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 6d8c698dd..bf25acfc7 100644
--- a/core/src/main/java/com/google/adk/events/EventActions.java
+++ b/core/src/main/java/com/google/adk/events/EventActions.java
@@ -22,6 +22,7 @@
 import com.google.adk.sessions.State;
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import java.util.HashSet;
+import java.util.Map;
 import java.util.Objects;
 import java.util.Optional;
 import java.util.Set;
@@ -383,7 +384,7 @@ public Builder compaction(EventCompaction value) {
     @CanIgnoreReturnValue
     public Builder merge(EventActions other) {
       other.skipSummarization().ifPresent(this::skipSummarization);
-      this.stateDelta.putAll(other.stateDelta());
+      other.stateDelta().forEach((key, value) -> stateDelta.merge(key, value, Builder::deepMerge));
       this.artifactDelta.putAll(other.artifactDelta());
       this.deletedArtifactIds.addAll(other.deletedArtifactIds());
       other.transferToAgent().ifPresent(this::transferToAgent);
@@ -395,6 +396,34 @@ public Builder merge(EventActions other) {
       return this;
     }
 
+    private static Object deepMerge(Object target, Object source) {
+      if (!(target instanceof Map) || !(source instanceof Map)) {
+        // If one of them is not a map, the source value overwrites the target.
+        return source;
+      }
+
+      Map<?, ?> targetMap = (Map<?, ?>) target;
+      Map<?, ?> sourceMap = (Map<?, ?>) source;
+
+      if (!targetMap.isEmpty() && !sourceMap.isEmpty()) {
+        Object targetKey = targetMap.keySet().iterator().next();
+        Object sourceKey = sourceMap.keySet().iterator().next();
+        if (targetKey != null
+            && sourceKey != null
+            && !targetKey.getClass().equals(sourceKey.getClass())) {
+          throw new IllegalArgumentException(
+              String.format(
+                  "Cannot merge maps with different key types: %s vs %s",
+                  targetKey.getClass().getName(), sourceKey.getClass().getName()));
+        }
+      }
+
+      // Create a new map to prevent UnsupportedOperationException from immutable maps
+      Map<Object, Object> mergedMap = new ConcurrentHashMap<>(targetMap);
+      sourceMap.forEach((key, value) -> mergedMap.merge(key, value, Builder::deepMerge));
+      return mergedMap;
+    }
+
     public EventActions build() {
       return new EventActions(this);
     }
diff --git a/core/src/test/java/com/google/adk/events/EventActionsTest.java b/core/src/test/java/com/google/adk/events/EventActionsTest.java
index 94cd399df..28123bab8 100644
--- a/core/src/test/java/com/google/adk/events/EventActionsTest.java
+++ b/core/src/test/java/com/google/adk/events/EventActionsTest.java
@@ -17,12 +17,14 @@
 package com.google.adk.events;
 
 import static com.google.common.truth.Truth.assertThat;
+import static org.junit.Assert.assertThrows;
 
 import com.google.adk.sessions.State;
 import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.ImmutableSet;
 import com.google.genai.types.Content;
 import com.google.genai.types.Part;
+import java.util.Map;
 import java.util.concurrent.ConcurrentHashMap;
 import org.junit.Test;
 import org.junit.runner.RunWith;
@@ -130,4 +132,37 @@ public void jsonSerialization_works() throws Exception {
     assertThat(deserialized).isEqualTo(eventActions);
     assertThat(deserialized.deletedArtifactIds()).containsExactly("d1", "d2");
   }
+
+  @Test
+  @SuppressWarnings("unchecked") // the nested map is known to be Map<String, Object>
+  public void merge_deeplyMergesStateDelta() {
+    EventActions eventActions1 = EventActions.builder().build();
+    eventActions1.stateDelta().put("a", 1);
+    eventActions1.stateDelta().put("b", ImmutableMap.of("nested1", 10, "nested2", 20));
+    eventActions1.stateDelta().put("c", 100);
+    EventActions eventActions2 = EventActions.builder().build();
+    eventActions2.stateDelta().put("a", 2);
+    eventActions2.stateDelta().put("b", ImmutableMap.of("nested2", 22, "nested3", 30));
+    eventActions2.stateDelta().put("d", 200);
+
+    EventActions merged = eventActions1.toBuilder().merge(eventActions2).build();
+
+    assertThat(merged.stateDelta().keySet()).containsExactly("a", "b", "c", "d");
+    assertThat(merged.stateDelta()).containsEntry("a", 2);
+    assertThat((Map<String, Object>) merged.stateDelta().get("b"))
+        .containsExactly("nested1", 10, "nested2", 22, "nested3", 30);
+    assertThat(merged.stateDelta()).containsEntry("c", 100);
+    assertThat(merged.stateDelta()).containsEntry("d", 200);
+  }
+
+  @Test
+  public void merge_failsOnMismatchedKeyTypesNestedInStateDelta() {
+    EventActions eventActions1 = EventActions.builder().build();
+    eventActions1.stateDelta().put("nested", ImmutableMap.of("a", 1));
+    EventActions eventActions2 = EventActions.builder().build();
+    eventActions2.stateDelta().put("nested", ImmutableMap.of(1, 2));
+
+    assertThrows(
+        IllegalArgumentException.class, () -> eventActions1.toBuilder().merge(eventActions2));
+  }
 }