diff --git a/docs/plugins/backend-plugin.md b/docs/plugins/backend-plugin.md
index 73b8f9027a..d2e93bb08c 100644
--- a/docs/plugins/backend-plugin.md
+++ b/docs/plugins/backend-plugin.md
@@ -41,9 +41,11 @@ standalone mode. You can do a first-light test of your service:
 
 ```sh
 cd plugins/carmen-backend
-yarn start
+LEGACY_BACKEND_START=true yarn start
 ```
 
+> Note: `LEGACY_BACKEND_START=true` is needed while we transition fully to the [New Backend System](../backend-system/index.md). The templates have not been migrated yet; you can track this in [issue 21288](https://github.com/backstage/backstage/issues/21288)
+
 This will think for a bit, and then say `Listening on :7007`. In a different
 terminal window, now run