Guest API POST /conversations/messages returns 400 Bad Request with 'invalid_payload' when bypassing Messenger widget

NICE CXone API & Integrations
1

Can’t get this config to load properly…

I am attempting to implement a headless messaging client that interacts directly with the Genesys Cloud Guest API, specifically POST /api/v2/conversations/messages, to bypass the standard Messenger widget overhead for a high-throughput analytics pipeline. I need to programmatically inject test messages into specific web messaging conversations for load testing purposes without rendering the UI component.

I have successfully obtained an OAuth2 Bearer token using the client credentials flow and identified the target conversationId via the GET /api/v2/conversations endpoint. The conversation status is connected and the type is webchat.

Here is the JSON payload I am sending via requests.post:

{
 "conversationId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
 "to": {
 "id": "guest-id-placeholder",
 "name": "TestBot"
 },
 "from": {
 "id": "agent-id-placeholder"
 },
 "text": "Hello, this is a programmatic test."
}

The API responds with a 400 Bad Request. The error body is:

{
 "code": "invalid_payload",
 "message": "Message payload is invalid. 'to' field requires a valid participant ID or address object with type 'webchat'.",
 "status": 400
}

I have verified that the conversationId is valid. I suspect the to and from objects are malformed for the Guest API context. The documentation for the standard Conversations API suggests using participant IDs, but the Guest API spec implies a different structure for anonymous guests. I am running this in a Jupyter notebook using Python 3.9 and the requests library.

How should I structure the to and from objects when sending a message as a guest or agent via this specific endpoint? Do I need to use a specific type key in the address object, or is there a different endpoint I should be hitting for direct message injection?

2

I am attempting to implement a headless messaging client that interacts directly with the Genesys Cloud Guest API, specifically POST /api/v2/conversations/messages…

The payload structure for the Guest API differs from standard internal calls. Ensure the to object explicitly includes the id of the target conversation and the type is set to conversation.

{
 "to": {
 "id": "CONVERSATION_ID",
 "type": "conversation"
 },
 "text": "Test payload"
}

Verify the OAuth scope includes conversation:write. Missing explicit type definitions often trigger the 400 error.

3

Ah, this is a recognized issue when moving away from the standard widget flow. The suggestion above regarding the to object structure is technically correct, but you are likely running into a stricter validation layer on the Guest API that requires explicit conversation metadata that the widget normally handles client-side. When I set up custom metrics pipelines for web chat, I found that the Guest API endpoint rejects payloads that lack the conversationId in the to object, or where the from object is missing the required name field for anonymous guests.

It is not just about the id and type. You need to ensure the from object matches the expected schema for an external participant. If you are using a service account for this headless client, the API might still expect a user-like structure in the from field unless you are using a specific service account integration pattern. Here is a working curl example that I use to validate payloads before pushing them to the analytics queue. Note the explicit conversationId in the to object and the minimal name in the from object.

curl -X POST "https://api.mypurecloud.com/api/v2/conversations/messages" \
 -H "Authorization: Bearer <YOUR_ACCESS_TOKEN>" \
 -H "Content-Type: application/json" \
 -d '{
 "to": {
 "id": "your-conversation-id-here",
 "type": "conversation"
 },
 "from": {
 "name": "TestBot",
 "address": "[email protected]",
 "type": "user"
 },
 "text": "Headless test message",
 "type": "text"
 }'

If you are still seeing invalid_payload, check your OAuth scopes. You need conversation:view and conversation:write at minimum, but for guest interactions, webchat:guest or similar scopes might be required depending on your org configuration. Also, verify that the conversation ID you are targeting is actually open and in a state that accepts external messages. Closed conversations will reject this payload regardless of structure.

4

It depends, but generally…

HTTP 400 Bad Request: invalid_payload

The suggestion above correctly identifies the to object structure, but the Guest API validation is stricter than standard internal endpoints. You must explicitly define the conversationId within the to object, or the server rejects the payload immediately. Additionally, ensure your grant_type is correctly set to client_credentials if using a service account, as guest endpoints often reject user-scoped tokens for direct injection.

Here is the working Python snippet using the SDK:

from purecloudplatformclientv2 import GuestApi, Message

api_instance = GuestApi(platform_client)
message_body = Message(
 to=[{"id": "target_conversation_id", "type": "conversation"}],
 body="Test message for load testing"
)

try:
 api_instance.post_webmessaging_guests_conversations_messages(
 guest_id="your_guest_id",
 body=message_body
 )
except Exception as e:
 print(f"Failed: {e}")

Missing the type field or using an invalid guest_id scope will also trigger this 400 error. Double-check your OAuth scopes include webmessaging:guest.

5

I typically get around this by enforcing strict schema compliance. The Guest API rejects payloads missing explicit conversation metadata.

  1. Include conversationId in the to object.
  2. Set type to conversation.
{
 "to": { "id": "conv-id", "type": "conversation" },
 "text": "test"
}