Guest API POST /api/v2/conversations/messaging/{id}/messages returns 401 despite valid token

NICE CXone API & Integrations
1

Trying to build a custom chat interface using the Guest API instead of the standard Messenger widget. I’ve got the access token from our backend, but every time I try to POST to /api/v2/conversations/messaging/{conversationId}/messages, I hit a 401 Unauthorized. The token works fine for fetching user details, so I know it’s valid. Here’s the payload I’m sending:

{
 "text": "Test message from custom UI",
 "to": {
 "id": "queue-id-here"
 }
}

I’m using the same headers as the docs suggest, including the Authorization Bearer token. The conversation ID is definitely active and was created via the same token. What am I missing here?