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For a responsive test, set the application viewport with cy.viewport(width, height), or define viewportWidth and viewportHeight in Cypress configuration. Those settings control the page under test. If you need to change the headless browser window used for screenshots or video, configure the separate before:browser:launch event.
The two meanings of browser size in Cypress
Cypress uses “viewport” and “screen” for different layers. Choosing the wrong layer is the most common reason a resize appears not to work.
| Need | Use | What changes |
|---|---|---|
| Resize the page during a test | cy.viewport(width, height) or a named preset |
The application viewport for the current test |
| Choose a default page size for runs | viewportWidth and viewportHeight in Cypress configuration |
The default application viewport, reset between tests |
| Use a different size for one test or suite | A test or suite configuration object | A scoped application viewport |
| Override dimensions from CI or a terminal | --config or the documented CYPRESS_VIEWPORT_WIDTH and CYPRESS_VIEWPORT_HEIGHT variables |
Run-level application configuration |
| Change the headless display used for screenshots or video | before:browser:launch |
The browser screen/rendering window, separately from the Cypress application viewport |
The Cypress application viewport defaults to 1000 by 660 pixels. A preview in the Cypress runner can be visually scaled to fit its pane; that scaling does not alter the dimensions reported to the application.
Resize the application in one test
Call cy.viewport() before the assertions that depend on layout. The numeric form takes width first and height second.
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describe('responsive navigation', () => {
it('shows the desktop navigation', () => {
cy.viewport(1280, 720)
cy.visit('/dashboard')
cy.get('[data-cy=desktop-nav]').should('be.visible')
cy.get('[data-cy=menu-button]').should('not.be.visible')
})
it('shows the mobile menu', () => {
cy.viewport(390, 844)
cy.visit('/dashboard')
cy.get('[data-cy=menu-button]').should('be.visible')
cy.get('[data-cy=desktop-nav]').should('not.be.visible')
})
})Place the command before cy.visit() when you want the initial page load to occur at that size. Cypress also resizes an already loaded page when the command runs, but components that calculate dimensions only during startup may need to be reloaded or explicitly exercised after the resize.
Use Cypress device presets and orientation
The preset form supplies a documented width and height for a named device profile:
it('renders the phone layout', () => {
cy.viewport('iphone-6')
cy.visit('/pricing')
cy.get('[data-cy=mobile-header]').should('be.visible')
})Add landscape as the second argument to swap the preset dimensions:
it('renders the phone landscape layout', () => {
cy.viewport('iphone-6', 'landscape')
cy.visit('/pricing')
})Presets are convenient for breakpoint tests, but they are not complete physical-device emulation. In particular, cy.viewport() does not simulate a device pixel ratio. Test high-density rendering separately if your application has DPR-specific behavior.
Set a default size in cypress.config.js or cypress.config.ts
Use configuration when most tests should start at the same dimensions. The default is restored between tests, so one test’s resize does not silently become the next test’s starting point.
const { defineConfig } = require('cypress')
module.exports = defineConfig({
viewportWidth: 1280,
viewportHeight: 720,
e2e: {
baseUrl: 'http://localhost:3000'
}
})The equivalent TypeScript configuration is:
import { defineConfig } from 'cypress'
export default defineConfig({
viewportWidth: 1280,
viewportHeight: 720,
e2e: {
baseUrl: 'http://localhost:3000'
}
})Use configuration for a stable project-wide baseline, then call cy.viewport() for a test that intentionally covers another breakpoint.
Apply a size only to a suite or test
A configuration object keeps special dimensions local instead of changing the project default:
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describe('tablet checkout', {
viewportWidth: 1024,
viewportHeight: 768
}, () => {
it('keeps the order summary visible', () => {
cy.visit('/checkout')
cy.get('[data-cy=order-summary]').should('be.visible')
})
})
it('uses a narrow phone width', {
viewportWidth: 375,
viewportHeight: 667
}, () => {
cy.visit('/checkout')
cy.get('[data-cy=menu-button]').should('be.visible')
})The scoped values apply to that suite or test and then return to the previous defaults. This is useful when a spec contains desktop, tablet and phone cases without repeatedly editing global configuration.
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Override the viewport from the command line or environment
For a one-off local run or a CI matrix, pass both dimensions through --config:
npx cypress run --config viewportWidth=1280,viewportHeight=720Cypress also documents environment-variable overrides:
CYPRESS_VIEWPORT_WIDTH=1280 CYPRESS_VIEWPORT_HEIGHT=720 npx cypress runThese are run-level values. A later cy.viewport() call or a more specific test configuration can still select a different size where the test requires it.
Change the headless browser screen for screenshots and video
If the page has the expected responsive layout but the captured screenshot or video has the wrong outer dimensions, you are changing the wrong setting. The headless browser screen is configured in the before:browser:launch event, not with cy.viewport().
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const { defineConfig } = require('cypress')
module.exports = defineConfig({
e2e: {
setupNodeEvents(on, config) {
on('before:browser:launch', (browser, launchOptions) => {
if (browser.family === 'chromium' && browser.isHeadless) {
launchOptions.args.push('--window-size=1440,900')
}
if (browser.name === 'electron') {
launchOptions.preferences.width = 1440
launchOptions.preferences.height = 900
}
return launchOptions
})
}
}
})Cypress documents a 1280 by 720 headless screen and device pixel ratio 1 as browser-launch defaults. Those values describe the browser display used for rendering; they are not the 1000 by 660 application-viewport defaults. Keep both settings explicit when reproducible visual output matters.
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Browser launch APIs vary by browser family. Inspect the supplied launch object before changing it, preserve existing arguments and preferences, and return the object from the callback. Do not expect a window-size argument to change media-query behavior if the application viewport remains configured to another size.
Build reliable responsive tests
Test breakpoints, not every possible pixel
Select widths immediately below, at and above each breakpoint that changes behavior. For example, if navigation switches at 768 pixels, cover a narrow mobile width, 767, 768 and a representative desktop width. This catches off-by-one CSS conditions without creating an unmaintainable test for every width.
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Check whether the menu button, navigation, grid or dialog is visible and usable. Avoid asserting internal CSS values unless the value itself is the requirement. A behavior assertion survives harmless changes to class names and spacing.
Set the size before visiting
Putting cy.viewport() before cy.visit() makes the first document load occur under the intended media-query conditions and avoids testing a desktop-first initialization by accident.
Keep viewport changes isolated
Use one viewport per test when possible. Multiple resizes in one test can be valid for a resize interaction, but they make failures harder to diagnose and can leave application state created at the wrong dimensions.
Remember that preview size is not test size
Open Mode may shrink the preview to fit the runner pane. Read the dimensions from the Cypress command log or application behavior, not from the apparent size of the preview on your monitor.
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The layout stays desktop after cy.viewport()
- Confirm the command runs before the layout assertion and, preferably, before
cy.visit(). - Check that the assertion is made against the application under test, not a separate browser chrome element.
- Look for CSS breakpoints that differ from the width you selected; a 768-pixel breakpoint will not activate at 800 pixels.
- If the application calculates dimensions only at startup, reload after resizing or trigger the same resize handler your production code uses.
The screenshot or video has the wrong outer size
- Set the headless browser screen in
before:browser:launch. - Keep the launch setting separate from
viewportWidthandviewportHeight; both may need values. - Run in the same browser family and headless mode as CI, because launch options are browser-specific.
Cypress.config() throws when changing viewport dimensions
In Cypress 16.0.0 and later, changing viewportWidth or viewportHeight through Cypress.config() while a test is executing throws an error. Use cy.viewport() for a runtime resize, or put the values in a test or suite configuration object.
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A mobile preset does not look like a real phone
A preset supplies dimensions and orientation only. It does not reproduce a physical device’s device pixel ratio or every browser and operating-system behavior. Use a real-device or dedicated device-emulation strategy when those characteristics are part of the requirement.
The CI result differs from local runs
- Pin the same viewport values in configuration or the command line rather than relying on a runner window.
- Set the headless screen explicitly when comparing screenshots or videos.
- Use identical browser family, headless mode and application build.
- Check whether a CI environment variable is overriding the configuration.
Performance, repeatability and cost considerations
Changing a viewport is inexpensive compared with starting a new browser, but visiting a page after every size still incurs the page’s normal network, JavaScript and fixture setup time. Keep shared setup in hooks, and avoid capturing full-page artifacts for every breakpoint unless visual review requires them.
For visual regression, standardize application viewport, headless screen, browser family, device pixel ratio and fonts. A mismatch in any one of these can produce image differences that are unrelated to responsive CSS. Cypress’s runner scaling should never be used as a visual-baseline setting.
For parallel CI, represent each target width as a deliberate test case or matrix value. Record the selected dimensions in the test name or artifact metadata so a failure identifies its responsive scenario immediately.
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FAQ
Can I use a named preset and then fine-tune its width?
Use the preset for its documented dimensions, then issue a numeric cy.viewport() call when the test reaches the custom case. Keep the custom value visible in the test so readers can see which breakpoint is being exercised.
Should screenshot baselines use the application viewport or the headless screen?
They should use both as fixed inputs: the application viewport determines responsive layout, while the headless screen determines the rendered browser surface and artifact dimensions. Record both in the baseline setup.
Does changing viewport dimensions emulate device pixel ratio?
No. Cypress viewport presets and numeric dimensions set CSS viewport size and orientation; they do not simulate device pixel ratio. Add a separate DPR-aware test strategy when your rendering depends on high-density displays.
Frequently Asked Questions
Can I use a named preset and then fine-tune its width?
Use the preset for its documented dimensions, then issue a numeric cy.viewport() call for the custom case and keep that value explicit in the test.
Should screenshot baselines use the application viewport or the headless screen?
Fix both: the application viewport controls responsive layout, and the headless screen controls the rendered browser surface and artifact dimensions.
Does changing viewport dimensions emulate device pixel ratio?
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