refactor: extract Pydantic serialization into shared utility

Extract the duplicated Pydantic BaseModel serialization logic from
both lite_llm.py and tracing.py into a shared serialize_pydantic_model()
helper in utils/serialization_utils.py. This ensures future changes to
Pydantic serialization are made in a single place.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 2 people

src/google/adk/models/lite_llm.py

@@ -51,6 +51,8 @@
 from pydantic import Field
 from typing_extensions import override
 
+from ..utils.serialization_utils import serialize_pydantic_model
+
 from ..utils._google_client_headers import merge_tracking_headers
 from .base_llm import BaseLlm
 from .llm_request import LlmRequest
@@ -493,8 +495,9 @@ def _safe_json_serialize(obj) -> str:
   """
 
   try:
-    if isinstance(obj, BaseModel):
-      return json.dumps(obj.model_dump(), default=str)
+    serialized = serialize_pydantic_model(obj)
+    if serialized is not None:
+      return serialized
     # Try direct JSON serialization first
     return json.dumps(obj, ensure_ascii=False)
   except (TypeError, OverflowError):

src/google/adk/telemetry/tracing.py

@@ -65,6 +65,7 @@
 
 from .. import version
 from ..utils.model_name_utils import is_gemini_model
+from ..utils.serialization_utils import serialize_pydantic_model
 from ._experimental_semconv import is_experimental_semconv
 from ._experimental_semconv import maybe_log_completion_details
 from ._experimental_semconv import set_operation_details_attributes_from_request
@@ -117,8 +118,9 @@ def _safe_json_serialize(obj) -> str:
   """
 
   try:
-    if isinstance(obj, BaseModel):
-      return json.dumps(obj.model_dump(), default=str)
+    serialized = serialize_pydantic_model(obj)
+    if serialized is not None:
+      return serialized
     # Try direct JSON serialization first
     return json.dumps(
         obj, ensure_ascii=False, default=lambda o: '<not serializable>'

src/google/adk/utils/serialization_utils.py

@@ -0,0 +1,35 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Shared serialization utilities for Pydantic models."""
+
+import json
+from typing import Any
+from typing import Optional
+
+from pydantic import BaseModel
+
+
+def serialize_pydantic_model(obj: Any) -> Optional[str]:
+  """Serialize a Pydantic BaseModel to a JSON string.
+
+  Args:
+    obj: The object to check and serialize.
+
+  Returns:
+    A JSON string if the object is a Pydantic BaseModel, or None otherwise.
+  """
+  if isinstance(obj, BaseModel):
+    return json.dumps(obj.model_dump(), default=str)
+  return None