fix: recurse into JSON-native containers in A2A _serialize_value()

The previous change passed `dict` and `list` values through
`_serialize_value()` unchanged. A non-JSON-serializable value nested inside one
of those containers (e.g. a `datetime`, `uuid.UUID`, or a nested Pydantic
model) was therefore returned raw. That is inconsistent with the stated intent
that only non-JSON-serializable types are stringified, and it risks a
`TypeError` when the A2A metadata is later JSON-encoded.

Recurse into `dict` and `list` elements so nested non-JSON-serializable values
are still stringified, while JSON-native scalars continue to pass through as-is.

Co-authored-by: Haran Rajkumar <haranrk@google.com>
PiperOrigin-RevId: 935043902

Author: haranrk

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

@@ -259,8 +259,19 @@ 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)):
+  # Recurse into JSON-native containers so nested non-JSON-serializable
+  # values (e.g. datetime) are still stringified, then pass through other
+  # JSON-native scalars as-is.
+  if isinstance(value, dict):
+    # JSON object keys must be strings, so stringify any non-string key to
+    # avoid a downstream TypeError when the metadata is JSON-encoded.
+    return {
+        (k if isinstance(k, str) else str(k)): _serialize_value(v)
+        for k, v in value.items()
+    }
+  if isinstance(value, list):
+    return [_serialize_value(item) for item in value]
+  if isinstance(value, (int, float, bool, str)):
     return value
 
   return str(value)

tests/unittests/a2a/converters/test_from_adk.py

@@ -179,3 +179,37 @@ def test_non_json_type_stringified(self) -> None:
     dt = datetime(2025, 1, 1)
     result = self.serialize(dt)
     assert isinstance(result, str)
+
+  def test_nested_non_json_value_in_dict_stringified(self) -> None:
+    """A non-JSON-native value nested in a dict is stringified."""
+    from datetime import datetime
+
+    dt = datetime(2025, 1, 1)
+    value = {"when": dt, "count": 1, "label": "x"}
+    result = self.serialize(value)
+    assert result == {"when": str(dt), "count": 1, "label": "x"}
+    assert isinstance(result["when"], str)
+    assert isinstance(result["count"], int)
+    assert isinstance(result["label"], str)
+
+  def test_nested_non_json_value_in_list_stringified(self) -> None:
+    """A non-JSON-native value nested in a list is stringified."""
+    from datetime import datetime
+
+    dt = datetime(2025, 1, 1)
+    value = [dt, 1, "x"]
+    result = self.serialize(value)
+    assert result == [str(dt), 1, "x"]
+    assert isinstance(result[0], str)
+    assert isinstance(result[1], int)
+    assert isinstance(result[2], str)
+
+  def test_non_string_dict_key_stringified(self) -> None:
+    """Non-string dict keys are stringified so the result is JSON-encodable."""
+    from datetime import datetime
+
+    dt = datetime(2025, 1, 1)
+    value = {dt: "when", 1: "one", "label": "x"}
+    result = self.serialize(value)
+    assert result == {str(dt): "when", "1": "one", "label": "x"}
+    assert all(isinstance(k, str) for k in result)