diff --git a/compose/config/config_schema_v3.5.json b/compose/config/config_schema_v3.5.json
index fa95d6a24..5400cd99f 100644
--- a/compose/config/config_schema_v3.5.json
+++ b/compose/config/config_schema_v3.5.json
@@ -64,6 +64,7 @@
     }
   },
 
+  "patternProperties": {"^x-": {}},
   "additionalProperties": false,
 
   "definitions": {