Guest API POST /api/v2/conversations/messaging/{id}/messages 400 Bad Request on custom payload

NICE CXone API & Integrations
1

Has anyone figured out why the Guest API rejects a valid message payload with a 400 error when bypassing the Messenger widget?

I am building a custom web chat interface using the Guest API directly, skipping the standard JavaScript widget. The goal is to have full control over the UI and message formatting. Authentication is handled via Client Credentials with the conversations:messaging:guest scope. The token is valid and I can successfully initiate a conversation using POST /api/v2/conversations/messaging.

The issue occurs when attempting to send the first message. I am using the following Python requests code:

import requests

headers = {
 'Authorization': f'Bearer {access_token}',
 'Content-Type': 'application/json'
}

payload = {
 "to": {
 "id": "system",
 "externalId": "default-channel-id"
 },
 "text": "Hello from custom client"
}

response = requests.post(
 f'https://api.mypurecloud.com/api/v2/conversations/messaging/{conversation_id}/messages',
 headers=headers,
 json=payload
)
print(response.status_code)
print(response.text)

The response is consistently a 400 Bad Request with the body:

{
 "code": "bad.request",
 "message": "The request body could not be parsed or is invalid.",
 "status": 400
}

Environment details:

  • Region: North America (api.mypurecloud.com)
  • Python 3.9, requests 2.28.1
  • Conversation ID is confirmed active and owned by the guest identity used in the initiation step.
  • The to field structure matches the schema for system-to-guest routing.

I have verified the token scopes using the introspection endpoint. The conversations:messaging:guest scope is present. I suspect the to object might be malformed or that the Guest API expects a different structure when not using the widget’s implicit context. The documentation is sparse on direct POST requests to the messages endpoint for guests.

Is the to field required here? Should it be omitted? Or is there a specific format for the externalId that I am missing?