diff --git a/tests/unittests/sessions/test__session_util.py b/tests/unittests/sessions/test__session_util.py
new file mode 100644
index 0000000000..24cc02f338
--- /dev/null
+++ b/tests/unittests/sessions/test__session_util.py
@@ -0,0 +1,91 @@
+# 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 __future__ import annotations
+
+from google.adk.sessions._session_util import decode_model
+from google.adk.sessions._session_util import extract_state_delta
+from pydantic import BaseModel
+import pytest
+
+
+class _SampleModel(BaseModel):
+  name: str
+  value: int
+
+
+class TestDecodeModel:
+
+  def test_returns_none_for_none_input(self):
+    assert decode_model(None, _SampleModel) is None
+
+  def test_decodes_dict_into_model_instance(self):
+    result = decode_model({"name": "foo", "value": 7}, _SampleModel)
+
+    assert result == _SampleModel(name="foo", value=7)
+
+  def test_raises_for_invalid_data(self):
+    with pytest.raises(Exception):
+      decode_model({"name": "foo"}, _SampleModel)
+
+
+class TestExtractStateDelta:
+
+  def test_returns_empty_deltas_for_empty_state(self):
+    assert extract_state_delta({}) == {"app": {}, "user": {}, "session": {}}
+
+  def test_returns_empty_deltas_for_none_state(self):
+    assert extract_state_delta(None) == {"app": {}, "user": {}, "session": {}}
+
+  def test_routes_app_prefixed_keys_with_prefix_stripped(self):
+    deltas = extract_state_delta({"app:theme": "dark"})
+
+    assert deltas["app"] == {"theme": "dark"}
+    assert deltas["user"] == {}
+    assert deltas["session"] == {}
+
+  def test_routes_user_prefixed_keys_with_prefix_stripped(self):
+    deltas = extract_state_delta({"user:lang": "en"})
+
+    assert deltas["user"] == {"lang": "en"}
+    assert deltas["app"] == {}
+    assert deltas["session"] == {}
+
+  def test_routes_unprefixed_keys_to_session(self):
+    deltas = extract_state_delta({"turn": 3})
+
+    assert deltas["session"] == {"turn": 3}
+    assert deltas["app"] == {}
+    assert deltas["user"] == {}
+
+  def test_skips_temp_prefixed_keys(self):
+    deltas = extract_state_delta({"temp:scratch": "ignore_me"})
+
+    assert deltas == {"app": {}, "user": {}, "session": {}}
+
+  def test_routes_mixed_keys_into_correct_buckets(self):
+    state = {
+        "app:theme": "dark",
+        "user:lang": "en",
+        "temp:scratch": "ignore_me",
+        "turn": 3,
+    }
+
+    deltas = extract_state_delta(state)
+
+    assert deltas == {
+        "app": {"theme": "dark"},
+        "user": {"lang": "en"},
+        "session": {"turn": 3},
+    }