diff --git a/src/google/adk/a2a/converters/from_adk_event.py b/src/google/adk/a2a/converters/from_adk_event.py
index d64c1940fa..b5bb260014 100644
--- a/src/google/adk/a2a/converters/from_adk_event.py
+++ b/src/google/adk/a2a/converters/from_adk_event.py
@@ -17,6 +17,7 @@
 from collections.abc import Callable
 from datetime import datetime
 from datetime import timezone
+import json
 import logging
 from typing import Any
 from typing import Dict
@@ -266,6 +267,27 @@ def _serialize_value(value: Any) -> Optional[Any]:
       logger.warning("Failed to serialize Pydantic model, falling back: %s", e)
       return str(value)
 
+  # 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, default=str))
+  except (TypeError, ValueError):
+    pass
+
   return str(value)
 
 
diff --git a/tests/unittests/a2a/converters/test_event_round_trip.py b/tests/unittests/a2a/converters/test_event_round_trip.py
index 00036f6af7..40a7d8c1d7 100644
--- a/tests/unittests/a2a/converters/test_event_round_trip.py
+++ b/tests/unittests/a2a/converters/test_event_round_trip.py
@@ -23,8 +23,10 @@
 from a2a.types import TaskStatusUpdateEvent
 from google.adk.a2a.converters.from_adk_event import convert_event_to_a2a_events
 from google.adk.a2a.converters.from_adk_event import create_error_status_event
+from google.adk.a2a.converters.to_adk_event import _parse_adk_metadata_value
 from google.adk.a2a.converters.to_adk_event import convert_a2a_artifact_update_to_event
 from google.adk.a2a.converters.to_adk_event import convert_a2a_status_update_to_event
+from google.adk.a2a.converters.utils import _get_adk_metadata_key
 from google.adk.agents.invocation_context import InvocationContext
 from google.adk.events.event import Event
 from google.genai import types as genai_types
@@ -206,3 +208,83 @@ def test_round_trip_function_response_event():
   assert restored_event.content.parts[0].function_response.response == {
       "result": "success"
   }
+
+
+def test_round_trip_custom_metadata_preserves_structured_values():
+  original_custom_metadata = {
+      "flag": True,
+      "count": 42,
+      "nested": {"key": "val"},
+      "tags": ["a", "b"],
+  }
+  original_event = Event(
+      invocation_id="test_invocation",
+      author="test_agent",
+      branch="main",
+      content=genai_types.Content(
+          role="model",
+          parts=[genai_types.Part.from_text(text="Hello world!")],
+      ),
+      custom_metadata=original_custom_metadata,
+  )
+  agents_artifacts: Dict[str, str] = {}
+
+  a2a_events = convert_event_to_a2a_events(
+      event=original_event,
+      agents_artifacts=agents_artifacts,
+      task_id="task1",
+      context_id="context1",
+  )
+
+  assert len(a2a_events) == 1
+  a2a_event = a2a_events[0]
+  assert isinstance(a2a_event, TaskArtifactUpdateEvent)
+
+  serialized_metadata = a2a_event.artifact.metadata[
+      _get_adk_metadata_key("custom_metadata")
+  ]
+
+  assert not isinstance(serialized_metadata, str)
+  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)
+