Open Messaging API: Sending structured cards via webhook endpoint

Quick question about sending structured messages via the Open Messaging API.

I’m trying to push a quick reply card to a web messaging session from my Node.js Lambda function. I have the conversationId and participantId from the EventBridge trigger, and I’m hitting the POST /api/v2/conversations/messaging/conversations/{conversationId}/messages endpoint. The request succeeds with a 200 OK, but the client never renders the card structure; it just shows the text content as plain text.

Here is the JSON payload I’m sending:

{
 "type": "application/vnd.genesys.message.card+json",
 "content": {
 "title": "Select Option",
 "buttons": [
 { "label": "Option 1", "value": "opt1" },
 { "label": "Option 2", "value": "opt2" }
 ]
 }
}

I’m using the standard bearer token from my Lambda layer. Is the type header correct for web messaging, or do I need to wrap this in a specific envelope for the guest API to parse it correctly? The docs are vague on the exact MIME type required for structured cards in this specific endpoint.