Set the user agent and viewport before the page loads, then capture the visible viewport or the full page. A user-agent string identifies the browser or device to the site; the viewport dimensions determine the responsive layout. For repeatable captures, use Playwright or Puppeteer. For a one-off check, use Chrome DevTools Device Mode.
What to set before you capture
Decide which browser engine, user-agent string, and viewport dimensions the test needs. A device preset can bundle a user agent with screen and viewport dimensions, touch support, and other emulation settings. If you need a custom user agent or exact dimensions, inspect and override the preset rather than assuming it matches your target. Playwright’s device emulation documentation explains the configurable parameters and viewport overrides: Playwright emulation.
- User agent: the browser/device identity sent to the site.
- Viewport: the page’s CSS layout area; this is what usually determines responsive breakpoints.
- Device scale factor and touch: relevant when the target behavior depends on pixel density or touch input.
- Capture extent: choose the visible viewport, a specific element, or the full scrollable page.
Changing only the user agent does not set a mobile-sized viewport. For a responsive layout test, set both deliberately.
Use Playwright for a repeatable screenshot
Install Playwright in a Node.js project and install its Chromium browser. The example below uses a custom user agent and viewport, applies them before navigation, waits for the page to load, and saves a full-page PNG.
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npm install playwright
npx playwright install chromiumconst { chromium } = require('playwright');
(async () => {
const browser = await chromium.launch();
const context = await browser.newContext({
userAgent: 'Mozilla/5.0 (iPhone; CPU iPhone OS 17_0 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/17.0 Mobile/15E148 Safari/604.1',
viewport: { width: 390, height: 844 },
deviceScaleFactor: 2,
isMobile: true,
hasTouch: true
});
const page = await context.newPage();
await page.goto('https://example.com', { waitUntil: 'networkidle' });
await page.screenshot({ path: 'responsive.png', fullPage: true });
await browser.close();
})();Replace the user-agent string, dimensions, and URL with the values for your test. The example’s string is an input, not a guarantee that the capture will match a particular physical iPhone. If you need a preset, spread the selected Playwright device descriptor into browser.newContext(), then place any overrides after it so your custom values take precedence. Check the descriptor’s user agent: Playwright notes that a preset may use a platform-specific string, and recommends leaving the preset user agent undefined when you want the running test platform’s own user agent.
Choose the screenshot extent
await page.screenshot({ path: 'viewport.png' })captures the visible viewport.await page.screenshot({ path: 'full.png', fullPage: true })captures the whole scrollable page.await page.locator('main').screenshot({ path: 'element.png' })captures one element.
Playwright documents viewport, full-page, and element screenshot options at Playwright screenshots.
Handle delayed and lazy-loaded content
networkidle can be a useful wait condition, but it is not proof that every image or animation is ready. Some sites keep network requests open, while lazy-loaded media may not load until it approaches the viewport. If a particular component matters, wait for it explicitly with await page.locator('.target').waitFor(). For content lower on the page, scroll through the document to trigger lazy loading before taking a full-page capture, or otherwise trigger the relevant media and verify the saved image.
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Use Puppeteer when your automation already uses it
Puppeteer’s Page.emulate(device) applies a known device profile’s metrics and user agent. Its documentation describes the method as a shortcut for Page.setUserAgent() and Page.setViewport(), and advises applying emulation before navigation because some sites do not expect a phone-sized page to be resized after loading: Puppeteer Page.emulate().
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(async () => {
const browser = await puppeteer.launch();
const page = await browser.newPage();
await page.setUserAgent(
'Mozilla/5.0 (iPhone; CPU iPhone OS 17_0 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/17.0 Mobile/15E148 Safari/604.1'
);
await page.setViewport({ width: 390, height: 844, deviceScaleFactor: 2 });
await page.goto('https://example.com', { waitUntil: 'networkidle2' });
await page.screenshot({ path: 'responsive.png', fullPage: true });
await browser.close();
})();For a built-in device profile, import the profile exposed by your installed Puppeteer version and call await page.emulate(profile) before page.goto(). For an arbitrary user agent or custom dimensions, set the user agent and viewport explicitly as above. The available device profiles can change between releases; use the profile names provided by the version installed in your project.
Capture manually with Chrome DevTools
Device Mode is convenient for an individual visual check. Open the page in Chrome, open DevTools, and enable the device toolbar. Select Responsive or a device preset, then set the viewport dimensions. Device Mode also provides controls for device pixel ratio, user-agent string, and device type; set the fields that matter to your test. Chrome’s guide is at Chrome DevTools Device Mode.
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Use the DevTools screenshot controls to capture the page in the emulated view. Device-specific screenshot capture can include a frame when the device frame is enabled. The precise controls can vary with Chrome versions, so consult the current Device Mode guide if a menu label differs.
Choose the right workflow and verify the result
| Workflow | Best fit | What to control |
|---|---|---|
| Playwright | Repeatable captures across URLs or regression checks | Browser engine, device/context settings, user agent, viewport, and screenshot extent |
| Puppeteer | Projects already using Puppeteer or a known device profile | Apply device emulation or explicit user-agent and viewport settings before navigation |
| Chrome DevTools Device Mode | A quick manual inspection | Responsive dimensions, device pixel ratio, user-agent string, and device type |
These workflows emulate selected browser parameters; they do not establish that every behavior matches a physical phone. If the question depends on hardware-specific behavior, confirm it on the target device. For practical visual checks, make sure the captured page has the expected breakpoint, content, images, and menus before treating the screenshot as evidence.
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Check the viewport width, not just the user-agent string. Make sure the chosen preset or explicit context sets the intended dimensions and that custom overrides are applied before navigation.
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The first load differs from later captures
Apply emulation before opening the target URL. Puppeteer specifically cautions that resizing after navigation can affect pages that do not expect a phone-size change. With Playwright, create the context with the desired settings before creating or navigating the page.
Images or page sections are missing
Wait for the required element, trigger lazy loading by scrolling, and allow any relevant animation or network activity to finish. A full-page screenshot does not guarantee that off-screen lazy content was loaded beforehand.
The screenshot has the wrong size or extent
Distinguish viewport capture from full-page capture. Check the viewport width and height and, where relevant, device scale factor. A full-page capture is taller than the viewport by design; use the default viewport screenshot when only the visible region is required.
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The emulated page does not match a real device
Confirm the browser engine and device parameters required by the test. Emulation is useful for responsive checks, but physical-device behavior can depend on hardware or browser behavior outside the settings being emulated; verify on the device when that distinction matters.
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For a one-call capture, ScreenshotNeo is a website screenshot API and MCP server. It accepts a URL and returns a PNG, JPEG, WebP, or PDF. To request a particular viewport and user agent, use the supported request parameters; see the ScreenshotNeo API documentation for parameter names and options.
curl -G "https://api.screenshotneo.com/v1/shot"
-d access_key=YOUR_API_KEY
--data-urlencode url=https://example.com
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Frequently Asked Questions
Does changing the user agent alone make a page responsive?
No. The user agent identifies the browser or device to the site; set the viewport dimensions as well to test a specific responsive layout.
Should I test a screenshot on a real phone?
Use browser emulation for routine responsive checks. If the result depends on hardware-specific behavior, verify it on the target physical device.
Quick Recap
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