startChat() attributes missing in Architect flow Data Action

Stuck on injecting CRM ID via startChat(). The Architect flow requires this value for a Data Action lookup, but the payload drops silently.

sdk.startChat({ attributes: { crmId: "123" } });

The session creates, yet the flow expression ToString(event.attributes.crmId) returns empty. Does startChat ignore the attributes object, or must I use a specific Guest API endpoint to merge this data before the chat begins?