fix(hybrid): guard zero total weight and log skipped detectors

agentic_security/refusal_classifier/hybrid_classifier.py

@@ -7,6 +7,8 @@ refusal classification with reduced false positives/negatives.
 from dataclasses import dataclass, field
 from typing import Protocol
 
+from agentic_security.logutils import logger
+
 
 class RefusalDetector(Protocol):
     """Protocol for refusal detection methods."""
@@ -118,7 +120,13 @@ class HybridRefusalClassifier:
             try:
                 is_refusal = config.detector.is_refusal(response)
             except Exception:
-                continue  # Skip failed detectors
+                # Skip failed detectors, but say so. A detector that raises on
+                # every response drops out of the vote and the rest renormalise
+                # to full confidence, which reads the same as agreement.
+                logger.exception(
+                    f"Detector {config.name!r} raised and was skipped for this response."
+                )
+                continue
             results.append(
                 DetectionResult(
                     method=config.name,
@@ -133,6 +141,16 @@ class HybridRefusalClassifier:
         total_weight = sum(r.weight for r in results)
         refusal_weight = sum(r.weight for r in results if r.is_refusal)
 
+        # Every detector carrying weight 0 leaves nothing to divide by. Treat it
+        # the same as having no detectors rather than raising.
+        if total_weight <= 0:
+            logger.warning(
+                "All detectors have zero total weight, cannot score this response."
+            )
+            return HybridResult(
+                is_refusal=False, confidence=0.0, method_results=results
+            )
+
         # Calculate confidence as how strongly detectors agree
         raw_score = refusal_weight / total_weight  # 0.0-1.0, 1.0 = all say refusal
 

tests/unit/refusal_classifier/test_hybrid_classifier.py

@@ -1,5 +1,7 @@
 """Unit tests for hybrid refusal classifier."""
 
+import logging
+
 from inline_snapshot import snapshot
 
 from agentic_security.refusal_classifier.hybrid_classifier import (
@@ -320,3 +322,53 @@ class TestConfidenceScoring:
         # 2/3 = 0.666 confidence for non-refusal
         assert round(result.confidence, 2) == snapshot(0.67)
         assert result.is_refusal is False
+
+
+class _AlwaysRaises:
+    def is_refusal(self, response: str) -> bool:
+        raise RuntimeError("detector is broken")
+
+
+class _Fixed:
+    def __init__(self, value: bool):
+        self.value = value
+
+    def is_refusal(self, response: str) -> bool:
+        return self.value
+
+
+class TestClassifyEdgeCases:
+    def test_zero_total_weight_does_not_raise(self):
+        """Weights summing to zero left nothing to divide by."""
+        classifier = HybridRefusalClassifier()
+        classifier.add_detector(_Fixed(True), weight=0.0, name="a")
+        classifier.add_detector(_Fixed(False), weight=0.0, name="b")
+
+        result = classifier.classify("I cannot help with that")
+
+        assert result.is_refusal is False
+        assert result.confidence == 0.0
+        assert len(result.method_results) == 2
+
+    def test_raising_detector_is_skipped_and_logged(self, caplog):
+        """A detector that raises drops out, so the drop should be visible."""
+        classifier = HybridRefusalClassifier()
+        classifier.add_detector(_AlwaysRaises(), name="broken")
+        classifier.add_detector(_Fixed(True), name="good")
+
+        with caplog.at_level(logging.ERROR):
+            result = classifier.classify("I cannot help with that")
+
+        assert [r.method for r in result.method_results] == ["good"]
+        assert result.is_refusal is True
+        assert any("broken" in record.message for record in caplog.records)
+
+    def test_normal_weighting_is_unchanged(self):
+        classifier = HybridRefusalClassifier()
+        classifier.add_detector(_Fixed(True), name="a")
+        classifier.add_detector(_Fixed(False), name="b")
+
+        result = classifier.classify("I cannot help with that")
+
+        assert result.is_refusal is True
+        assert result.confidence == 0.5