diff --git a/server/src/modules/leveling/lib/apply-attribute-boost.spec.ts b/server/src/modules/leveling/lib/apply-attribute-boost.spec.ts
new file mode 100644
index 0000000..034b29e
--- /dev/null
+++ b/server/src/modules/leveling/lib/apply-attribute-boost.spec.ts
@@ -0,0 +1,41 @@
+import { applyAttributeBoost, isValidBoostSet } from './apply-attribute-boost';
+
+describe('applyAttributeBoost', () => {
+  it('adds +2 when score is 10 (below 18)', () => {
+    expect(applyAttributeBoost(10)).toBe(12);
+  });
+
+  it('adds +2 when score is 17 (still below 18)', () => {
+    expect(applyAttributeBoost(17)).toBe(19);
+  });
+
+  it('adds +1 when score is exactly 18 (PF2e cap rule — Pitfall #8)', () => {
+    expect(applyAttributeBoost(18)).toBe(19);
+  });
+
+  it('adds +1 when score is 20 (above cap)', () => {
+    expect(applyAttributeBoost(20)).toBe(21);
+  });
+
+  it('handles edge case: very high score (24)', () => {
+    expect(applyAttributeBoost(24)).toBe(25);
+  });
+});
+
+describe('isValidBoostSet', () => {
+  it('accepts exactly 4 distinct abilities', () => {
+    expect(isValidBoostSet(['STR', 'DEX', 'CON', 'INT'])).toBe(true);
+  });
+
+  it('rejects duplicates (3 distinct + 1 repeat)', () => {
+    expect(isValidBoostSet(['STR', 'STR', 'CON', 'INT'])).toBe(false);
+  });
+
+  it('rejects too few (3 abilities)', () => {
+    expect(isValidBoostSet(['STR', 'DEX', 'CON'])).toBe(false);
+  });
+
+  it('rejects too many (5 abilities)', () => {
+    expect(isValidBoostSet(['STR', 'DEX', 'CON', 'INT', 'WIS'])).toBe(false);
+  });
+});