fix(auth): resolve zero-cost model aliases

Alias pricing resolves through its target model group, but explicit-cost validation searched under the alias name and enforced budgets on free models.\n\nFixes #35369

litellm/proxy/auth/auth_checks.py

@@ -263,8 +263,9 @@ def _is_cost_explicitly_configured(model: str, llm_router: "Router") -> bool:
     fields. _get_model_info_helper() then defaults missing costs to 0.
     This function detects that scenario by checking the raw model_cost entry.
     """
+    resolved_model = llm_router._get_model_from_alias(model) or model
     for deployment in llm_router.model_list:
-        if deployment.get("model_name") != model:
+        if deployment.get("model_name") != resolved_model:
             continue
         model_id = deployment.get("model_info", {}).get("id")
         if model_id is None:

tests/proxy_unit_tests/test_zero_cost_model_budget_bypass.py

@@ -108,6 +108,26 @@ class TestIsModelCostZero:
         )
         assert result is True
 
+    @pytest.mark.parametrize(
+        "alias_target",
+        [
+            "on-prem-model",
+            {"model": "on-prem-model", "hidden": False},
+        ],
+    )
+    def test_zero_cost_model_group_alias(
+        self, mock_router_with_zero_cost_model, alias_target
+    ):
+        mock_router_with_zero_cost_model.model_group_alias = {
+            "free-alias": alias_target
+        }
+
+        result = _is_model_cost_zero(
+            model="free-alias", llm_router=mock_router_with_zero_cost_model
+        )
+
+        assert result is True
+
     def test_paid_model_in_router(self, mock_router_with_zero_cost_model):
         """Test that a paid model is correctly identified as non-zero cost."""
         with patch("litellm.get_model_info") as mock_get_model_info: