diff --git a/lib/svg/layout.massive-scaling.test.ts b/lib/svg/layout.massive-scaling.test.ts
new file mode 100644
index 000000000..87015e137
--- /dev/null
+++ b/lib/svg/layout.massive-scaling.test.ts
@@ -0,0 +1,113 @@
+import { describe, expect, it } from 'vitest';
+import { computeTowers, projectIsometric } from './layout';
+
+const makeCalendar = (weeksCount: number, countFactory: (week: number, day: number) => number) => {
+  const weeks = Array.from({ length: weeksCount }, (_, weekIndex) => ({
+    contributionDays: Array.from({ length: 7 }, (_, dayIndex) => {
+      const dayNumber = weekIndex * 7 + dayIndex + 1;
+
+      return {
+        date: new Date(Date.UTC(2026, 0, dayNumber)).toISOString().slice(0, 10),
+        contributionCount: countFactory(weekIndex, dayIndex),
+      };
+    }),
+  }));
+
+  return {
+    totalContributions: weeks.reduce(
+      (total, week) =>
+        total + week.contributionDays.reduce((sum, day) => sum + day.contributionCount, 0),
+      0
+    ),
+    weeks,
+  };
+};
+
+describe('layout massive scaling', () => {
+  it('limits rendering to the latest fourteen weeks for massive calendars', () => {
+    const calendar = makeCalendar(1000, (week, day) => week + day + 1);
+
+    const towers = computeTowers(calendar);
+
+    expect(towers).toHaveLength(98);
+    expect(towers[0].row).toBe(0);
+    expect(towers.at(-1)?.row).toBe(13);
+  });
+
+  it('keeps projected coordinates finite and unique for dense visible grids', () => {
+    const calendar = makeCalendar(250, (week, day) => (week + 1) * (day + 1));
+
+    const towers = computeTowers(calendar);
+    const coordinateKeys = new Set(towers.map((tower) => `${tower.x}:${tower.y}`));
+
+    expect(coordinateKeys.size).toBe(towers.length);
+
+    towers.forEach((tower) => {
+      expect(Number.isFinite(tower.x)).toBe(true);
+      expect(Number.isFinite(tower.y)).toBe(true);
+      expect(Number.isFinite(tower.h)).toBe(true);
+      expect(tower.x).toBe(projectIsometric(tower.row, tower.col).x);
+      expect(tower.y).toBe(projectIsometric(tower.row, tower.col).y);
+    });
+  });
+
+  it('caps tower heights for extreme high contribution counts', () => {
+    const calendar = makeCalendar(52, () => Number.MAX_SAFE_INTEGER);
+
+    const linearTowers = computeTowers(calendar, 'linear');
+    const logTowers = computeTowers(calendar, 'log');
+
+    linearTowers.forEach((tower) => {
+      expect(tower.h).toBeLessThanOrEqual(60);
+      expect(tower.intensityLevel).toBe(4);
+      expect(tower.hasCommits).toBe(true);
+    });
+
+    logTowers.forEach((tower) => {
+      expect(tower.h).toBeLessThanOrEqual(80);
+      expect(tower.intensityLevel).toBe(4);
+      expect(tower.hasCommits).toBe(true);
+    });
+  });
+
+  it('scales LoC mode safely for large addition and deletion values', () => {
+    const calendar = {
+      totalContributions: 0,
+      weeks: Array.from({ length: 120 }, (_, weekIndex) => ({
+        contributionDays: Array.from({ length: 7 }, (_, dayIndex) => {
+          const dayNumber = weekIndex * 7 + dayIndex + 1;
+
+          return {
+            date: new Date(Date.UTC(2026, 0, dayNumber)).toISOString().slice(0, 10),
+            contributionCount: 1,
+            locAdditions: 100_000 + weekIndex,
+            locDeletions: 50_000 + dayIndex,
+          };
+        }),
+      })),
+    };
+
+    const towers = computeTowers(calendar, 'log', '', 'loc');
+
+    expect(towers).toHaveLength(98);
+
+    towers.forEach((tower) => {
+      expect(tower.contributionCount).toBeGreaterThan(100_000);
+      expect(tower.tooltip).toContain('est. lines of code');
+      expect(tower.h).toBeLessThanOrEqual(80);
+      expect(Number.isFinite(tower.h)).toBe(true);
+    });
+  });
+
+  it('computes massive layouts within a safe performance margin', () => {
+    const calendar = makeCalendar(5000, (week, day) => (week + 1) * (day + 1));
+    const startedAt = performance.now();
+
+    const towers = computeTowers(calendar, 'log');
+
+    const duration = performance.now() - startedAt;
+
+    expect(towers).toHaveLength(98);
+    expect(duration).toBeLessThan(100);
+  });
+});