diff --git a/scripts/lib/inspection.js b/scripts/lib/inspection.js
index 495a36524..709a6bb73 100644
--- a/scripts/lib/inspection.js
+++ b/scripts/lib/inspection.js
@@ -60,6 +60,14 @@ function groupFailures(skillRuns) {
   return groups;
 }
 
+function compareTimestampsDescending(a, b) {
+  const timeA = Date.parse(a);
+  const timeB = Date.parse(b);
+  if (Number.isNaN(timeA)) return Number.isNaN(timeB) ? 0 : 1;
+  if (Number.isNaN(timeB)) return -1;
+  return timeB - timeA;
+}
+
 /**
  * Detect recurring failure patterns from skill runs.
  *
@@ -79,7 +87,7 @@ function detectPatterns(skillRuns, options = {}) {
     }
 
     const sortedRuns = [...group.runs].sort(
-      (a, b) => (Date.parse(b.createdAt) || 0) - (Date.parse(a.createdAt) || 0)
+      (a, b) => compareTimestampsDescending(a.createdAt, b.createdAt)
     );
 
     const firstSeen = sortedRuns[sortedRuns.length - 1].createdAt || null;
@@ -106,7 +114,7 @@ function detectPatterns(skillRuns, options = {}) {
   // Sort by count descending, then by lastSeen descending
   return patterns.sort((a, b) => {
     if (b.count !== a.count) return b.count - a.count;
-    return (Date.parse(b.lastSeen) || 0) - (Date.parse(a.lastSeen) || 0);
+    return compareTimestampsDescending(a.lastSeen, b.lastSeen);
   });
 }
 
diff --git a/tests/lib/inspection.test.js b/tests/lib/inspection.test.js
index a1bf21154..c9557faa6 100644
--- a/tests/lib/inspection.test.js
+++ b/tests/lib/inspection.test.js
@@ -149,6 +149,36 @@ async function runTests() {
     assert.strictEqual(patterns[2].lastSeen, null);
   })) passed += 1; else failed += 1;
 
+  if (await test('detectPatterns keeps undated runs and patterns stable after dated ones', async () => {
+    const historical = '1969-12-31T23:59:59Z';
+    const epoch = '1970-01-01T00:00:00Z';
+    const runs = [
+      { ...makeSkillRun({ id: 'missing-a' }), createdAt: null },
+      makeSkillRun({ id: 'historical', createdAt: historical }),
+      { ...makeSkillRun({ id: 'missing-b' }), createdAt: null },
+      makeSkillRun({ id: 'epoch', createdAt: epoch }),
+    ];
+
+    const [pattern] = detectPatterns(runs, { threshold: 2 });
+    assert.deepStrictEqual(pattern.runIds, ['epoch', 'historical', 'missing-a', 'missing-b']);
+    assert.strictEqual(pattern.lastSeen, epoch);
+    const patternRuns = [
+      { ...makeSkillRun({ id: 'a1', skillId: 'undated-a' }), createdAt: null },
+      { ...makeSkillRun({ id: 'a2', skillId: 'undated-a' }), createdAt: null },
+      makeSkillRun({ id: 'h1', skillId: 'historical', createdAt: '1969-12-31T23:59:58Z' }),
+      makeSkillRun({ id: 'h2', skillId: 'historical', createdAt: '1969-12-31T23:59:59Z' }),
+      { ...makeSkillRun({ id: 'b1', skillId: 'undated-b' }), createdAt: null },
+      { ...makeSkillRun({ id: 'b2', skillId: 'undated-b' }), createdAt: null },
+    ];
+
+    const patterns = detectPatterns(patternRuns, { threshold: 2 });
+    assert.deepStrictEqual(patterns.map(pattern => pattern.skillId), ['historical', 'undated-a', 'undated-b']);
+    assert.deepStrictEqual(patterns[1].runIds, ['a1', 'a2']);
+    assert.deepStrictEqual(patterns[2].runIds, ['b1', 'b2']);
+    assert.strictEqual(patterns[1].firstSeen, null);
+    assert.strictEqual(patterns[1].lastSeen, null);
+  })) passed += 1; else failed += 1;
+
   if (await test('detectPatterns uses default threshold', async () => {
     const runs = Array.from({ length: DEFAULT_FAILURE_THRESHOLD }, (_, i) =>
       makeSkillRun({ id: `r${i}`, failureReason: 'permission denied' })