How does test isolation work in Cypress?

Test isolation makes each test start from a known state. With the default enabled, Cypress clears the browser context between tests: cookies, localStorage, sessionStorage, the page and the application under test. A test that relied on the previous test visiting a page will fail, which is intentional.

describe("isolation", () => {
  it("stores a draft", () => {
    cy.visit("/editor");
    cy.window().then((win) => {
      win.localStorage.setItem("draft", "hello");
    });
  });

  it("starts without the draft", () => {
    cy.visit("/editor");
    cy.window().then((win) => {
      expect(win.localStorage.getItem("draft")).to.be.null;
    });
  });
});

What isolation changes

• Cookies, localStorage and sessionStorage are cleared between tests
• The page is reset, so a test must visit its own starting URL
• The application is not restarted, so server state can still persist
• cy.session is the supported way to reuse authentication without fighting isolation
• testIsolation can be disabled, but doing so makes tests order-dependent

Isolation covers browser state, not backend state. If a test creates a record, that record remains unless the test or a hook removes it. Use a unique identifier per test, seed through an API, or reset the database with cy.task. Do not assume isolation gives you a clean backend.