Open Messaging API: Quick Reply buttons returning 400 Bad Request

Trying to send structured quick replies via the Open Messaging API. I’m using the POST /api/v2/webmessaging/guests/sessions/{sessionId}/messages endpoint. The basic text message works fine, but adding the actions array for buttons triggers a 400 Bad Request.

Here’s the payload:

{
 "type": "text",
 "text": "Choose an option",
 "actions": [
 { "type": "quickReply", "label": "Option 1" }
 ]
}

The error response is vague. Am I missing a required field in the action object? Checked the docs but they’re sparse on JSON structure examples.