint (POST /admin/api/2025-07/flow/triggers/receive.json) or the equivalent GraphQL trigger mutation when the event occurs in your system, passing a payload that matches the schema declared in the extension
3. For a custom action: declare your action endpoint URL in the TOML so Flow sends an HTTP POST to your app when the action runs; your endpoint receives the action payload and must respond with 200 within the timeout window
4. Define the payload properties (fields) your trigger or action exposes — these become the variables merchants can reference inside Flow workflow steps
5. Deploy via `shopify app deploy`; the trigger and action appear in the Flow editor for merchants who have installed your app
6. Test using the draft version in a development store before promoting to production; look for the 'Draft' badge in the Flow editor to confirm you are testing the unpublished version