diff --git a/src/google/adk/runners.py b/src/google/adk/runners.py
index 9802801dca..886629e01d 100644
--- a/src/google/adk/runners.py
+++ b/src/google/adk/runners.py
@@ -177,7 +177,7 @@ def __init__(
       *,
       app: Optional[App] = None,
       app_name: Optional[str] = None,
-      agent: Optional[BaseAgent] = None,
+      agent: Optional[BaseAgent | 'BaseNode'] = None,
       node: Any = None,
       plugins: Optional[List[BasePlugin]] = None,
       artifact_service: Optional[BaseArtifactService] = None,
diff --git a/tests/unittests/test_runners.py b/tests/unittests/test_runners.py
index 925769951d..5ad81ab3b2 100644
--- a/tests/unittests/test_runners.py
+++ b/tests/unittests/test_runners.py
@@ -1409,6 +1409,22 @@ def test_resolve_app_with_agent_wraps_in_app(self):
     assert runner.app_name == "test_app"
     assert runner.agent is self.root_agent
 
+  def test_resolve_app_with_base_node_via_agent_param(self):
+    """Test that a BaseNode root passed via `agent` is accepted."""
+    from google.adk.workflow._base_node import BaseNode
+
+    node = BaseNode(name="test_node")
+    runner = Runner(
+        app_name="test_app",
+        agent=node,
+        session_service=self.session_service,
+        artifact_service=self.artifact_service,
+    )
+    assert runner.app is not None
+    assert runner.app.root_agent is node
+    assert runner.app_name == "test_app"
+    assert runner.agent is node
+
   def test_resolve_app_with_node_wraps_in_app(self):
     """Test that a bare node is wrapped into an App."""
     from google.adk.workflow._base_node import BaseNode