diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 65a00c979691..f3a585e32db3 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -6869,6 +6869,23 @@ static int validate_ops(struct scx_sched *sch, const struct sched_ext_ops *ops)
 	if (!sch->is_cid_type && (ops->cpu_acquire || ops->cpu_release))
 		pr_warn("ops->cpu_acquire/release() are deprecated, use sched_switch TP instead\n");
 
+	/*
+	 * Sub-scheduler support is tied to the cid-form struct_ops. A sub-sched
+	 * attaches through a cid-form-only interface (sub_attach/sub_detach),
+	 * and a root that accepts sub-scheds must expose cid-form state to
+	 * them. Reject cpu-form schedulers on either side.
+	 */
+	if (!sch->is_cid_type) {
+		if (scx_parent(sch)) {
+			scx_error(sch, "sub-sched requires cid-form struct_ops");
+			return -EINVAL;
+		}
+		if (ops->sub_attach || ops->sub_detach) {
+			scx_error(sch, "sub_attach/sub_detach requires cid-form struct_ops");
+			return -EINVAL;
+		}
+	}
+
 	return 0;
 }