Guest API send message returns 400 Bad Request

Trying to use the Guest API to send a message without the widget. I have a valid session ID and agent ID. Here is the payload I am sending to /api/v2/conversations/guest/messages:

{
 "sessionId": "12345",
 "text": "Hello"
}

The response is always 400 Bad Request with message Invalid session. I checked the docs but can’t find the issue. List of things tried:

  • Verified session is active
  • Checked timezone settings
  • Used correct auth header
  • Tried different message formats