diff --git a/src/google/adk/cli/fast_api.py b/src/google/adk/cli/fast_api.py
index 7d93b54360..ab103edacf 100644
--- a/src/google/adk/cli/fast_api.py
+++ b/src/google/adk/cli/fast_api.py
@@ -375,7 +375,13 @@ async def _get_a2a_runner_async() -> Runner:
           )
 
           for new_route in routes:
-            app.router.routes.append(new_route)
+            app.add_api_route(
+                new_route.path,
+                new_route.endpoint,
+                name=new_route.name,
+                methods=new_route.methods,
+                tags=["A2A Endpoints"],
+            )
 
           logger.info("Successfully configured A2A agent: %s", app_name)