POST /interactions/message returns 400 when sending canned response

We’re trying to send a canned response during an active chat via the Conversations API. The POST to /api/v2/conversations/interactions/message fails with a 400 Bad Request despite valid JSON. Here is the payload we’re sending:

{
 "text": "{{canned_response}}",
 "to": {"id": "chat_session_id"}
}

Any idea what’s missing?