fix: preserve JSON-native types in A2A _serialize_value()

`_serialize_value()` in `from_adk_event.py` was calling `str()` on all
non-Pydantic values, which corrupted JSON-native metadata types (`dict`,
`list`, `int`, `float`, `bool`) into their Python string representations
during A2A event conversion.

Before: `{"count": 42}` became `{"count": "42"}` and `{"tags": ["a", "b"]}`
became `{"tags": "['a', 'b']"}`.

After: JSON-native types (`dict`, `list`, `int`, `float`, `bool`, `str`) are
passed through as-is. Only non-JSON-serializable types (e.g. `datetime`) are
still stringified.

Closes #5183

Co-authored-by: Haran Rajkumar <haranrk@google.com>
PiperOrigin-RevId: 934496131

Author: 2 people

src/google/adk/a2a/converters/from_adk_event.py

@@ -259,6 +259,10 @@ 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)
 
 

tests/unittests/a2a/converters/test_from_adk.py

@@ -130,3 +130,52 @@ def test_convert_event_to_a2a_events_with_actions(self):
     metadata = result[0].status.message.metadata
     assert "adk_actions" in metadata
     assert metadata["adk_actions"]["artifactDelta"] == {"image": 0}
+
+
+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)