diff --git a/src/google/adk/cli/fast_api.py b/src/google/adk/cli/fast_api.py
index 4ef8ae6c..875c3cb7 100644
--- a/src/google/adk/cli/fast_api.py
+++ b/src/google/adk/cli/fast_api.py
@@ -276,7 +276,6 @@ def get_fast_api_app(
     memory_service = InMemoryMemoryService()
 
   # Build the Session service
-  agent_engine_id = ""
   if session_service_uri:
     if session_service_uri.startswith("agentengine://"):
       # Create vertex session service
@@ -285,8 +284,9 @@ def get_fast_api_app(
         raise click.ClickException("Agent engine id can not be empty.")
       envs.load_dotenv_for_agent("", agents_dir)
       session_service = VertexAiSessionService(
-          os.environ["GOOGLE_CLOUD_PROJECT"],
-          os.environ["GOOGLE_CLOUD_LOCATION"],
+          project=os.environ["GOOGLE_CLOUD_PROJECT"],
+          location=os.environ["GOOGLE_CLOUD_LOCATION"],
+          agent_engine_id=agent_engine_id,
       )
     else:
       session_service = DatabaseSessionService(db_url=session_service_uri)
@@ -357,8 +357,6 @@ def get_fast_api_app(
   async def get_session(
       app_name: str, user_id: str, session_id: str
   ) -> Session:
-    # Connect to managed session if agent_engine_id is set.
-    app_name = agent_engine_id if agent_engine_id else app_name
     session = await session_service.get_session(
         app_name=app_name, user_id=user_id, session_id=session_id
     )
@@ -371,8 +369,6 @@ def get_fast_api_app(
       response_model_exclude_none=True,
   )
   async def list_sessions(app_name: str, user_id: str) -> list[Session]:
-    # Connect to managed session if agent_engine_id is set.
-    app_name = agent_engine_id if agent_engine_id else app_name
     list_sessions_response = await session_service.list_sessions(
         app_name=app_name, user_id=user_id
     )
@@ -393,8 +389,6 @@ def get_fast_api_app(
       session_id: str,
       state: Optional[dict[str, Any]] = None,
   ) -> Session:
-    # Connect to managed session if agent_engine_id is set.
-    app_name = agent_engine_id if agent_engine_id else app_name
     if (
         await session_service.get_session(
             app_name=app_name, user_id=user_id, session_id=session_id
@@ -419,8 +413,6 @@ def get_fast_api_app(
       user_id: str,
       state: Optional[dict[str, Any]] = None,
   ) -> Session:
-    # Connect to managed session if agent_engine_id is set.
-    app_name = agent_engine_id if agent_engine_id else app_name
     logger.info("New session created")
     return await session_service.create_session(
         app_name=app_name, user_id=user_id, state=state
@@ -660,8 +652,6 @@ def get_fast_api_app(
 
   @app.delete("/apps/{app_name}/users/{user_id}/sessions/{session_id}")
   async def delete_session(app_name: str, user_id: str, session_id: str):
-    # Connect to managed session if agent_engine_id is set.
-    app_name = agent_engine_id if agent_engine_id else app_name
     await session_service.delete_session(
         app_name=app_name, user_id=user_id, session_id=session_id
     )
@@ -677,7 +667,6 @@ def get_fast_api_app(
       artifact_name: str,
       version: Optional[int] = Query(None),
   ) -> Optional[types.Part]:
-    app_name = agent_engine_id if agent_engine_id else app_name
     artifact = await artifact_service.load_artifact(
         app_name=app_name,
         user_id=user_id,
@@ -700,7 +689,6 @@ def get_fast_api_app(
       artifact_name: str,
       version_id: int,
   ) -> Optional[types.Part]:
-    app_name = agent_engine_id if agent_engine_id else app_name
     artifact = await artifact_service.load_artifact(
         app_name=app_name,
         user_id=user_id,
@@ -719,7 +707,6 @@ def get_fast_api_app(
   async def list_artifact_names(
       app_name: str, user_id: str, session_id: str
   ) -> list[str]:
-    app_name = agent_engine_id if agent_engine_id else app_name
     return await artifact_service.list_artifact_keys(
         app_name=app_name, user_id=user_id, session_id=session_id
     )
@@ -731,7 +718,6 @@ def get_fast_api_app(
   async def list_artifact_versions(
       app_name: str, user_id: str, session_id: str, artifact_name: str
   ) -> list[int]:
-    app_name = agent_engine_id if agent_engine_id else app_name
     return await artifact_service.list_versions(
         app_name=app_name,
         user_id=user_id,
@@ -745,7 +731,6 @@ def get_fast_api_app(
   async def delete_artifact(
       app_name: str, user_id: str, session_id: str, artifact_name: str
   ):
-    app_name = agent_engine_id if agent_engine_id else app_name
     await artifact_service.delete_artifact(
         app_name=app_name,
         user_id=user_id,
@@ -755,10 +740,8 @@ def get_fast_api_app(
 
   @app.post("/run", response_model_exclude_none=True)
   async def agent_run(req: AgentRunRequest) -> list[Event]:
-    # Connect to managed session if agent_engine_id is set.
-    app_name = agent_engine_id if agent_engine_id else req.app_name
     session = await session_service.get_session(
-        app_name=app_name, user_id=req.user_id, session_id=req.session_id
+        app_name=req.app_name, user_id=req.user_id, session_id=req.session_id
     )
     if not session:
       raise HTTPException(status_code=404, detail="Session not found")
@@ -776,11 +759,9 @@ def get_fast_api_app(
 
   @app.post("/run_sse")
   async def agent_run_sse(req: AgentRunRequest) -> StreamingResponse:
-    # Connect to managed session if agent_engine_id is set.
-    app_name = agent_engine_id if agent_engine_id else req.app_name
     # SSE endpoint
     session = await session_service.get_session(
-        app_name=app_name, user_id=req.user_id, session_id=req.session_id
+        app_name=req.app_name, user_id=req.user_id, session_id=req.session_id
     )
     if not session:
       raise HTTPException(status_code=404, detail="Session not found")
@@ -818,8 +799,6 @@ def get_fast_api_app(
   async def get_event_graph(
       app_name: str, user_id: str, session_id: str, event_id: str
   ):
-    # Connect to managed session if agent_engine_id is set.
-    app_name = agent_engine_id if agent_engine_id else app_name
     session = await session_service.get_session(
         app_name=app_name, user_id=user_id, session_id=session_id
     )
@@ -875,8 +854,6 @@ def get_fast_api_app(
   ) -> None:
     await websocket.accept()
 
-    # Connect to managed session if agent_engine_id is set.
-    app_name = agent_engine_id if agent_engine_id else app_name
     session = await session_service.get_session(
         app_name=app_name, user_id=user_id, session_id=session_id
     )
@@ -940,7 +917,7 @@ def get_fast_api_app(
       return runner_dict[app_name]
     root_agent = agent_loader.load_agent(app_name)
     runner = Runner(
-        app_name=agent_engine_id if agent_engine_id else app_name,
+        app_name=app_name,
         agent=root_agent,
         artifact_service=artifact_service,
         session_service=session_service,
diff --git a/src/google/adk/sessions/vertex_ai_session_service.py b/src/google/adk/sessions/vertex_ai_session_service.py
index 5d6bed2e..258dcd93 100644
--- a/src/google/adk/sessions/vertex_ai_session_service.py
+++ b/src/google/adk/sessions/vertex_ai_session_service.py
@@ -22,7 +22,6 @@ from typing import Optional
 import urllib.parse
 
 from dateutil import parser
-from google.genai import types
 from typing_extensions import override
 
 from google import genai
@@ -40,15 +39,27 @@ logger = logging.getLogger('google_adk.' + __name__)
 
 
 class VertexAiSessionService(BaseSessionService):
-  """Connects to the managed Vertex AI Session Service."""
+  """Connects to the Vertex AI Agent Engine Session Service using GenAI API client.
+
+  https://cloud.google.com/vertex-ai/generative-ai/docs/agent-engine/sessions/overview
+  """
 
   def __init__(
       self,
-      project: str = None,
-      location: str = None,
+      project: Optional[str] = None,
+      location: Optional[str] = None,
+      agent_engine_id: Optional[str] = None,
   ):
-    self.project = project
-    self.location = location
+    """Initializes the VertexAiSessionService.
+
+    Args:
+      project: The project id of the project to use.
+      location: The location of the project to use.
+      agent_engine_id: The resource ID of the agent engine to use.
+    """
+    self._project = project
+    self._location = location
+    self._agent_engine_id = agent_engine_id
 
   @override
   async def create_session(
@@ -64,14 +75,13 @@ class VertexAiSessionService(BaseSessionService):
           'User-provided Session id is not supported for'
           ' VertexAISessionService.'
       )
-
-    reasoning_engine_id = _parse_reasoning_engine_id(app_name)
+    reasoning_engine_id = self._get_reasoning_engine_id(app_name)
+    api_client = self._get_api_client()
 
     session_json_dict = {'user_id': user_id}
     if state:
       session_json_dict['session_state'] = state
 
-    api_client = _get_api_client(self.project, self.location)
     api_response = await api_client.async_request(
         http_method='POST',
         path=f'reasoningEngines/{reasoning_engine_id}/sessions',
@@ -130,10 +140,10 @@ class VertexAiSessionService(BaseSessionService):
       session_id: str,
       config: Optional[GetSessionConfig] = None,
   ) -> Optional[Session]:
-    reasoning_engine_id = _parse_reasoning_engine_id(app_name)
+    reasoning_engine_id = self._get_reasoning_engine_id(app_name)
+    api_client = self._get_api_client()
 
     # Get session resource
-    api_client = _get_api_client(self.project, self.location)
     get_session_api_response = await api_client.async_request(
         http_method='GET',
         path=f'reasoningEngines/{reasoning_engine_id}/sessions/{session_id}',
@@ -203,14 +213,14 @@ class VertexAiSessionService(BaseSessionService):
   async def list_sessions(
       self, *, app_name: str, user_id: str
   ) -> ListSessionsResponse:
-    reasoning_engine_id = _parse_reasoning_engine_id(app_name)
+    reasoning_engine_id = self._get_reasoning_engine_id(app_name)
+    api_client = self._get_api_client()
 
     path = f'reasoningEngines/{reasoning_engine_id}/sessions'
     if user_id:
       parsed_user_id = urllib.parse.quote(f'''"{user_id}"''', safe='')
       path = path + f'?filter=user_id={parsed_user_id}'
 
-    api_client = _get_api_client(self.project, self.location)
     api_response = await api_client.async_request(
         http_method='GET',
         path=path,
@@ -236,8 +246,9 @@ class VertexAiSessionService(BaseSessionService):
   async def delete_session(
       self, *, app_name: str, user_id: str, session_id: str
   ) -> None:
-    reasoning_engine_id = _parse_reasoning_engine_id(app_name)
-    api_client = _get_api_client(self.project, self.location)
+    reasoning_engine_id = self._get_reasoning_engine_id(app_name)
+    api_client = self._get_api_client()
+
     try:
       await api_client.async_request(
           http_method='DELETE',
@@ -253,8 +264,8 @@ class VertexAiSessionService(BaseSessionService):
     # Update the in-memory session.
     await super().append_event(session=session, event=event)
 
-    reasoning_engine_id = _parse_reasoning_engine_id(session.app_name)
-    api_client = _get_api_client(self.project, self.location)
+    reasoning_engine_id = self._get_reasoning_engine_id(session.app_name)
+    api_client = self._get_api_client()
     await api_client.async_request(
         http_method='POST',
         path=f'reasoningEngines/{reasoning_engine_id}/sessions/{session.id}:appendEvent',
@@ -262,15 +273,34 @@ class VertexAiSessionService(BaseSessionService):
     )
     return event
 
+  def _get_reasoning_engine_id(self, app_name: str):
+    if self._agent_engine_id:
+      return self._agent_engine_id
 
-def _get_api_client(project: str, location: str):
-  """Instantiates an API client for the given project and location.
+    if app_name.isdigit():
+      return app_name
 
-  It needs to be instantiated inside each request so that the event loop
-  management.
-  """
-  client = genai.Client(vertexai=True, project=project, location=location)
-  return client._api_client
+    pattern = r'^projects/([a-zA-Z0-9-_]+)/locations/([a-zA-Z0-9-_]+)/reasoningEngines/(\d+)$'
+    match = re.fullmatch(pattern, app_name)
+
+    if not bool(match):
+      raise ValueError(
+          f'App name {app_name} is not valid. It should either be the full'
+          ' ReasoningEngine resource name, or the reasoning engine id.'
+      )
+
+    return match.groups()[-1]
+
+  def _get_api_client(self):
+    """Instantiates an API client for the given project and location.
+
+    It needs to be instantiated inside each request so that the event loop
+    management can be properly propagated.
+    """
+    client = genai.Client(
+        vertexai=True, project=self._project, location=self._location
+    )
+    return client._api_client
 
 
 def _convert_event_to_json(event: Event) -> Dict[str, Any]:
@@ -366,19 +396,3 @@ def _from_api_event(api_event: Dict[str, Any]) -> Event:
     )
 
   return event
-
-
-def _parse_reasoning_engine_id(app_name: str):
-  if app_name.isdigit():
-    return app_name
-
-  pattern = r'^projects/([a-zA-Z0-9-_]+)/locations/([a-zA-Z0-9-_]+)/reasoningEngines/(\d+)$'
-  match = re.fullmatch(pattern, app_name)
-
-  if not bool(match):
-    raise ValueError(
-        f'App name {app_name} is not valid. It should either be the full'
-        ' ReasoningEngine resource name, or the reasoning engine id.'
-    )
-
-  return match.groups()[-1]
diff --git a/tests/unittests/sessions/test_vertex_ai_session_service.py b/tests/unittests/sessions/test_vertex_ai_session_service.py
index 92f6a29d..6a9e0b46 100644
--- a/tests/unittests/sessions/test_vertex_ai_session_service.py
+++ b/tests/unittests/sessions/test_vertex_ai_session_service.py
@@ -245,8 +245,14 @@ class MockApiClient:
       raise ValueError(f'Unsupported http method: {http_method}')
 
 
-def mock_vertex_ai_session_service():
+def mock_vertex_ai_session_service(agent_engine_id: Optional[str] = None):
   """Creates a mock Vertex AI Session service for testing."""
+  if agent_engine_id:
+    return VertexAiSessionService(
+        project='test-project',
+        location='test-location',
+        agent_engine_id=agent_engine_id,
+    )
   return VertexAiSessionService(
       project='test-project', location='test-location'
   )
@@ -265,7 +271,7 @@ def mock_get_api_client():
       '2': (MOCK_EVENT_JSON_2, 'my_token'),
   }
   with mock.patch(
-      'google.adk.sessions.vertex_ai_session_service._get_api_client',
+      'google.adk.sessions.vertex_ai_session_service.VertexAiSessionService._get_api_client',
       return_value=api_client,
   ):
     yield
@@ -273,8 +279,12 @@ def mock_get_api_client():
 
 @pytest.mark.asyncio
 @pytest.mark.usefixtures('mock_get_api_client')
-async def test_get_empty_session():
-  session_service = mock_vertex_ai_session_service()
+@pytest.mark.parametrize('agent_engine_id', [None, '123'])
+async def test_get_empty_session(agent_engine_id):
+  if agent_engine_id:
+    session_service = mock_vertex_ai_session_service(agent_engine_id)
+  else:
+    session_service = mock_vertex_ai_session_service()
   with pytest.raises(ValueError) as excinfo:
     await session_service.get_session(
         app_name='123', user_id='user', session_id='0'