fixed grader

tasks/hard.py

@@ -411,15 +411,17 @@ class HardTaskGrader:
         """
         Fraction of chains that were successfully stopped (any position).
 
-        Returns 1.0 when no chains exist (nothing to detect).
+        Returns 0.99 when no chains exist (nothing to detect).
         """
         if not self._chains:
-            return 1.0
+            return 0.99
         stopped = sum(
             1 for c in self._chains.values()
             if c.completed and not c.hit_failure
         )
-        return stopped / len(self._chains)
+        raw = stopped / len(self._chains)
+        # Clamp to (0, 1)
+        return max(0.01, min(raw, 0.99))
 
     def calculate_stability_score(self) -> float:
         """Return the stability multiplier for the current failure count."""
@@ -568,11 +570,13 @@ class HardTaskGrader:
 
     @staticmethod
     def _stability_score(failures: int) -> float:
-        """Step-function stability multiplier."""
+        """Step-function stability multiplier clamped to (0, 1)."""
         for threshold, score in _STABILITY_BY_FAILURES:
             if failures <= threshold:
-                return score
-        return _STABILITY_FLOOR
+                # Clamp stability scores to strict (0, 1)
+                return max(0.01, min(score, 0.99))
+        # Return floor clamped to (0, 1)
+        return max(0.01, min(_STABILITY_FLOOR, 0.99))
 
 
 # ---------------------------------------------------------------------------

tasks/medium.py

@@ -236,14 +236,16 @@ class MediumTaskGrader:
         Fraction of INVESTIGATE + ESCALATE actions that were productive.
 
         Productive = action on an alert with true_severity ≥ 0.50.
-        Returns 1.0 when no costly actions were taken (no waste, but also no work).
+        Returns 0.99 when no costly actions were taken (or 0.99 for perfect efficiency).
         """
         costly = [h for h in self._action_history
                   if h["action"] in ("INVESTIGATE", "ESCALATE")]
         if not costly:
-            return 1.0
+            return 0.99
         productive = sum(1 for h in costly if h["true_severity"] >= _MEDIUM_LOWER)
-        return productive / len(costly)
+        raw = productive / len(costly)
+        # Clamp to (0, 1)
+        return max(0.01, min(raw, 0.99))
 
     # ------------------------------------------------------------------
     # Metrics