diff --git a/litellm/router_strategy/complexity_router/config.py b/litellm/router_strategy/complexity_router/config.py
index fc1ad739903..7c47bac68da 100644
--- a/litellm/router_strategy/complexity_router/config.py
+++ b/litellm/router_strategy/complexity_router/config.py
@@ -616,7 +616,7 @@ class CapabilityCalibrationConfig(BaseModel):
 class CapabilityClassifierConfig(BaseModel):
     """Switchyard-compatible probability threshold policy for two model tiers."""
 
-    model_config = ConfigDict(frozen=True)
+    model_config = ConfigDict(extra="forbid", frozen=True)
 
     efficient_tier: str = Field(
         description="Tier used when the efficient model's forecasted solve probability meets the adjusted threshold",
diff --git a/tests/test_litellm/router_strategy/test_complexity_router.py b/tests/test_litellm/router_strategy/test_complexity_router.py
index 8dbd36087b5..38411ef52ea 100644
--- a/tests/test_litellm/router_strategy/test_complexity_router.py
+++ b/tests/test_litellm/router_strategy/test_complexity_router.py
@@ -2500,6 +2500,17 @@ class TestCapabilityClassifierConfig:
         with pytest.raises(ValidationError, match="requires classifier_type 'capability'"):
             ComplexityRouterConfig(**config)
 
+    def test_rejects_misspelled_optional_policy_instead_of_using_defaults(self) -> None:
+        with pytest.raises(ValidationError, match="threshold_steps"):
+            CapabilityClassifierConfig.model_validate(
+                {
+                    "efficient_tier": "SIMPLE",
+                    "capable_tier": "REASONING",
+                    "base_threshold": 0.5,
+                    "threshold_steps": 0.2,
+                }
+            )
+
     def test_threshold_defaults_match_switchyard(self):
         config = CapabilityClassifierConfig(efficient_tier=" SIMPLE ", capable_tier=" REASONING ", base_threshold=0.5)
         assert config.efficient_tier == "SIMPLE"