diff --git a/app/api/streak/route.test.ts b/app/api/streak/route.test.ts
index c81ca19b..4aebf7b8 100644
--- a/app/api/streak/route.test.ts
+++ b/app/api/streak/route.test.ts
@@ -650,6 +650,14 @@ describe('GET /api/streak', () => {
         expect(response.status).toBe(400);
         expect(body.details.fieldErrors.date[0]).toContain('Invalid "date" format');
       });
+
+      it('returns 400 when an invalid ISO8601 calendar date format like "2026-15-40" is supplied (Variation 4)', async () => {
+        const response = await GET(makeRequest({ user: 'octocat', date: '2026-15-40' }));
+        const body = await response.json();
+
+        expect(response.status).toBe(400);
+        expect(body.details.fieldErrors.date[0]).toContain('Invalid "date" format. Use ISO 8601.');
+      });
     });
   });
 
diff --git a/lib/validations.test.ts b/lib/validations.test.ts
index 6cf61a05..4ed8a45e 100644
--- a/lib/validations.test.ts
+++ b/lib/validations.test.ts
@@ -1061,3 +1061,31 @@ describe('streakParamsSchema user maxLength validation boundaries (Variation 3)'
     expect(parseResult.success).toBe(true);
   });
 });
+
+/* ==========================================================================
+ * DATE PARAMETER — QUERY VALIDATION BOUNDARIES (VARIATION 4)
+ * ========================================================================== */
+
+describe('streakParamsSchema — date query validation boundaries (Variation 4)', () => {
+  it('rejects an invalid date format like "2026-15-40"', () => {
+    const result = streakParamsSchema.safeParse({
+      user: 'octocat',
+      date: '2026-15-40',
+    });
+
+    expect(result.success).toBe(false);
+    if (!result.success) {
+      const messages = result.error.issues.map((i) => i.message).join(' ');
+      expect(messages).toContain('Invalid "date" format. Use ISO 8601.');
+    }
+  });
+
+  it('accepts a valid ISO8601 date', () => {
+    const result = streakParamsSchema.safeParse({
+      user: 'octocat',
+      date: '2026-05-30',
+    });
+
+    expect(result.success).toBe(true);
+  });
+});