diff --git a/tests/unit/vertex_adk/test_agent_engine_templates_adk.py b/tests/unit/vertex_adk/test_agent_engine_templates_adk.py
index 86840722f0..3f48a0e23e 100644
--- a/tests/unit/vertex_adk/test_agent_engine_templates_adk.py
+++ b/tests/unit/vertex_adk/test_agent_engine_templates_adk.py
@@ -546,12 +546,12 @@ async def test_streaming_agent_run_with_events_force_flush_otel(
     async def test_async_create_session(self, get_project_id_mock: mock.Mock):
         app = agent_engines.AdkApp(agent=_TEST_AGENT)
         session1 = await app.async_create_session(user_id=_TEST_USER_ID)
-        assert session1.user_id == _TEST_USER_ID
+        assert session1["user_id"] == _TEST_USER_ID
         session2 = await app.async_create_session(
             user_id=_TEST_USER_ID, session_id="test_session_id"
         )
-        assert session2.user_id == _TEST_USER_ID
-        assert session2.id == "test_session_id"
+        assert session2["user_id"] == _TEST_USER_ID
+        assert session2["id"] == "test_session_id"
 
     @pytest.mark.asyncio
     async def test_async_get_session(self, get_project_id_mock: mock.Mock):
@@ -559,10 +559,10 @@ async def test_async_get_session(self, get_project_id_mock: mock.Mock):
         session1 = await app.async_create_session(user_id=_TEST_USER_ID)
         session2 = await app.async_get_session(
             user_id=_TEST_USER_ID,
-            session_id=session1.id,
+            session_id=session1["id"],
         )
-        assert session2.user_id == _TEST_USER_ID
-        assert session1.id == session2.id
+        assert session2["user_id"] == _TEST_USER_ID
+        assert session1["id"] == session2["id"]
 
     @pytest.mark.asyncio
     async def test_async_list_sessions(self, get_project_id_mock: mock.Mock):
@@ -572,12 +572,12 @@ async def test_async_list_sessions(self, get_project_id_mock: mock.Mock):
         session = await app.async_create_session(user_id=_TEST_USER_ID)
         response1 = await app.async_list_sessions(user_id=_TEST_USER_ID)
         assert len(response1.sessions) == 1
-        assert response1.sessions[0].id == session.id
+        assert response1.sessions[0].id == session["id"]
         session2 = await app.async_create_session(user_id=_TEST_USER_ID)
         response2 = await app.async_list_sessions(user_id=_TEST_USER_ID)
         assert len(response2.sessions) == 2
-        assert response2.sessions[0].id == session.id
-        assert response2.sessions[1].id == session2.id
+        assert response2.sessions[0].id == session["id"]
+        assert response2.sessions[1].id == session2["id"]
 
     @pytest.mark.asyncio
     async def test_async_delete_session(self, get_project_id_mock: mock.Mock):
@@ -592,7 +592,7 @@ async def test_async_delete_session(self, get_project_id_mock: mock.Mock):
         assert len(response1.sessions) == 1
         await app.async_delete_session(
             user_id=_TEST_USER_ID,
-            session_id=session.id,
+            session_id=session["id"],
         )
         response0 = await app.async_list_sessions(user_id=_TEST_USER_ID)
         assert not response0.sessions
@@ -600,22 +600,22 @@ async def test_async_delete_session(self, get_project_id_mock: mock.Mock):
     def test_create_session(self, get_project_id_mock: mock.Mock):
         app = agent_engines.AdkApp(agent=_TEST_AGENT)
         session1 = app.create_session(user_id=_TEST_USER_ID)
-        assert session1.user_id == _TEST_USER_ID
+        assert session1["user_id"] == _TEST_USER_ID
         session2 = app.create_session(
             user_id=_TEST_USER_ID, session_id="test_session_id"
         )
-        assert session2.user_id == _TEST_USER_ID
-        assert session2.id == "test_session_id"
+        assert session2["user_id"] == _TEST_USER_ID
+        assert session2["id"] == "test_session_id"
 
     def test_get_session(self, get_project_id_mock: mock.Mock):
         app = agent_engines.AdkApp(agent=_TEST_AGENT)
         session1 = app.create_session(user_id=_TEST_USER_ID)
         session2 = app.get_session(
             user_id=_TEST_USER_ID,
-            session_id=session1.id,
+            session_id=session1["id"],
         )
-        assert session2.user_id == _TEST_USER_ID
-        assert session1.id == session2.id
+        assert session2["user_id"] == _TEST_USER_ID
+        assert session1["id"] == session2["id"]
 
     def test_list_sessions(self, get_project_id_mock: mock.Mock):
         app = agent_engines.AdkApp(agent=_TEST_AGENT)
@@ -624,12 +624,12 @@ def test_list_sessions(self, get_project_id_mock: mock.Mock):
         session = app.create_session(user_id=_TEST_USER_ID)
         response1 = app.list_sessions(user_id=_TEST_USER_ID)
         assert len(response1.sessions) == 1
-        assert response1.sessions[0].id == session.id
+        assert response1.sessions[0].id == session["id"]
         session2 = app.create_session(user_id=_TEST_USER_ID)
         response2 = app.list_sessions(user_id=_TEST_USER_ID)
         assert len(response2.sessions) == 2
-        assert response2.sessions[0].id == session.id
-        assert response2.sessions[1].id == session2.id
+        assert response2.sessions[0].id == session["id"]
+        assert response2.sessions[1].id == session2["id"]
 
     def test_delete_session(self, get_project_id_mock: mock.Mock):
         app = agent_engines.AdkApp(agent=_TEST_AGENT)
@@ -638,7 +638,7 @@ def test_delete_session(self, get_project_id_mock: mock.Mock):
         session = app.create_session(user_id=_TEST_USER_ID)
         response1 = app.list_sessions(user_id=_TEST_USER_ID)
         assert len(response1.sessions) == 1
-        app.delete_session(user_id=_TEST_USER_ID, session_id=session.id)
+        app.delete_session(user_id=_TEST_USER_ID, session_id=session["id"])
         response0 = app.list_sessions(user_id=_TEST_USER_ID)
         assert not response0.sessions
 
diff --git a/vertexai/agent_engines/templates/adk.py b/vertexai/agent_engines/templates/adk.py
index 2387cf2f38..20dc1c38ba 100644
--- a/vertexai/agent_engines/templates/adk.py
+++ b/vertexai/agent_engines/templates/adk.py
@@ -1318,7 +1318,9 @@ async def async_get_session(
             raise RuntimeError(
                 "Session not found. Please create it using .create_session()"
             )
-        return session
+        if hasattr(session, "model_dump"):
+            return session.model_dump(mode="json")
+        return session.dict() if hasattr(session, "dict") else session
 
     def get_session(
         self,
@@ -1464,7 +1466,9 @@ async def async_create_session(
             state=state,
             **kwargs,
         )
-        return session
+        if hasattr(session, "model_dump"):
+            return session.model_dump(mode="json")
+        return session.dict() if hasattr(session, "dict") else session
 
     def create_session(
         self,