diff --git a/src/google/adk/cli/fast_api.py b/src/google/adk/cli/fast_api.py
index 8f78c15f9b..0ff5e9c20b 100644
--- a/src/google/adk/cli/fast_api.py
+++ b/src/google/adk/cli/fast_api.py
@@ -38,6 +38,9 @@
 from ..auth.credential_service.in_memory_credential_service import InMemoryCredentialService
 from ..evaluation.local_eval_set_results_manager import LocalEvalSetResultsManager
 from ..evaluation.local_eval_sets_manager import LocalEvalSetsManager
+from ..memory.base_memory_service import BaseMemoryService
+from ..memory.in_memory_memory_service import InMemoryMemoryService
+from ..memory.vertex_ai_memory_bank_service import VertexAiMemoryBankService
 from ..runners import Runner
 from .adk_web_server import AdkWebServer
 from .service_registry import load_services_module
@@ -79,6 +82,7 @@ def get_fast_api_app(
     artifact_service_uri: Optional[str] = None,
     memory_service_uri: Optional[str] = None,
     use_local_storage: bool = True,
+    memory_service: Optional[BaseMemoryService] = None,
     eval_storage_uri: Optional[str] = None,
     allow_origins: Optional[list[str]] = None,
     web: bool,
@@ -161,13 +165,13 @@ def get_fast_api_app(
   load_services_module(agents_dir)
 
   # Build the Memory service
-  try:
+  if memory_service:
+    pass
+  else:
     memory_service = create_memory_service_from_options(
         base_dir=agents_dir,
         memory_service_uri=memory_service_uri,
     )
-  except ValueError as exc:
-    raise click.ClickException(str(exc)) from exc
 
   # Build the Session service
   session_service = create_session_service_from_options(
diff --git a/tests/unittests/cli/test_fast_api.py b/tests/unittests/cli/test_fast_api.py
index d6ccf6e218..a96fee8457 100755
--- a/tests/unittests/cli/test_fast_api.py
+++ b/tests/unittests/cli/test_fast_api.py
@@ -1717,5 +1717,75 @@ async def run_async_session_not_found(self, **kwargs):
   assert "Session not found" in response.json()["detail"]
 
 
+def test_get_fast_api_app_with_custom_memory_service(
+    mock_session_service,
+    mock_artifact_service,
+    mock_agent_loader,
+    mock_eval_sets_manager,
+    mock_eval_set_results_manager,
+):
+  """Test that custom memory_service is used directly when provided."""
+  custom_memory_service = MagicMock()
+
+  with (
+      patch.object(signal, "signal", autospec=True, return_value=None),
+      patch.object(
+          fast_api_module,
+          "create_session_service_from_options",
+          autospec=True,
+          return_value=mock_session_service,
+      ),
+      patch.object(
+          fast_api_module,
+          "create_artifact_service_from_options",
+          autospec=True,
+          return_value=mock_artifact_service,
+      ),
+      patch.object(
+          fast_api_module,
+          "create_memory_service_from_options",
+          autospec=True,
+      ) as mock_create_memory_service,
+      patch.object(
+          fast_api_module,
+          "AgentLoader",
+          autospec=True,
+          return_value=mock_agent_loader,
+      ),
+      patch.object(
+          fast_api_module,
+          "LocalEvalSetsManager",
+          autospec=True,
+          return_value=mock_eval_sets_manager,
+      ),
+      patch.object(
+          fast_api_module,
+          "LocalEvalSetResultsManager",
+          autospec=True,
+          return_value=mock_eval_set_results_manager,
+      ),
+      patch.object(
+          fast_api_module,
+          "load_services_module",
+          autospec=True,
+          return_value=None,
+      ),
+  ):
+    app = get_fast_api_app(
+        agents_dir=".",
+        web=True,
+        session_service_uri="",
+        artifact_service_uri="",
+        memory_service_uri="",
+        memory_service=custom_memory_service,
+        allow_origins=["*"],
+        a2a=False,
+        host="127.0.0.1",
+        port=8000,
+    )
+
+    mock_create_memory_service.assert_not_called()
+
+
 if __name__ == "__main__":
   pytest.main(["-xvs", __file__])