diff --git a/lib/lib.ts b/lib/lib.ts
index c4590b7..67bfada 100644
--- a/lib/lib.ts
+++ b/lib/lib.ts
@@ -57,13 +57,14 @@ export const testActor = <T>(
     fn: TestFunction<{ run: ReturnType<typeof createStartRunFn<T>> }>,
     testOptions?: ActorTestOptions,
 ) => {
-    const options = {
+    const { fails, ...restOptions } = {
         ...DEFAULT_TEST_ACTOR_OPTIONS,
         ...testOptions,
     };
     const name = `${actorName}: ${testName}`;
     const shouldRun = !!RUN_ALL_PLATFORM_TESTS || config.has(actorName);
-    vitestTest.runIf(shouldRun)(name, options, async <TYPE extends TestContext>(context: TYPE) => {
+    const vitestTestFn = fails && shouldRun ? vitestTest.fails : vitestTest.runIf(shouldRun);
+    vitestTestFn(name, restOptions, async <TYPE extends TestContext>(context: TYPE) => {
         const { expect, ...rest } = context;
         await fn({
             expect: extendExpect(expect),
@@ -86,14 +87,15 @@ export const testStandbyActor = <I = any, O = any>(
     fn: TestFunction<{ callStandby: ReturnType<typeof createStartStandbyFn<I, O>> }>,
     testOptions?: ActorTestOptions,
 ) => {
-    const options = {
+    const { fails, ...restOptions } = {
         ...DEFAULT_TEST_ACTOR_OPTIONS,
         ...testOptions,
     };
     const name = `${actorName}: ${testName}`;
     const shouldRun = !!RUN_ALL_PLATFORM_TESTS || config.has(actorName);
+    const vitestTestFn = fails && shouldRun ? vitestTest.fails : vitestTest.runIf(shouldRun);
 
-    vitestTest.runIf(shouldRun)(name, options, async <T extends TestContext>(context: T) => {
+    vitestTestFn(name, restOptions, async <T extends TestContext>(context: T) => {
         const standbyTask = await createStandbyTask(actorName, config.get(actorName)?.buildNumber);
         const { expect, ...rest } = context;
 
diff --git a/lib/types.ts b/lib/types.ts
index 6b301f3..96406a9 100644
--- a/lib/types.ts
+++ b/lib/types.ts
@@ -144,6 +144,12 @@ export type ActorTestOptions = Omit<TestOptions, 'retry' | 'timeout'> & {
      * @default 60 * 60 * 1000 // 1 hour
      */
     timeout?: ActorCallOptions['timeout'];
+    /**
+     * Mark this test as expected to fail (wraps the underlying vitest test with `test.fails`).
+     * The test passes as long as it keeps failing, and alerts you (by failing) if it unexpectedly starts passing.
+     * Use this to keep a sentinel for known regressions without inverting assertions by hand.
+     */
+    fails?: boolean;
 };
 
 declare module 'vitest' {
diff --git a/test/unit/test-actor.test.ts b/test/unit/test-actor.test.ts
new file mode 100644
index 0000000..5ed9a66
--- /dev/null
+++ b/test/unit/test-actor.test.ts
@@ -0,0 +1,115 @@
+import type * as Vitest from 'vitest';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+
+// Spy handles: mockFails is called directly as vitestTest.fails(name, opts, fn),
+// mockRunIf is called as vitestTest.runIf(condition) and returns mockRunIfFn.
+const mockRunIfFn = vi.fn();
+const mockRunIf = vi.fn(() => mockRunIfFn);
+const mockFails = vi.fn();
+
+// Mock the vitest module so lib.ts picks up our spies when dynamically imported.
+// We spread the real module so our test file's own `it`, `describe`, `expect` etc. still work.
+// We only replace `test` — `it` remains the real registration function used by this file.
+vi.mock('vitest', async (importOriginal) => {
+    const actual = await importOriginal<typeof Vitest>();
+    return {
+        ...actual,
+        test: Object.assign(vi.fn(), {
+            fails: mockFails,
+            runIf: mockRunIf,
+        }),
+    };
+});
+
+const ACTOR_BUILD = { actorName: 'my-actor', actorId: 'abc123', buildNumber: '1.0', buildId: 'build1' };
+// eslint-disable-next-line @typescript-eslint/no-empty-function
+const noop = () => {};
+
+describe('testActor - fails option', () => {
+    beforeEach(() => {
+        // Fresh module so env vars read at lib.ts module level are re-evaluated each test.
+        vi.resetModules();
+        mockRunIf.mockClear();
+        mockRunIfFn.mockClear();
+        mockFails.mockClear();
+    });
+
+    afterEach(() => {
+        vi.unstubAllEnvs();
+    });
+
+    it('calls vitestTest.fails when fails:true and actor is in the build list', async () => {
+        vi.stubEnv('ACTOR_BUILDS', JSON.stringify([ACTOR_BUILD]));
+
+        const { testActor } = await import('../../lib/lib.js');
+        testActor('my-actor', 'test name', noop, { fails: true });
+
+        expect(mockFails).toHaveBeenCalledOnce();
+        expect(mockRunIf).not.toHaveBeenCalled();
+    });
+
+    it('calls vitestTest.runIf(false) when fails:true but actor is not in the build list', async () => {
+        vi.stubEnv('ACTOR_BUILDS', JSON.stringify([]));
+
+        const { testActor } = await import('../../lib/lib.js');
+        testActor('my-actor', 'test name', noop, { fails: true });
+
+        expect(mockRunIf).toHaveBeenCalledWith(false);
+        expect(mockFails).not.toHaveBeenCalled();
+    });
+
+    it('calls vitestTest.runIf(true) when no fails option and actor is in the build list', async () => {
+        vi.stubEnv('ACTOR_BUILDS', JSON.stringify([ACTOR_BUILD]));
+
+        const { testActor } = await import('../../lib/lib.js');
+        testActor('my-actor', 'test name', noop);
+
+        expect(mockRunIf).toHaveBeenCalledWith(true);
+        expect(mockFails).not.toHaveBeenCalled();
+    });
+
+    it('strips the fails key from options forwarded to vitest', async () => {
+        vi.stubEnv('ACTOR_BUILDS', JSON.stringify([ACTOR_BUILD]));
+
+        const { testActor } = await import('../../lib/lib.js');
+        testActor('my-actor', 'test name', noop, { fails: true, retry: 2 });
+
+        const [, options] = mockFails.mock.calls[0] as [string, Record<string, unknown>, unknown];
+        expect(options).not.toHaveProperty('fails');
+        expect(options).toHaveProperty('retry', 2);
+    });
+
+    it('calls vitestTest.fails when fails:true and RUN_ALL_PLATFORM_TESTS is set (no build config)', async () => {
+        vi.stubEnv('ACTOR_BUILDS', JSON.stringify([]));
+        vi.stubEnv('RUN_ALL_PLATFORM_TESTS', '1');
+
+        const { testActor } = await import('../../lib/lib.js');
+        testActor('my-actor', 'test name', noop, { fails: true });
+
+        expect(mockFails).toHaveBeenCalledOnce();
+        expect(mockRunIf).not.toHaveBeenCalled();
+    });
+});
+
+describe('testStandbyActor - fails option', () => {
+    beforeEach(() => {
+        vi.resetModules();
+        mockRunIf.mockClear();
+        mockRunIfFn.mockClear();
+        mockFails.mockClear();
+    });
+
+    afterEach(() => {
+        vi.unstubAllEnvs();
+    });
+
+    it('calls vitestTest.fails when fails:true and actor is in the build list', async () => {
+        vi.stubEnv('ACTOR_BUILDS', JSON.stringify([ACTOR_BUILD]));
+
+        const { testStandbyActor } = await import('../../lib/lib.js');
+        testStandbyActor('my-actor', 'test name', noop, { fails: true });
+
+        expect(mockFails).toHaveBeenCalledOnce();
+        expect(mockRunIf).not.toHaveBeenCalled();
+    });
+});