Use cy.viewport(width, height) to change the application viewport while a Cypress test is running. For a project-wide default, set viewportWidth and viewportHeight in Cypress configuration. These settings control the page’s CSS layout area; they are separate from the headless browser display size used for screenshots and videos.
Choose the right Cypress size control
“Screen size” can mean two different things in Cypress. Decide which one you need before changing a setting:
- Application viewport: the width and height available to your page’s responsive CSS and JavaScript. Use
cy.viewport(), test configuration, project configuration, CLI options, or environment variables. - Browser display size: the outer display canvas used by a headless browser. Configure it in the
before:browser:launchevent when the requirement concerns captured screenshots or videos.
Changing the browser display size does not change viewportWidth or viewportHeight. Conversely, changing the application viewport does not emulate every property of a physical device, such as its device-pixel ratio.
Change the application viewport inside a test
Call cy.viewport() before the actions and assertions that should run at the new size:
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describe('responsive header', () => {
it('shows the mobile navigation', () => {
cy.viewport(390, 844)
cy.visit('/')
cy.get('[data-testid="desktop-nav"]').should('not.be.visible')
cy.get('[data-testid="mobile-menu"]').should('be.visible')
})
})The command accepts a width and height in pixels. It can be called again later in the same test, which makes it suitable for checking several breakpoints:
it('checks desktop and mobile layouts', () => {
cy.visit('/')
cy.viewport(1440, 900)
cy.get('[data-testid="desktop-nav"]').should('be.visible')
cy.viewport(390, 844)
cy.get('[data-testid="mobile-menu"]').should('be.visible')
})Set the viewport before cy.visit() when the initial page load itself must occur at that size. If you change it after visiting, Cypress resizes the existing application and the page can react through its normal resize handlers.
Use Cypress device presets
Named presets are shorthand for documented width and height pairs:
cy.viewport('iphone-6')
cy.viewport('iphone-6', 'landscape')For landscape orientation, Cypress reverses the preset’s width and height. The current API documentation lists these preset dimensions:
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iphone-5 |
320 × 568 |
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414 × 736 |
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414 × 896 |
macbook-11 |
1366 × 768 |
macbook-13 |
1280 × 800 |
macbook-15 |
1440 × 900 |
macbook-16 |
1536 × 960 |
samsung-note9 |
414 × 846 |
samsung-s10 |
360 × 760 |
These are Cypress configuration presets, not measurements of current device market share or complete device emulation. The available names can change between Cypress versions, so verify the current API list when a preset is part of a long-lived test suite.
Test a responsive matrix of sizes
Prefer explicit dimensions that exercise your application’s actual CSS breakpoints. A device name is useful shorthand, but the assertion should verify behavior: navigation visibility, column count, menu state, or another user-visible change.
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const viewports = [
{ name: 'mobile', width: 390, height: 844 },
{ name: 'tablet', width: 768, height: 1024 },
{ name: 'desktop', width: 1440, height: 900 },
]
describe('layout at supported breakpoints', () => {
for (const size of viewports) {
it(`renders the ${size.name} layout`, () => {
cy.viewport(size.width, size.height)
cy.visit('/')
if (size.width < 768) {
cy.get('[data-testid="mobile-menu"]').should('be.visible')
} else {
cy.get('[data-testid="desktop-nav"]').should('be.visible')
}
})
}
})Include widths just below, at, and just above important breakpoints when you need boundary coverage. Avoid asserting only that an element exists; assert whether it is visible, hidden, stacked, or reordered as your design requires.
Set a default viewport in project configuration
For a baseline used by most tests, add the dimensions to cypress.config.js:
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const { defineConfig } = require('cypress')
module.exports = defineConfig({
viewportWidth: 1000,
viewportHeight: 660,
})Cypress documents 1000 × 660 pixels as the default application viewport. A project setting prevents every test from repeating the same command, while an individual test can still override it with cy.viewport().
Apply a size to one suite or test
Use test configuration when an entire suite shares a size and should return to the previous defaults afterward:
describe('tablet layout', {
viewportWidth: 768,
viewportHeight: 1024,
}, () => {
it('shows the tablet navigation', () => {
cy.visit('/')
cy.get('[data-testid="tablet-nav"]').should('be.visible')
})
})
it('uses a large desktop viewport', {
viewportWidth: 1536,
viewportHeight: 960,
}, () => {
cy.visit('/')
cy.get('[data-testid="desktop-nav"]').should('be.visible')
})This scope-specific approach keeps responsive intent next to the tests that depend on it. Cypress restores the prior defaults after the configured suite or test completes.
Override the viewport for a command-line run or CI job
Run the same test suite with different defaults without editing the configuration file:
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cypress run --config viewportWidth=1280,viewportHeight=720Cypress also maps configuration keys to environment variables. In a Unix-like shell:
export CYPRESS_VIEWPORT_WIDTH=800
export CYPRESS_VIEWPORT_HEIGHT=600
cypress runThese overrides are useful when a CI job needs a controlled baseline. Record the dimensions in the job configuration so a failed run can be reproduced locally.
Cypress 16: do not mutate viewport config during a running test
Beginning with Cypress 16.0.0, changing viewportWidth or viewportHeight through Cypress.config() while a test is executing is disallowed. Earlier behavior could affect the next test rather than the current one. Replace code such as:
// Do not use this during a test in Cypress 16+
Cypress.config('viewportWidth', 1280)
Cypress.config('viewportHeight', 720)with an in-test command:
cy.viewport(1280, 720)Or put the values in suite/test configuration, project configuration, a CLI --config override, or environment variables.
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When the preview looks smaller than the configured size
In Cypress Open Mode, the runner scales and centers the preview to fit the available pane. That visual scaling does not alter the application’s calculated viewport or responsive behavior. The Cypress interface displays the current size and scale. To investigate a layout failure, trust measurements and assertions in the page rather than the apparent physical size of the runner pane.
Control the headless browser display size
If your goal is the canvas of a headless screenshot or video, configure the browser launch options in the before:browser:launch event. This is separate from the page viewport:
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// cypress.config.js
const { defineConfig } = require('cypress')
module.exports = defineConfig({
e2e: {
setupNodeEvents(on, config) {
on('before:browser:launch', (browser, launchOptions) => {
if (browser.isHeadless) {
launchOptions.args.push('--window-size=1440,900')
}
return launchOptions
})
},
},
})The exact launch argument depends on the browser and execution mode. Consult Cypress’s browser-launch API for the supported event details. Keep the application dimensions explicit with cy.viewport() or configuration even when you also set a display size.
Screenshot behavior and reliable capture
Use Cypress’s screenshot API after the page reaches the state you intend to capture:
cy.get('[data-testid="report"]').should('be.visible')
cy.screenshot('report-desktop', { capture: 'fullPage' })A full-page capture can include content beyond the current viewport, but it does not turn a device preset into physical-device emulation. If a screenshot is the deliverable, distinguish the page layout viewport from the browser’s display canvas and document both settings in CI.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Troubleshooting viewport problems
The layout does not change after calling cy.viewport()
- Call the command before the assertion that depends on the new size.
- If the initial load must use the size, call it before
cy.visit(). - Check that your application listens to resize events and that the breakpoint is actually crossed.
- Assert on responsive behavior rather than a device label.
Cypress.config() throws an error
On Cypress 16 and later, runtime changes to viewportWidth and viewportHeight are not permitted. Use cy.viewport() or scoped test configuration instead.
The Open Mode preview appears too small
The runner is scaling the preview to fit its pane. Check the displayed viewport dimensions and your application’s computed layout; the scaling itself does not change test behavior.
The screenshot or video has the wrong outer dimensions
You may have changed only the application viewport. Configure the headless browser display through before:browser:launch, then keep the application viewport setting separate.
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A named preset is unknown
Preset names are version-sensitive. Check the current Cypress viewport API, or replace the name with an explicit width and height.
CI results differ from local results
Compare Cypress versions, browser choice, project defaults, CLI overrides, environment variables, and headless display settings. Print or otherwise record the intended viewport for each job so the effective configuration is visible in logs.
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FAQ
Does cy.viewport() change device pixel ratio?
No. It changes the application’s CSS viewport dimensions; it is not complete physical-device emulation.
Can I use a different viewport for each test?
Yes. Call cy.viewport() in each test or provide dimensions in that test’s configuration.
Should I test named devices or numeric breakpoints?
Use names for readable shorthand, but include numeric widths that correspond to your application’s actual responsive breakpoints.
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