diff --git a/tests/offscreen/codec-check.test.ts b/tests/offscreen/codec-check.test.ts
new file mode 100644
index 0000000..ea52caa
--- /dev/null
+++ b/tests/offscreen/codec-check.test.ts
@@ -0,0 +1,43 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+
+interface ChromeStub {
+  runtime: { sendMessage: ReturnType<typeof vi.fn> };
+}
+
+interface GlobalWithChrome {
+  chrome?: ChromeStub;
+  MediaRecorder?: { isTypeSupported: (mime: string) => boolean };
+}
+
+describe('codec strict mode', () => {
+  beforeEach(() => {
+    vi.resetModules();
+    (globalThis as unknown as GlobalWithChrome).chrome = {
+      runtime: { sendMessage: vi.fn() },
+    };
+  });
+
+  it('throws on unsupported vp9 and emits RECORDING_ERROR', async () => {
+    (globalThis as unknown as GlobalWithChrome).MediaRecorder = {
+      isTypeSupported: vi.fn().mockReturnValue(false),
+    };
+    const mod = await import('../../src/offscreen/recorder');
+    expect(() => mod.assertCodecSupported()).toThrow(/vp9 unsupported/);
+    const stub = (globalThis as unknown as GlobalWithChrome).chrome!;
+    expect(stub.runtime.sendMessage).toHaveBeenCalledWith(
+      expect.objectContaining({ type: 'RECORDING_ERROR' })
+    );
+  });
+
+  it('does not throw when vp9 IS supported', async () => {
+    (globalThis as unknown as GlobalWithChrome).MediaRecorder = {
+      isTypeSupported: vi.fn().mockReturnValue(true),
+    };
+    const mod = await import('../../src/offscreen/recorder');
+    expect(() => mod.assertCodecSupported()).not.toThrow();
+    const stub = (globalThis as unknown as GlobalWithChrome).chrome!;
+    expect(stub.runtime.sendMessage).not.toHaveBeenCalledWith(
+      expect.objectContaining({ type: 'RECORDING_ERROR' })
+    );
+  });
+});