From 9879baeabf707c0a9cefd553c207aec8db77674e Mon Sep 17 00:00:00 2001
From: Tin Chi Lo <tin@berri.ai>
Date: Thu, 30 Jul 2026 23:41:42 -0700
Subject: [PATCH] fix(complexity_router): stop default_model rescuing an
 unservable default_tier under plugins

The validator accepted default_model as evidence that an explicitly configured
default_tier could be served. The plugin path never consults it: candidates come from
the tier pool alone, and a pool that is empty raises rather than falling through to a
model the plugins never vetted. So a config with plugins, an explicit default_tier
absent from tiers, and a default_model passed validation and then failed on its first
no-signal request, which is exactly what validating at load is meant to prevent

default_model now counts only when no plugins are configured, and the error names which
of the two remedies applies
---
 .../complexity_router/config.py               | 24 +++++++++++++++----
 .../router_strategy/test_complexity_router.py | 21 ++++++++++++++++
 2 files changed, 40 insertions(+), 5 deletions(-)

diff --git a/litellm/router_strategy/complexity_router/config.py b/litellm/router_strategy/complexity_router/config.py
index 6064cbb0ce8..b1eb424b2d4 100644
--- a/litellm/router_strategy/complexity_router/config.py
+++ b/litellm/router_strategy/complexity_router/config.py
@@ -476,14 +476,28 @@ class ComplexityRouterConfig(BaseModel):
     def _validate_default_tier_is_servable(self) -> "ComplexityRouterConfig":
         if "default_tier" not in self.model_fields_set:
             return self
-        if self.tiers.get(self.default_tier.value) or self.default_model:
+        if self.tiers.get(self.default_tier.value):
             return self
+        # default_model rescues this only without plugins. The plugin path never falls back
+        # to it, since a model the plugins did not vet must not serve, so accepting it here
+        # would validate a config whose every no-signal request fails at routing time.
+        if self.default_model and not self.plugins:
+            return self
+        remedy = (
+            "Add it to tiers, or name a tier that is configured"
+            if self.plugins
+            else "Add it to tiers, name a tier that is configured, or set default_model in complexity_router_config"
+        )
+        because = (
+            "routing plugins are configured, so default_model is not consulted: a model the plugins "
+            "never vetted must not serve"
+            if self.plugins
+            else "the deployment-level complexity_router_default_model does not count here, because "
+            "falling through to it would serve every no-signal request from a model this tier never names"
+        )
         raise ValueError(
             f"default_tier {self.default_tier.value} is not a non-empty entry in tiers "
-            f"({sorted(self.tiers)}). Add it to tiers, name a tier that is configured, or set "
-            f"default_model in complexity_router_config; the deployment-level "
-            f"complexity_router_default_model does not count here, because falling through to it "
-            f"would serve every no-signal request from a model this tier never names"
+            f"({sorted(self.tiers)}). {remedy}; {because}"
         )
 
     @model_validator(mode="after")
diff --git a/tests/test_litellm/router_strategy/test_complexity_router.py b/tests/test_litellm/router_strategy/test_complexity_router.py
index 6c01bf88ded..d37eb5bdeb7 100644
--- a/tests/test_litellm/router_strategy/test_complexity_router.py
+++ b/tests/test_litellm/router_strategy/test_complexity_router.py
@@ -4893,6 +4893,27 @@ class TestNoSignalDefaultTier:
                 },
             )
 
+    def test_default_model_does_not_rescue_the_default_tier_when_plugins_are_configured(self, mock_router_instance):
+        """The plugin path builds candidates from the tier pool alone and never consults
+        default_model, since a model the plugins did not vet must not serve. Accepting
+        default_model here would validate a config whose every no-signal request raises."""
+
+        class NoOpPlugin:
+            async def run(self, context):
+                return context
+
+        with pytest.raises(ValidationError, match="routing plugins are configured"):
+            ComplexityRouter(
+                model_name="test-router",
+                litellm_router_instance=mock_router_instance,
+                complexity_router_config={
+                    "tiers": {"SIMPLE": "simple-model", "MEDIUM": "medium-model"},
+                    "default_tier": "COMPLEX",
+                    "default_model": "fallback-model",
+                    "plugins": [NoOpPlugin()],
+                },
+            )
+
     def test_default_tier_outside_tiers_is_allowed_with_default_model(self, mock_router_instance):
         router = ComplexityRouter(
             model_name="test-router",