diff --git a/src/lib/blueprint.ts b/src/lib/blueprint.ts
index ea6cac8..30ca3e8 100644
--- a/src/lib/blueprint.ts
+++ b/src/lib/blueprint.ts
@@ -1504,6 +1504,12 @@ const createProperty = (
 ): Property => {
   const parsedProp = parsePropertySchema(name, prop, parentPaths, schemas)
 
+  if (name.startsWith('ext_')) {
+    throw new Error(
+      `Property cannot start with ext_ : ${parentPaths.join('.')}.${name}`,
+    )
+  }
+
   const propertyGroupKey = parsedProp['x-property-group-key'] as string
   validateGroupKey(propertyGroupKey, name, parentPaths, propertyGroups)