Advanced cy.intercept() Patterns for Stubbing and Spying APIs

The cy.intercept() command in Cypress is powerful — but most tutorials stop at simple request stubbing. In real-world apps, APIs are dynamic, paginated, and unpredictable.

Let’s look at advanced patterns that help you fully control and observe API traffic using cy.intercept().

🔁 Pattern 1: Mock Paginated API Responses

When testing “Load More” or infinite scroll, you often need to return different data per page.

let page = 1;
cy.intercept('GET', '/api/posts*', (req) => {
  req.reply({ fixture: `posts-page-${page}.json` });
  page++;
}).as('getPosts');

🧪 Pattern 2: Spy on Requests and Assert Payloads

Use .intercept() with req.continue() to observe and assert real network traffic.

cy.intercept('POST', '/api/comments', (req) => {
  expect(req.body).to.have.property('text', 'Nice article!');
  req.continue();
}).as('postComment');

💥 Pattern 3: Simulate API Failures or Delays

Test how your UI handles failure gracefully.

cy.intercept('GET', '/api/dashboard', {
  statusCode: 500,
  body: { error: 'Internal Server Error' },
  delayMs: 2000
}).as('getDashboard');

🧬 Pattern 4: Conditional Responses Based on Request Body

Sometimes your app sends different payloads, and you want to return different mocks.

cy.intercept('POST', '/api/login', (req) => {
  if (req.body.username === 'admin') {
    req.reply({ token: 'admin-token' });
  } else {
    req.reply({ statusCode: 401 });
  }
});

🪝 Pattern 5: Modify Requests on the Fly

Need to force in test data?

cy.intercept('POST', '/api/orders', (req) => {
  req.body.items.push({ id: 'bonus-item' });
  req.continue();
});

🧠 Best Practices

• Use req.reply() to control responses, and req.continue() to let them pass through
• Always alias intercepted requests with .as('alias') for easier waiting
• Use fixtures for reusability and version control of mocked data
• Combine with cy.wait('@alias') to assert call behavior

🚀 Final Thoughts

cy.intercept() is more than just a stubber — it’s your gateway to full API control in test environments. These advanced patterns let you simulate real-world edge cases, spy on behavior, and validate data at the network layer.

🔗 Next in Series: Cypress Testing Library for Stable Selectors