> For the complete documentation index, see [llms.txt](https://docs.hello-lisa.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.hello-lisa.com/developers/guides/player-communication/message-api-reference/app.md).

# App

The LiSA Player sends a set of essential messages to facilitate communication with the host app or environment, including:

* [Heartbeat](/developers/guides/player-communication/message-api-reference/app/app-heartbeat.md) — Signals the player's continued availability.
* [Message Acknowledge](/developers/guides/player-communication/message-api-reference/app/app-message-acknowledge.md) — Confirms receipt of a message.
* [Message Error](/developers/guides/player-communication/message-api-reference/app/app-message-error.md) — Indicates an issue with a received message.
* [Listen](/developers/guides/player-communication/message-api-reference/app/app-listen.md) — Notifies the host that the player is ready for communication.

The host app or environment must wait for the [Listen](/developers/guides/player-communication/message-api-reference/app/app-listen.md) message before sending any messages to the LiSA Player. Otherwise, the player may not receive them.