diff --git a/samples/agent/adk/contact_lookup/a2ui_schema.py b/samples/agent/adk/contact_lookup/a2ui_schema.py
index 4b6038fd..f3163fce 100644
--- a/samples/agent/adk/contact_lookup/a2ui_schema.py
+++ b/samples/agent/adk/contact_lookup/a2ui_schema.py
@@ -20,6 +20,7 @@
   "title": "A2UI Message Schema",
   "description": "Describes a JSON payload for an A2UI (Agent to UI) message, which is used to dynamically construct and update user interfaces. A message MUST contain exactly ONE of the action properties: 'beginRendering', 'surfaceUpdate', 'dataModelUpdate', or 'deleteSurface'.",
   "type": "object",
+  "additionalProperties": false
   "properties": {
     "beginRendering": {
       "type": "object",