diff --git a/optuna_dashboard/_serializer.py b/optuna_dashboard/_serializer.py
index 7fae23ea..04190f59 100644
--- a/optuna_dashboard/_serializer.py
+++ b/optuna_dashboard/_serializer.py
@@ -15,6 +15,7 @@ from . import _note as note
 from ._form_widget import get_form_widgets_json
 from ._named_objectives import get_objective_names
 from .artifact._backend import list_trial_artifacts
+from .preferential._study import _SYSTEM_ATTR_PREFERENTIAL_STUDY
 
 
 if TYPE_CHECKING:
@@ -143,6 +144,7 @@ def serialize_study_detail(
     serialized["union_user_attrs"] = [{"key": a[0], "sortable": a[1]} for a in union_user_attrs]
     serialized["has_intermediate_values"] = has_intermediate_values
     serialized["note"] = note.get_note_from_system_attrs(system_attrs, None)
+    serialized["is_preferential"] = system_attrs.get(_SYSTEM_ATTR_PREFERENTIAL_STUDY, False)
     objective_names = get_objective_names(system_attrs)
     if objective_names:
         serialized["objective_names"] = objective_names
diff --git a/python_tests/test_serializers.py b/python_tests/test_serializers.py
index 4ea8c9bd..7a038b08 100644
--- a/python_tests/test_serializers.py
+++ b/python_tests/test_serializers.py
@@ -1,19 +1,43 @@
 from __future__ import annotations
 
-from unittest import TestCase
-
+import optuna
 from optuna_dashboard._serializer import serialize_attrs
+from optuna_dashboard._serializer import serialize_study_detail
+from optuna_dashboard._storage import get_study_summaries
+from optuna_dashboard.preferential import create_study
 
 
-class SerializeAttrsTestCase(TestCase):
-    def test_serialize_bytes(self) -> None:
-        serialized = serialize_attrs({"bytes": b"This is a bytes object."})
-        self.assertEqual(serialized[0]["value"], "<binary object>")
+def test_serialize_bytes() -> None:
+    serialized = serialize_attrs({"bytes": b"This is a bytes object."})
+    assert serialized[0]["value"] == "<binary object>"
 
-    def test_serialize_dict(self) -> None:
-        serialized = serialize_attrs(
-            {
-                "key": {"foo": "bar"},
-            }
-        )
-        self.assertLessEqual(len(serialized), 1)
+
+def test_serialize_dict() -> None:
+    serialized = serialize_attrs(
+        {
+            "key": {"foo": "bar"},
+        }
+    )
+    assert len(serialized) <= 1
+
+
+def test_get_study_detail_is_preferential() -> None:
+    storage = optuna.storages.InMemoryStorage()
+    study = create_study(storage=storage)
+    study_summaries = get_study_summaries(storage)
+    assert len(study_summaries) == 1
+
+    study_summary = study_summaries[0]
+    study_detail = serialize_study_detail(study_summary, [], study.trials, [], [], [], False)
+    assert study_detail["is_preferential"]
+
+
+def test_get_study_detail_is_not_preferential() -> None:
+    storage = optuna.storages.InMemoryStorage()
+    study = optuna.create_study(storage=storage)
+    study_summaries = get_study_summaries(storage)
+    assert len(study_summaries) == 1
+
+    study_summary = study_summaries[0]
+    study_detail = serialize_study_detail(study_summary, [], study.trials, [], [], [], False)
+    assert not study_detail["is_preferential"]
diff --git a/python_tests/wsgi_client.py b/python_tests/wsgi_client.py
index 3a6b078e..84fe8aac 100644
--- a/python_tests/wsgi_client.py
+++ b/python_tests/wsgi_client.py
@@ -6,12 +6,21 @@ from typing import Optional
 from typing import Union
 
 from bottle import Bottle
+from optuna_dashboard._storage import trials_cache
+from optuna_dashboard._storage import trials_cache_lock
+from optuna_dashboard._storage import trials_last_fetched_at
 
 
 if typing.TYPE_CHECKING:
     from _typeshed.wsgi import WSGIEnvironment
 
 
+def clear_inmemory_cache() -> None:
+    with trials_cache_lock:
+        trials_cache.clear()
+        trials_last_fetched_at.clear()
+
+
 def create_wsgi_env(
     path: str,
     method: str,
@@ -66,6 +75,8 @@ def send_request(
     headers = headers or {}
     queries = queries or {}
     env = create_wsgi_env(path, method, content_type, bytes_body, queries, headers)
+
+    clear_inmemory_cache()
     response_body = b""
     iterable_body = app(env, start_response)
     for b in iterable_body: