diff --git a/src/google/adk/a2a/converters/from_adk_event.py b/src/google/adk/a2a/converters/from_adk_event.py
index d64c1940fa..e68996ea87 100644
--- a/src/google/adk/a2a/converters/from_adk_event.py
+++ b/src/google/adk/a2a/converters/from_adk_event.py
@@ -266,6 +266,10 @@ def _serialize_value(value: Any) -> Optional[Any]:
       logger.warning("Failed to serialize Pydantic model, falling back: %s", e)
       return str(value)
 
+  # Pass through JSON-native types as-is
+  if isinstance(value, (dict, list, int, float, bool, str)):
+    return value
+
   return str(value)
 
 
diff --git a/tests/unittests/a2a/converters/test_from_adk.py b/tests/unittests/a2a/converters/test_from_adk.py
index 23546c58b0..f657ea3176 100644
--- a/tests/unittests/a2a/converters/test_from_adk.py
+++ b/tests/unittests/a2a/converters/test_from_adk.py
@@ -106,3 +106,52 @@ def test_convert_event_to_a2a_events_none_artifacts(self):
     """Test convert_event_to_a2a_events with None agents_artifacts."""
     with pytest.raises(ValueError, match="Agents artifacts cannot be None"):
       convert_event_to_a2a_events(self.mock_event, None)
+
+
+class TestSerializeValue:
+  """Tests for _serialize_value preserving JSON-native types."""
+
+  def setup_method(self) -> None:
+    from google.adk.a2a.converters.from_adk_event import _serialize_value
+
+    self.serialize = _serialize_value
+
+  def test_dict_preserved(self) -> None:
+    value = {"key": "val", "nested": {"a": 1}}
+    result = self.serialize(value)
+    assert result == value
+    assert isinstance(result, dict)
+
+  def test_list_preserved(self) -> None:
+    value = [1, "two", {"three": 3}]
+    result = self.serialize(value)
+    assert result == value
+    assert isinstance(result, list)
+
+  def test_int_preserved(self) -> None:
+    result = self.serialize(42)
+    assert result == 42
+    assert isinstance(result, int)
+
+  def test_float_preserved(self) -> None:
+    result = self.serialize(3.14)
+    assert result == 3.14
+    assert isinstance(result, float)
+
+  def test_bool_preserved(self) -> None:
+    assert self.serialize(True) is True
+    assert self.serialize(False) is False
+
+  def test_string_preserved(self) -> None:
+    assert self.serialize("hello") == "hello"
+
+  def test_none_returns_none(self) -> None:
+    assert self.serialize(None) is None
+
+  def test_non_json_type_stringified(self) -> None:
+    """Non-JSON-native types should still be converted to str."""
+    from datetime import datetime
+
+    dt = datetime(2025, 1, 1)
+    result = self.serialize(dt)
+    assert isinstance(result, str)