Studio Session Handover failing with 400 Bad Request when passing payload to Cognigy

We’re seeing intermittent failures when trying to hand over a voice interaction from a NICE Cognigy bot to a CXone Studio flow. The bot initiates the handover via the Session API, but Studio drops the call with a 400 Bad Request error. I’ve verified the bot is passing the required sessionId and userId in the POST payload to the Studio webhook endpoint.

Here’s the payload structure we’re sending:

{
 "sessionId": "abc-123-def",
 "userId": "user-999",
 "context": {
 "intent": "billing_inquiry",
 "amount": 150.00
 }
}

I checked the Studio logs, and it looks like the context object is being rejected. The error message in the Studio trace just says Invalid payload structure. I’ve tried removing the nested context object and flattening the keys, but that breaks the downstream logic we have in Studio.

Has anyone run into this specific schema mismatch with the Cognigy voicebot integration? I’m wondering if the Session API expects a different format for custom data attributes. The documentation is pretty light on the exact JSON schema requirements for the handover endpoint.