diff --git a/client/tests/integration/api/client.test.ts b/client/tests/integration/api/client.test.ts
index 529fc715..0b71af53 100644
--- a/client/tests/integration/api/client.test.ts
+++ b/client/tests/integration/api/client.test.ts
@@ -334,12 +334,14 @@ describe('API client interceptors', () => {
   // ── backupApi.download ───────────────────────────────────────────────────────
 
   it('FE-API-013: backupApi.download creates a temp anchor and clicks it', async () => {
-    // backupApi.download uses native fetch (not axios), so mock it directly to
-    // avoid jsdom/MSW interception differences across environments.
+    // backupApi.download uses native fetch (not axios). Mock fetch directly and
+    // use a plain-object Response duck-type to avoid MSW patching the Response
+    // constructor (which calls blob.stream() — not implemented in jsdom's Blob).
     const blob = new Blob(['zip-bytes'], { type: 'application/zip' });
-    vi.spyOn(globalThis, 'fetch').mockResolvedValueOnce(
-      new Response(blob, { status: 200 })
-    );
+    vi.spyOn(globalThis, 'fetch').mockResolvedValueOnce({
+      ok: true,
+      blob: () => Promise.resolve(blob),
+    } as unknown as Response);
     const createObjectURL = vi.spyOn(URL, 'createObjectURL').mockReturnValue('blob:mock-url');
     const revokeObjectURL = vi.spyOn(URL, 'revokeObjectURL').mockImplementation(() => {});
 
diff --git a/client/tests/unit/api/authUrl.test.ts b/client/tests/unit/api/authUrl.test.ts
index 23e5f23a..16e3aead 100644
--- a/client/tests/unit/api/authUrl.test.ts
+++ b/client/tests/unit/api/authUrl.test.ts
@@ -9,6 +9,12 @@ const flushPromises = () => new Promise<void>(r => setTimeout(r, 10));
 beforeEach(() => {
   clearImageQueue();
   vi.restoreAllMocks(); // restore any vi.spyOn() wrappers from the previous test
+
+  // jsdom's URL.createObjectURL returns '' and may be non-configurable, so
+  // Object.defineProperty in setup.ts can fail silently on CI. Assign directly
+  // here (after restoreAllMocks) so every test in this file gets a fresh mock.
+  URL.createObjectURL = vi.fn(() => 'blob:mock') as typeof URL.createObjectURL;
+  URL.revokeObjectURL = vi.fn() as typeof URL.revokeObjectURL;
 });
 
 // ── getAuthUrl ─────────────────────────────────────────────────────────────────
@@ -193,7 +199,6 @@ describe('clearImageQueue', () => {
     it('removes queued items so they never execute after active slots drain', async () => {
       const resolvers: Array<() => void> = [];
       const createObjectURLSpy = vi.spyOn(URL, 'createObjectURL');
-      createObjectURLSpy.mockClear(); // vi.spyOn returns the same vi.fn() set in setup.ts; reset accumulated calls from prior tests
 
       vi.spyOn(globalThis, 'fetch').mockImplementation(async () => {
         await new Promise<void>(r => resolvers.push(r));