diff --git a/.changeset/mosaic-feature-cookie-isolation.md b/.changeset/mosaic-feature-cookie-isolation.md
new file mode 100644
index 00000000000..a845151cc84
--- /dev/null
+++ b/.changeset/mosaic-feature-cookie-isolation.md
@@ -0,0 +1,2 @@
+---
+---
diff --git a/packages/mosaic/docs/testing.md b/packages/mosaic/docs/testing.md
index 8ddb38e614e..e1b8cfc278d 100644
--- a/packages/mosaic/docs/testing.md
+++ b/packages/mosaic/docs/testing.md
@@ -178,6 +178,9 @@ in [Test placement](#test-placement).
   `ClerkContextProvider` and `MosaicProvider`, loads Clerk, and returns
   `{ clerk, navigate, rerender, ... }`.
   `navigate` is the router Clerk was loaded with, called with the path.
+  It loads Clerk with `standardBrowser: false`. A test's Clerk is never
+  disposed, and its cookie service would otherwise write cookies into later
+  tests.
 
 The setup file (`vitest.setup.browser.mts`) starts the worker and focuses the
 window. Before each test it moves FAPI to a new host, so a request a previous
diff --git a/packages/mosaic/src/__tests__/feature/fake-fapi-isolation.feature.test.tsx b/packages/mosaic/src/__tests__/feature/fake-fapi-isolation.feature.test.tsx
index 700da58ae8a..9ed44df552f 100644
--- a/packages/mosaic/src/__tests__/feature/fake-fapi-isolation.feature.test.tsx
+++ b/packages/mosaic/src/__tests__/feature/fake-fapi-isolation.feature.test.tsx
@@ -36,4 +36,12 @@ describe('fake FAPI isolation between tests', () => {
     expect(memberships.requests).toHaveLength(0);
     memberships.release();
   });
+
+  it('does not let any Clerk write session cookies', async () => {
+    const user = fapiUser({ id: 'user_1' });
+    serveFapi({ client: fapiClient([fapiSession({ id: 'sess_1', user })]) });
+    const { clerk } = await renderWithClerk(<div />);
+    expect(await clerk.session?.getToken()).toEqual(expect.any(String));
+    expect(document.cookie).not.toMatch(/(?:^|; )__session/);
+  });
 });
diff --git a/packages/mosaic/src/__tests__/feature/render.tsx b/packages/mosaic/src/__tests__/feature/render.tsx
index 933e4dbffca..cbcf75842b2 100644
--- a/packages/mosaic/src/__tests__/feature/render.tsx
+++ b/packages/mosaic/src/__tests__/feature/render.tsx
@@ -33,7 +33,14 @@ export async function renderWithClerk(
   );
   let current = ui;
   const view = render(wrap(current));
-  await act(() => clerk.load({ routerPush: to => navigate(to), routerReplace: to => navigate(to), ...options }));
+  await act(() =>
+    clerk.load({
+      standardBrowser: false,
+      routerPush: to => navigate(to),
+      routerReplace: to => navigate(to),
+      ...options,
+    }),
+  );
   view.rerender(wrap(current));
 
   return {