diff --git a/scripts/build-milestones.js b/scripts/build-milestones.js
index 561ce12..a4fbf96 100644
--- a/scripts/build-milestones.js
+++ b/scripts/build-milestones.js
@@ -63,8 +63,20 @@ milestones.forEach((m, i) => {
     console.error(`ERROR: milestone[${i}] (id="${m.id}") reference must be a string URL.`);
     process.exit(1);
   }
+
+  if (m.extinctionMarker !== undefined && m.extinctionMarker !== true) {
+    console.error(`ERROR: milestone[${i}] (id="${m.id}") extinctionMarker must be true if set.`);
+    process.exit(1);
+  }
 });
 
+const extinctionMarkers = milestones.filter((m) => m.extinctionMarker === true);
+if (extinctionMarkers.length > 1) {
+  const ids = extinctionMarkers.map((m) => `"${m.id}"`).join(', ');
+  console.error(`ERROR: exactly one milestone may have extinctionMarker: true, but found ${extinctionMarkers.length}: ${ids}`);
+  process.exit(1);
+}
+
 // ── Generate JS ───────────────────────────────────────────────────────────────
 function jsString(s) {
   // Emit as a template-literal-safe single-quoted JS string.
diff --git a/tests/death-clock.test.js b/tests/death-clock.test.js
index 5cee049..3ab889e 100644
--- a/tests/death-clock.test.js
+++ b/tests/death-clock.test.js
@@ -447,6 +447,11 @@ describe('Constants', () => {
     }
   });
 
+  test('at most one milestone has extinctionMarker: true', () => {
+    const marked = MILESTONES.filter((m) => m.extinctionMarker === true);
+    expect(marked.length).toBeLessThanOrEqual(1);
+  });
+
   test('HISTORICAL_DATA is a non-empty array', () => {
     expect(Array.isArray(HISTORICAL_DATA)).toBe(true);
     expect(HISTORICAL_DATA.length).toBeGreaterThan(0);