diff --git a/linkml_model/model/schema/extended_types.yaml b/linkml_model/model/schema/extended_types.yaml
index 0298fc62..28f08a50 100644
--- a/linkml_model/model/schema/extended_types.yaml
+++ b/linkml_model/model/schema/extended_types.yaml
@@ -2,8 +2,14 @@ title: Core LinkML metamodel extended types
 name: extended_types
 id: https://w3id.org/linkml/extended_types
 
-description: Extensions to core LinkML types
+description: >-
+  Extensions to core LinkML types.
+
+  Status: Experimental. This schema is not yet fully supported by the LinkML
+  toolkit — in particular, the `union_of` construct on types is not handled
+  by `PythonGenerator`.
 license: https://creativecommons.org/publicdomain/zero/1.0/
+status: testing
 
 prefixes:
   linkml: https://w3id.org/linkml/