fix: guard decode_model against non-dict session values

Merge #6383

Closes #6348

PiperOrigin-RevId: 947276063

Author: Brumbelow

src/google/adk/sessions/_session_util.py

@@ -29,7 +29,13 @@ def decode_model(
     data: Optional[dict[str, Any]], model_cls: Type[M]
 ) -> Optional[M]:
   """Decodes a pydantic model object from a JSON dictionary."""
-  if data is None:
+  # Guard against primitive non-dict values (e.g. a legacy/corrupted "null" string
+  # persisted in place of SQL NULL). Passing those to model_validate would
+  # raise a ValidationError and break session replay in get_session().
+  # We allow dicts and other objects (like PydanticNamespace in tests).
+  if data is None or isinstance(
+      data, (str, int, float, bool, list, set, tuple, bytes)
+  ):
     return None
   return model_cls.model_validate(data)
 

tests/unittests/sessions/test_session_util.py

@@ -0,0 +1,35 @@
+# Copyright 2026 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.
+
+from google.adk.sessions._session_util import decode_model
+from google.genai import types
+
+
+def test_decode_model_returns_none_for_none():
+  assert decode_model(None, types.Content) is None
+
+
+def test_decode_model_validates_dict():
+  result = decode_model(
+      {"role": "user", "parts": [{"text": "hello"}]}, types.Content
+  )
+  assert isinstance(result, types.Content)
+  assert result.role == "user"
+  assert result.parts[0].text == "hello"
+
+
+def test_decode_model_returns_none_for_non_dict_value():
+  # A transcription field persisted as the JSON string "null" instead of SQL
+  # NULL should decode to None rather than crash session replay.
+  assert decode_model("null", types.Transcription) is None