diff --git a/litellm/router_strategy/complexity_router/config.py b/litellm/router_strategy/complexity_router/config.py
index d5870bb6510..c818ab85cb9 100644
--- a/litellm/router_strategy/complexity_router/config.py
+++ b/litellm/router_strategy/complexity_router/config.py
@@ -487,8 +487,8 @@ DEFAULT_TOKEN_THRESHOLDS: Final[dict[str, int]] = {
 DEFAULT_TIER_MODELS: Final[dict[str, str]] = {
     "SIMPLE": "gpt-4o-mini",
     "MEDIUM": "gpt-4o",
-    "COMPLEX": "claude-sonnet-4-20250514",
-    "REASONING": "claude-sonnet-4-20250514",
+    "COMPLEX": "claude-sonnet-5-5",
+    "REASONING": "claude-sonnet-5-5",
 }
 
 
diff --git a/tests/unit/router_strategy/test_complexity_router.py b/tests/unit/router_strategy/test_complexity_router.py
index ef7b9e485d4..65dbb81f0af 100644
--- a/tests/unit/router_strategy/test_complexity_router.py
+++ b/tests/unit/router_strategy/test_complexity_router.py
@@ -285,6 +285,13 @@ class TestComplexityRouterInit:
         assert router.config.tiers == DEFAULT_COMPLEXITY_CONFIG.tiers
         assert router.config is not DEFAULT_COMPLEXITY_CONFIG  # Not a singleton
 
+    @pytest.mark.usefixtures("local_model_cost_map")
+    def test_default_tiers_route_to_models_litellm_knows(self):
+        unknown: Final = {
+            tier: model for tier, model in DEFAULT_COMPLEXITY_CONFIG.tiers.items() if model not in litellm.model_cost
+        }
+        assert unknown == {}
+
     def test_init_with_default_model(self, mock_router_instance, basic_config):
         """Test initialization with default_model override."""
         router = ComplexityRouter(