How does Playwright isolate tests and where does isolation break?

Playwright's default isolation is strong at the browser layer. The page and context fixtures are per test, so one test cannot see another test's cookies, localStorage or open tabs. That removes a large class of order-dependent failures.

What is isolated by default

• Browser context, including cookies and localStorage
• Pages, popups and dialogs opened by the test
• Network routes registered on that context
• Test-scoped fixture instances
• In-flight requests, which are closed with the context

test.describe.configure({ mode: "serial" });

test("creates an order", async ({ page }) => {
  await page.goto("/orders/new");
  await page.getByRole("button", { name: "Create" }).click();
});

test("updates the order", async ({ page }) => {
  await page.goto("/orders");
  await page.getByRole("button", { name: "Edit" }).click();
});

Isolation breaks outside the browser. If both tests use the same account, truncate the same table or write the same file, they can interfere even with separate contexts. Serial mode hides the symptom by forcing order, but it also skips later tests when one fails and prevents parallelism.

test("shows the invoice", async ({ page, request }) => {
  const created = await request.post("/api/invoices", {
    data: { total: 120 },
  });
  const invoice = await created.json();

  await page.goto("/invoices/" + invoice.id);
  await expect(page.getByText("120")).toBeVisible();

  await request.delete("/api/invoices/" + invoice.id);
});