Guest API POST /webmessaging/v1/sessions fails with 400 Bad Request

NICE CXone API & Integrations
1

Hey folks,

Trying to build a custom chat interface using the Genesys Cloud Guest API instead of the widget. When I hit POST /webmessaging/v1/sessions with the required headers, I get a 400 Bad Request.

Here’s the payload:

{
 "brandId": "my-brand-id",
 "attributes": {}
}

The Authorization header has a valid OAuth token. Any idea what’s missing?