parentSessionId property
ParentSessionID makes this a subagent of that session: it inherits the parent's root, so one flow stays one tree. The parent must exist IN THE SAME ORG — a foreign or unknown id is a 400, never a tree across tenants. Empty opens a root session.
Please note: This property should have been non-nullable! Since the specification file does not include a default value (using the "default:" property), however, the generated source code must fall back to having a nullable type. Consider adding a "default:" property in the specification file to hide this note.
Implementation
String? parentSessionId;