diff --git a/src/tools/testmanagement-utils/create-testcase.ts b/src/tools/testmanagement-utils/create-testcase.ts
index 1119fda..446e2fe 100644
--- a/src/tools/testmanagement-utils/create-testcase.ts
+++ b/src/tools/testmanagement-utils/create-testcase.ts
@@ -320,6 +320,10 @@ export async function createTestCase(
 ): Promise<CallToolResult> {
   const testCaseParams: TestCaseCreateRequest = { ...params };
 
+  testCaseParams.tags = Array.from(
+    new Set([...(testCaseParams.tags ?? []), "MCP Generated"]),
+  );
+
   if (
     testCaseParams.priority !== undefined ||
     testCaseParams.case_type !== undefined
diff --git a/tests/tools/create-testcase-tag.test.ts b/tests/tools/create-testcase-tag.test.ts
new file mode 100644
index 0000000..01bf621
--- /dev/null
+++ b/tests/tools/create-testcase-tag.test.ts
@@ -0,0 +1,80 @@
+import { beforeEach, describe, it, expect, vi, Mock } from 'vitest';
+import { apiClient } from '../../src/lib/apiClient';
+import {
+  createTestCase,
+  TestCaseCreateRequest,
+} from '../../src/tools/testmanagement-utils/create-testcase';
+
+// Reach the real createTestCase implementation; stub only its external deps so
+// we can assert on the payload it builds (PMAA-166: MCP-origin tag stamping).
+vi.mock('../../src/lib/apiClient', () => ({
+  apiClient: { get: vi.fn(), post: vi.fn() },
+}));
+vi.mock('../../src/lib/tm-base-url', () => ({
+  getTMBaseURL: vi.fn(async () => 'https://test-management.browserstack.com'),
+}));
+vi.mock('../../src/lib/get-auth', () => ({
+  getBrowserStackAuth: vi.fn(() => 'fake-user:fake-key'),
+}));
+vi.mock('../../src/tools/testmanagement-utils/TCG-utils/api', () => ({
+  projectIdentifierToId: vi.fn(async () => '999'),
+  fetchFormFields: vi.fn(),
+  normalizeDefaultFieldValue: vi.fn(),
+}));
+vi.mock('../../src/logger', () => ({
+  default: { error: vi.fn(), warn: vi.fn(), info: vi.fn(), debug: vi.fn() },
+}));
+
+const mockConfig = {
+  'browserstack-username': 'fake-user',
+  'browserstack-access-key': 'fake-key',
+} as any;
+
+const baseArgs: TestCaseCreateRequest = {
+  project_identifier: 'proj-123',
+  folder_id: 'fold-456',
+  name: 'Sample Test Case',
+  test_case_steps: [{ step: 'Step 1', result: 'Result 1' }],
+};
+
+function mockCreateSuccess() {
+  (apiClient.post as Mock).mockResolvedValue({
+    data: {
+      data: {
+        success: true,
+        test_case: {
+          identifier: 'TC-001',
+          title: 'Sample Test Case',
+          template: undefined,
+        },
+      },
+    },
+  });
+}
+
+function sentTags(): string[] {
+  const req = (apiClient.post as Mock).mock.calls[0][0];
+  return req.body.test_case.tags;
+}
+
+describe('createTestCase MCP-origin tag (PMAA-166)', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    mockCreateSuccess();
+  });
+
+  it('stamps "MCP Generated" when the caller passes no tags', async () => {
+    await createTestCase({ ...baseArgs }, mockConfig);
+    expect(sentTags()).toEqual(['MCP Generated']);
+  });
+
+  it('preserves user tags and appends "MCP Generated"', async () => {
+    await createTestCase({ ...baseArgs, tags: ['smoke', 'regression'] }, mockConfig);
+    expect(sentTags()).toEqual(['smoke', 'regression', 'MCP Generated']);
+  });
+
+  it('does not duplicate "MCP Generated" if already present', async () => {
+    await createTestCase({ ...baseArgs, tags: ['MCP Generated'] }, mockConfig);
+    expect(sentTags()).toEqual(['MCP Generated']);
+  });
+});