Sending canned responses via Genesys Conversations API returns 400

Trying to send a canned response during an active chat interaction using the Conversations API. Here’s the POST to /api/v2/conversations/messaging/{conversationId}/messages:

{
 "text": "Here is the info you requested.",
 "cannedResponseId": "123-abc-def"
}

Getting a 400 Bad Request with message: "Invalid message format". I’m using the JS SDK with OAuth tokens. Is there a specific field I’m missing for canned responses versus plain text?