diff --git a/src/google/adk/evaluation/local_eval_service.py b/src/google/adk/evaluation/local_eval_service.py
index f443bb70..fa50f70d 100644
--- a/src/google/adk/evaluation/local_eval_service.py
+++ b/src/google/adk/evaluation/local_eval_service.py
@@ -68,14 +68,19 @@ class LocalEvalService(BaseEvalService):
       self,
       root_agent: BaseAgent,
       eval_sets_manager: EvalSetsManager,
-      metric_evaluator_registry: MetricEvaluatorRegistry = DEFAULT_METRIC_EVALUATOR_REGISTRY,
-      session_service: BaseSessionService = InMemorySessionService(),
-      artifact_service: BaseArtifactService = InMemoryArtifactService(),
+      metric_evaluator_registry: Optional[MetricEvaluatorRegistry] = None,
+      session_service: Optional[BaseSessionService] = None,
+      artifact_service: Optional[BaseArtifactService] = None,
       eval_set_results_manager: Optional[EvalSetResultsManager] = None,
       session_id_supplier: Callable[[], str] = _get_session_id,
   ):
     self._root_agent = root_agent
     self._eval_sets_manager = eval_sets_manager
+    metric_evaluator_registry = (
+        metric_evaluator_registry or DEFAULT_METRIC_EVALUATOR_REGISTRY
+    )
+    session_service = session_service or InMemorySessionService()
+    artifact_service = artifact_service or InMemoryArtifactService()
     self._metric_evaluator_registry = metric_evaluator_registry
     self._session_service = session_service
     self._artifact_service = artifact_service
diff --git a/src/google/adk/runners.py b/src/google/adk/runners.py
index f0f92047..d18127b1 100644
--- a/src/google/adk/runners.py
+++ b/src/google/adk/runners.py
@@ -188,7 +188,7 @@ class Runner:
       user_id: str,
       session_id: str,
       new_message: types.Content,
-      run_config: RunConfig = RunConfig(),
+      run_config: Optional[RunConfig] = None,
   ) -> Generator[Event, None, None]:
     """Runs the agent.
 
@@ -205,6 +205,7 @@ class Runner:
     Yields:
       The events generated by the agent.
     """
+    run_config = run_config or RunConfig()
     event_queue = queue.Queue()
 
     async def _invoke_run_async():
@@ -248,7 +249,7 @@ class Runner:
       session_id: str,
       new_message: types.Content,
       state_delta: Optional[dict[str, Any]] = None,
-      run_config: RunConfig = RunConfig(),
+      run_config: Optional[RunConfig] = None,
   ) -> AsyncGenerator[Event, None]:
     """Main entry method to run the agent in this runner.
 
@@ -261,6 +262,7 @@ class Runner:
     Yields:
       The events generated by the agent.
     """
+    run_config = run_config or RunConfig()
 
     async def _run_with_trace(
         new_message: types.Content,
@@ -426,7 +428,7 @@ class Runner:
       user_id: Optional[str] = None,
       session_id: Optional[str] = None,
       live_request_queue: LiveRequestQueue,
-      run_config: RunConfig = RunConfig(),
+      run_config: Optional[RunConfig] = None,
       session: Optional[Session] = None,
   ) -> AsyncGenerator[Event, None]:
     """Runs the agent in live mode (experimental feature).
@@ -452,6 +454,7 @@ class Runner:
     .. NOTE::
         Either `session` or both `user_id` and `session_id` must be provided.
     """
+    run_config = run_config or RunConfig()
     if session is None and (user_id is None or session_id is None):
       raise ValueError(
           'Either session or user_id and session_id must be provided.'
@@ -601,7 +604,7 @@ class Runner:
       *,
       new_message: Optional[types.Content] = None,
       live_request_queue: Optional[LiveRequestQueue] = None,
-      run_config: RunConfig = RunConfig(),
+      run_config: Optional[RunConfig] = None,
   ) -> InvocationContext:
     """Creates a new invocation context.
 
@@ -614,6 +617,7 @@ class Runner:
     Returns:
         The new invocation context.
     """
+    run_config = run_config or RunConfig()
     invocation_id = new_invocation_context_id()
 
     if run_config.support_cfc and isinstance(self.agent, LlmAgent):
@@ -645,9 +649,10 @@ class Runner:
       session: Session,
       *,
       live_request_queue: Optional[LiveRequestQueue] = None,
-      run_config: RunConfig = RunConfig(),
+      run_config: Optional[RunConfig] = None,
   ) -> InvocationContext:
     """Creates a new invocation context for live multi-agent."""
+    run_config = run_config or RunConfig()
 
     # For live multi-agent, we need model's text transcription as context for
     # next agent.
diff --git a/tests/unittests/cli/test_fast_api.py b/tests/unittests/cli/test_fast_api.py
index b05bf3c4..d0d84cac 100755
--- a/tests/unittests/cli/test_fast_api.py
+++ b/tests/unittests/cli/test_fast_api.py
@@ -21,6 +21,7 @@ import sys
 import tempfile
 import time
 from typing import Any
+from typing import Optional
 from unittest.mock import MagicMock
 from unittest.mock import patch
 
@@ -120,8 +121,9 @@ async def dummy_run_async(
     session_id,
     new_message,
     state_delta=None,
-    run_config: RunConfig = RunConfig(),
+    run_config: Optional[RunConfig] = None,
 ):
+  run_config = run_config or RunConfig()
   yield _event_1()
   await asyncio.sleep(0)