fix(complexity_router): resolve custom-tier classifier replies case-insensitively

An exact membership check meant a reply like "research" or " RESEARCH " for a
defined RESEARCH tier was treated as unknown and rerouted to the fallback tier,
while the built-in path already resolves labels case-insensitively. Custom tier
names are validated unique case-insensitively at config write, so folding is
unambiguous; the decision records the canonical configured name, never the
model's spelling.

litellm/router_strategy/complexity_router/complexity_router.py

@@ -999,9 +999,11 @@ class ComplexityRouter(CustomLogger):
             raise ValueError("LLM classifier returned empty content")
         raw_tier: Final = _TierReply.model_validate_json(content).tier
         if self.config.has_custom_tiers:
-            if raw_tier not in self.config.tier_names():
+            folded: Final = raw_tier.strip().casefold()
+            matched: Final = next((name for name in self.config.tier_names() if name.casefold() == folded), None)
+            if matched is None:
                 raise ValueError(f"LLM classifier returned an unknown tier: {raw_tier!r}")
-            return raw_tier
+            return matched
         tier: Final = self.config.tier_for_label(raw_tier)
         if tier is None:
             raise ValueError(f"LLM classifier returned an unrecognized tier: {raw_tier!r}")

tests/test_litellm/router_strategy/test_complexity_router.py

@@ -5940,6 +5940,31 @@ class TestTierDefinitionsClassifier:
         assert "score" not in result.routing_decision
         assert "tier_boundaries" not in result.routing_decision
 
+    @pytest.mark.asyncio
+    @pytest.mark.parametrize(
+        "reply",
+        [
+            pytest.param("research", id="different-case"),
+            pytest.param(" RESEARCH ", id="surrounding-whitespace"),
+        ],
+    )
+    async def test_custom_tier_reply_resolves_case_insensitively_to_the_canonical_name(
+        self, custom_tier_router, mock_router_instance, reply
+    ):
+        """A model that answers in the wrong case or with stray whitespace still names a
+        defined tier, matching the built-in path's label resolution; rejecting it would
+        silently reroute the request to the fallback tier. The decision records the
+        canonical configured name, never the model's spelling."""
+        mock_router_instance.acompletion = AsyncMock(return_value=_llm_response(f'{{"tier": "{reply}"}}'))
+        result = await custom_tier_router.async_pre_routing_hook(
+            model="custom-tier-router",
+            request_kwargs={},
+            messages=[{"role": "user", "content": "compare these two consensus protocols"}],
+        )
+        assert result.model == "deep-model"
+        assert result.routing_decision["tier"] == "RESEARCH"
+        assert result.routing_decision["cause"] == "llm_classifier"
+
     @pytest.mark.asyncio
     async def test_classifier_failure_routes_to_the_fallback_tier(self, custom_tier_router, mock_router_instance):
         """The heuristic scorer cannot produce a custom tier, so it must never be the