Genesys Cloud Web Messaging Guest SDK: Proactive notification fails with 404

Trying to send a proactive notification to a user via the Web Messaging Guest SDK. The customer had an active session earlier today, so I’m using the conversationId from that history.

const notification = {
 type: "notification",
 text: "Checking in."
};

await client.conversations.postConversationsMessagingConversationsNotification({
 conversationId: "abc-123",
 body: notification
});

The SDK throws a 404 Not Found. The endpoint is POST /api/v2/conversations/messaging/conversations/{id}/notification. I’ve verified the ID is correct. The session isn’t closed, just idle. Am I missing a specific flag in the payload or is the endpoint wrong for proactive messages?