diff --git a/packages/gittensory-engine/src/phase7-calibration-loop.ts b/packages/gittensory-engine/src/phase7-calibration-loop.ts
index 35969f5ac6..bbd1eb2215 100644
--- a/packages/gittensory-engine/src/phase7-calibration-loop.ts
+++ b/packages/gittensory-engine/src/phase7-calibration-loop.ts
@@ -331,9 +331,11 @@ export function shouldScheduleHistoricalReplayRun(input: {
 
 function extractHistoricalReplayScore(
   compositeScore: number | GateVerdictCompositeCalibrationScore,
-): number {
-  if (typeof compositeScore === "number") return roundScore(compositeScore);
-  return roundScore(compositeScore.compositeScore);
+): number | null {
+  const rawScore =
+    typeof compositeScore === "number" ? compositeScore : compositeScore.compositeScore;
+  if (!Number.isFinite(rawScore)) return null;
+  return roundScore(rawScore);
 }
 
 /**
@@ -423,6 +425,10 @@ export function computePhase7CalibrationLoop(input: {
       replayHarnessHold = true;
       holdReasons.push("replay_run_stale");
       rejectedSources.push({ source: "historical_replay", reason: "replay_run_stale" });
+    } else if (accuracy === null) {
+      replayHarnessHold = true;
+      holdReasons.push("invalid_replay_score");
+      rejectedSources.push({ source: "historical_replay", reason: "invalid_replay_score" });
     } else {
       contributingSources.push("historical_replay");
     }
diff --git a/packages/gittensory-engine/test/phase7-calibration-loop.test.ts b/packages/gittensory-engine/test/phase7-calibration-loop.test.ts
index ad286f7779..1751e8a90d 100644
--- a/packages/gittensory-engine/test/phase7-calibration-loop.test.ts
+++ b/packages/gittensory-engine/test/phase7-calibration-loop.test.ts
@@ -345,6 +345,28 @@ test("computePhase7CalibrationLoop fails closed when the replay harness is degra
   }
 });
 
+test("REGRESSION: malformed historical replay scores fail closed instead of permitting autonomy increases", () => {
+  for (const compositeScore of [{} as never, Number.NaN, Number.POSITIVE_INFINITY] as const) {
+    const result = computePhase7CalibrationLoop({
+      config: enabledConfig({ autonomyIncreaseMinAccuracy: 0.99 }),
+      prOutcome: sufficientPrOutcome(1),
+      historicalReplay: {
+        ...healthyReplay(0.82),
+        compositeScore,
+      },
+      now: NOW,
+    });
+
+    assert.equal(result.bySource.historical_replay.accuracy, null);
+    assert.equal(result.combinedAccuracy, 1);
+    assert.equal(result.replayHarnessHold, true);
+    assert.equal(result.autonomyIncreasePermitted, false);
+    assert.ok(result.holdReasons.includes("invalid_replay_score"));
+    assert.ok(!result.audit.contributingSources.includes("historical_replay"));
+    assert.ok(result.audit.rejectedSources.some((row) => row.reason === "invalid_replay_score"));
+  }
+});
+
 test("computePhase7CalibrationLoop fails closed on stale replay rather than silently using pr_outcome only", () => {
   const result = computePhase7CalibrationLoop({
     config: enabledConfig(),