fix(router): honor reasoning_override_min_score on heuristic_v2

classifier_type=heuristic_v2 routed every request through the
TierSuccessPredictor, which reads outcome statistics and never the
ask's reasoning markers, so an operator who set
reasoning_override_min_score (0 = unconditional) saw 2+ exact
reasoning keywords silently ignored and the request classified by
the predictor alone. Route the ask through the shared scorer gate
first and short-circuit to REASONING when the reasoning-marker
override fires, so the documented classifier-wide contract holds on
every classification path.

Fixes #44527

Signed-off-by: apex-mochen <2756823972@qq.com>

litellm/router_strategy/complexity_router/complexity_router.py

@@ -1944,6 +1944,21 @@ class ComplexityRouter(CustomLogger):
         return await self._llm_classifier_outcome(prompt, system_prompt, request_kwargs, messages)
 
     def _classify_with_heuristic_v2(self, prompt: str) -> ClassificationOutcome:
+        # The reasoning-marker override is a classifier-wide contract, not a
+        # heuristic-scorer quirk: an operator who sets reasoning_override_min_score
+        # (0 = unconditional) expects 2+ markers to promote on every path that
+        # classifies. The success predictor reads outcome statistics, never the
+        # ask's markers, so honor the override here through the same gate the
+        # heuristic scorer uses -- otherwise the setting silently does nothing
+        # under heuristic_v2 while still being documented as classifier-wide.
+        overridden: Final = self._score_and_classify(prompt)
+        if overridden[3] == "reasoning_override":
+            return ClassificationOutcome(
+                tier=overridden[0],
+                score=overridden[1],
+                signals=overridden[2],
+                cause="reasoning_override",
+            )
         predictor: Final = self._tier_success_predictor
         if predictor is None:
             raise ValueError("heuristic v2 predictor is not configured")

tests/unit/router_strategy/test_complexity_router.py

@@ -661,6 +661,45 @@ class TestReasoningMarkerScoring:
         assert score == 0.25
         assert tier == ComplexityTier.REASONING
 
+    def test_heuristic_v2_honors_the_zero_floor_override(self, mock_router_instance, basic_config):
+        """#44527: reasoning_override_min_score=0 must promote on heuristic_v2 too.
+
+        The success predictor reads outcome statistics, never the ask's reasoning
+        markers, so before the fix a request carrying 2+ exact markers was still
+        classified by the predictor alone and the operator's explicit 0 floor was
+        silently ignored.
+        """
+        router = ComplexityRouter(
+            model_name="test-complexity-router",
+            litellm_router_instance=mock_router_instance,
+            complexity_router_config={
+                **basic_config,
+                "classifier_type": "heuristic_v2",
+                "heuristic_v2_artifact": _heuristic_v2_artifact(),
+                "reasoning_override_min_score": 0.0,
+            },
+        )
+        outcome = asyncio.run(router.aclassify("hi, step by step, pros and cons"))
+        assert outcome.cause == "reasoning_override"
+        assert outcome.tier == ComplexityTier.REASONING
+
+    def test_heuristic_v2_respects_a_floor_above_the_markers_score(self, mock_router_instance, basic_config):
+        """A floor above the markers' weighted score leaves heuristic_v2 to the predictor."""
+        router = ComplexityRouter(
+            model_name="test-complexity-router",
+            litellm_router_instance=mock_router_instance,
+            complexity_router_config={
+                **basic_config,
+                "classifier_type": "heuristic_v2",
+                "heuristic_v2_artifact": _heuristic_v2_artifact(),
+                "reasoning_override_min_score": 0.99,
+            },
+        )
+        outcome = asyncio.run(router.aclassify("hi, step by step, pros and cons"))
+        assert outcome.cause != "reasoning_override"
+        assert outcome.cause == "heuristic_v2"
+        assert outcome.tier == ComplexityTier.COMPLEX
+
     def test_system_prompt_reasoning_not_counted(self, complexity_router):
         """Reasoning markers in system prompt should not count for override."""
         user_prompt = "What is 2+2?"