fix: recursive serialization for nested non-JSON-native types

Address review feedback: _serialize_value() now recurses into dict/list
values so that nested non-serializable types (datetime, set, etc.) are
gracefully handled instead of being passed through raw.

- str/bool/int/float: return as-is (JSON leaf types)
- dict: recurse into values, coerce keys to str
- list/tuple: recurse into elements
- set/frozenset: convert to sorted list, recurse
- Other: try json.loads(json.dumps(value, default=str)) fallback
- Added regression test for datetime and set inside custom_metadata

Author: YizukiAme

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

@@ -250,11 +250,24 @@ def _serialize_value(value: Any) -> Optional[Any]:
       logger.warning("Failed to serialize Pydantic model, falling back: %s", e)
       return str(value)
 
-  if isinstance(value, (dict, list, str, bool, int, float)):
+  # JSON-native leaf types — return as-is
+  if isinstance(value, (str, bool, int, float)):
     return value
 
+  # Containers — recurse so nested non-serializable values are handled
+  if isinstance(value, dict):
+    return {str(k): _serialize_value(v) for k, v in value.items()}
+
+  if isinstance(value, (list, tuple)):
+    return [_serialize_value(item) for item in value]
+
+  # Common Python types with no JSON equivalent
+  if isinstance(value, (set, frozenset)):
+    return [_serialize_value(item) for item in sorted(value, key=str)]
+
+  # Other objects — try JSON normalization, then str() fallback
   try:
-    return json.loads(json.dumps(value))
+    return json.loads(json.dumps(value, default=str))
   except (TypeError, ValueError):
     pass
 

tests/unittests/a2a/converters/test_event_round_trip.py

@@ -248,3 +248,43 @@ def test_round_trip_custom_metadata_preserves_structured_values():
   assert (
       _parse_adk_metadata_value(serialized_metadata) == original_custom_metadata
   )
+
+
+def test_serialize_value_handles_non_serializable_nested_types():
+  """Regression: non-JSON-native types inside dicts/lists must not crash."""
+  from datetime import datetime
+  from datetime import timezone
+
+  from google.adk.a2a.converters.from_adk_event import _serialize_value
+
+  ts = datetime(2026, 1, 15, 12, 0, 0, tzinfo=timezone.utc)
+  value = {
+      "created_at": ts,
+      "tags": {"alpha", "beta"},
+      "normal": 42,
+      "nested_list": [True, ts],
+  }
+
+  result = _serialize_value(value)
+
+  # Result must be fully JSON-serializable (no crash)
+  import json
+
+  json_str = json.dumps(result)
+  parsed = json.loads(json_str)
+
+  # Leaf types preserved
+  assert parsed["normal"] == 42
+
+  # datetime falls back to str representation
+  assert isinstance(parsed["created_at"], str)
+  assert "2026" in parsed["created_at"]
+
+  # set becomes a sorted list of strings
+  assert isinstance(parsed["tags"], list)
+  assert set(parsed["tags"]) == {"alpha", "beta"}
+
+  # nested list with mixed types
+  assert parsed["nested_list"][0] is True
+  assert isinstance(parsed["nested_list"][1], str)
+