fix: disallow runaway subagent chains (#5659)

Author: alexhancock

crates/goose/src/agents/agent.rs

@@ -394,6 +394,20 @@ impl Agent {
         cancellation_token: Option<CancellationToken>,
         session: &Session,
     ) -> (String, Result<ToolCallResult, ErrorData>) {
+        if session.session_type == crate::session::SessionType::SubAgent
+            && (tool_call.name == DYNAMIC_TASK_TOOL_NAME_PREFIX
+                || tool_call.name == SUBAGENT_EXECUTE_TASK_TOOL_NAME)
+        {
+            return (
+                request_id,
+                Err(ErrorData::new(
+                    ErrorCode::INVALID_REQUEST,
+                    "Subagents cannot create other subagents".to_string(),
+                    None,
+                )),
+            );
+        }
+
         if tool_call.name == PLATFORM_MANAGE_SCHEDULE_TOOL_NAME {
             let arguments = tool_call
                 .arguments