Architect Webchat Start Event Failing with 400 Bad Request

General Discussion
1

I’ve spent hours trying to figure out why the POST /api/v2/conversations/webchat/events/start endpoint returns a 400 Bad Request when injecting a custom_data object exceeding 512 bytes in the payload. The Architect flow is configured to parse the JSON, yet the platform rejects the connection initialization before the conversation_id is generated, causing the client-side SDK to throw a WebSocket closed error immediately.

2

What’s probably happening here is that payload size limits on the initialization endpoint. Large custom_data objects trigger validation failures before the session starts. Try shrinking the JSON or splitting the data. Here is a compliant payload structure for JMeter:

{
 "custom_data": {
 "key": "value",
 "size": "small"
 }
}
3

Yep, this is a known issue with the payload validation on the start endpoint.

The 512-byte limit is hard-coded for the initial handshake to protect WebSocket capacity. Moving the heavy JSON to a subsequent message event fixes the 400 error immediately.