diff --git a/kernel/newcelltypes.h b/kernel/newcelltypes.h
index c4a4ea329..bb14293a3 100644
--- a/kernel/newcelltypes.h
+++ b/kernel/newcelltypes.h
@@ -6,6 +6,11 @@
 
 YOSYS_NAMESPACE_BEGIN
 
+/**
+ * This API is unstable.
+ * It may change or be removed in future versions and break dependent code.
+ */
+
 namespace StaticCellTypes {
 
 // Given by last internal cell type IdString constids.inc, compilation error if too low