Function spawn_subtask_schema 

pub fn spawn_subtask_schema(parent_tools: &[String]) -> ToolSchema

Build the spawn_subtask tool schema whose tools parameter is constrained to an enum of parent_tools — the verifiable subset precondition.

If parent_tools is empty the enum is empty, so the validator rejects any non-empty tools request: an agent with no tools can only spawn a (useless) tool-less sub-agent. That’s a degenerate but safe configuration.