Canned response injection losing trace context

How do you inject trace context into a canned response sent via POST /api/v2/conversations/messaging/messages? I’m using the Python SDK to push a canned reply during an active chat. The request succeeds with a 200 OK, but the span in Jaeger shows no child links. Here’s the payload:

{
 "toUserId": "user-123",
 "text": "Here is the info.",
 "type": "text",
 "traceparent": "00-traceid-spanid-01"
}

The traceparent header isn’t being picked up by the backend service. Is there a specific header format required?