diff --git a/app/api/streak/route.test.ts b/app/api/streak/route.test.ts
index c81ca19b..17a8174e 100644
--- a/app/api/streak/route.test.ts
+++ b/app/api/streak/route.test.ts
@@ -136,7 +136,21 @@ describe('GET /api/streak', () => {
       expect(response.status).toBe(400);
       const body = await response.json();
       expect(body.error).toBe('Invalid parameters');
-      expect(body.details).not.toBeNull();
+    });
+
+    it('returns 400 Bad Request when ?layout= is set to an unsupported format (Variation 4)', async () => {
+      const response = await GET(
+        makeRequest({
+          user: 'octocat',
+          layout: 'unsupported_layout',
+        })
+      );
+
+      expect(response.status).toBe(400);
+      const body = await response.json();
+      expect(body.details.fieldErrors.layout[0]).toContain(
+        'Invalid layout format. Supported values: default, compact, full.'
+      );
     });
 
     it('returns 400 when the user parameter is missing', async () => {
diff --git a/lib/validations.test.ts b/lib/validations.test.ts
index 6cf61a05..b3b0a05e 100644
--- a/lib/validations.test.ts
+++ b/lib/validations.test.ts
@@ -1061,3 +1061,71 @@ describe('streakParamsSchema user maxLength validation boundaries (Variation 3)'
     expect(parseResult.success).toBe(true);
   });
 });
+
+/* ==========================================================================
+ * LAYOUT PARAMETER — QUERY VALIDATION BOUNDARIES (VARIATION 4)
+ * ========================================================================== */
+
+describe('streakParamsSchema — layout query validation boundaries (Variation 4)', () => {
+  it('rejects unsupported_layout and marks the parse as failed', () => {
+    const result = streakParamsSchema.safeParse({
+      user: 'octocat',
+      layout: 'unsupported_layout',
+    });
+
+    expect(result.success).toBe(false);
+  });
+
+  it('surfaces a meaningful error message for unsupported_layout', () => {
+    const result = streakParamsSchema.safeParse({
+      user: 'octocat',
+      layout: 'unsupported_layout',
+    });
+
+    expect(result.success).toBe(false);
+    if (!result.success) {
+      const messages = result.error.issues.map((i) => i.message).join(' ');
+      expect(messages).toContain(
+        'Invalid layout format. Supported values: default, compact, full.'
+      );
+    }
+  });
+
+  it('accepts "default" as a valid layout value', () => {
+    const result = streakParamsSchema.safeParse({
+      user: 'octocat',
+      layout: 'default',
+    });
+
+    expect(result.success).toBe(true);
+  });
+
+  it('accepts "compact" as a valid layout value', () => {
+    const result = streakParamsSchema.safeParse({
+      user: 'octocat',
+      layout: 'compact',
+    });
+
+    expect(result.success).toBe(true);
+  });
+
+  it('accepts "full" as a valid layout value', () => {
+    const result = streakParamsSchema.safeParse({
+      user: 'octocat',
+      layout: 'full',
+    });
+
+    expect(result.success).toBe(true);
+  });
+
+  it('treats omitted layout as undefined (no validation error)', () => {
+    const result = streakParamsSchema.safeParse({
+      user: 'octocat',
+    });
+
+    expect(result.success).toBe(true);
+    if (result.success) {
+      expect(result.data.layout).toBeUndefined();
+    }
+  });
+});