diff --git a/core/src/main/java/com/google/adk/events/EventActions.java b/core/src/main/java/com/google/adk/events/EventActions.java
index 0b167de93..38e7d1d96 100644
--- a/core/src/main/java/com/google/adk/events/EventActions.java
+++ b/core/src/main/java/com/google/adk/events/EventActions.java
@@ -83,7 +83,7 @@ public void setSkipSummarization(boolean skipSummarization) {
   }
 
   @JsonProperty("stateDelta")
-  public ConcurrentMap<String, Object> stateDelta() {
+  public Map<String, Object> stateDelta() {
     return stateDelta;
   }