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Kernel browsers allow you to configure the viewport size and refresh rate when creating a browser session. The viewport configuration determines the initial browser window dimensions and display refresh rate. The refresh rate can be explicitly specified or automatically determined based on the width and height if they match a supported configuration.

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Default viewport

If the viewport parameter is omitted when creating a browser, the default configuration is 1920x1080 at 25Hz. 
// Uses default viewport (1920x1080@25Hz)
const defaultViewport = await kernel.browsers.create();

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Setting viewport configuration

You can configure the viewport when creating a browser by specifying the viewport parameter with width and height. The refresh_rate is optional and will be automatically determined from the dimensions if they match a supported configuration: 
import Kernel from '@onkernel/sdk';

const kernel = new Kernel();

// Explicitly specify refresh rate
const kernelBrowser = await kernel.browsers.create({
  viewport: {
    width: 1920,
    height: 1080,
    refresh_rate: 25
  }
});

// Auto-determine refresh rate from dimensions (25Hz for 1920x1080)
const kernelBrowserAuto = await kernel.browsers.create({
  viewport: {
    width: 1920,
    height: 1080
  }
});
The refresh_rate parameter only applies to live view sessions and is ignored for headless browsers.

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Supported viewport configurations

Kernel supports specific viewport configurations. The server will reject unsupported combinations. When you provide width and height without specifying refresh_rate, it will be automatically determined if the dimensions match one of the supported resolutions exactly. The following resolutions are supported:
ResolutionWidthHeightRefresh Rate
QHD2560144010 Hz
Full HD1920108025 Hz
WUXGA1920120025 Hz
WXGA+144090025 Hz
WXGA120080025 Hz
XGA102476860 Hz
Higher resolutions may affect the responsiveness of live view browser sessions.

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Example configurations

// Full HD (1920x1080) at 25Hz - explicit refresh rate
const fullHD = await kernel.browsers.create({
  viewport: {
    width: 1920,
    height: 1080,
    refresh_rate: 25
  }
});

// Full HD (1920x1080) - auto-determined 25Hz (Default configuration)
const fullHDAuto = await kernel.browsers.create({
  viewport: {
    width: 1920,
    height: 1080
  }
});

// QHD (2560x1440) - auto-determined 10Hz
// Note: May affect live view responsiveness
const qhd = await kernel.browsers.create({
  viewport: {
    width: 2560,
    height: 1440
  }
});

// XGA (1024x768) - auto-determined 60Hz
const xga = await kernel.browsers.create({
  viewport: {
    width: 1024,
    height: 768
  }
});

// WUXGA (1920x1200) at 25Hz - explicit refresh rate
const wuxga = await kernel.browsers.create({
  viewport: {
    width: 1920,
    height: 1200,
    refresh_rate: 25
  }
});

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Viewport constraints

Only the specific viewport configurations listed in the supported configurations table above are supported:
  • 2560x1440 (QHD) at 10 Hz
  • 1920x1080 (Full HD) at 25 Hz
  • 1920x1200 (WUXGA) at 25 Hz
  • 1440x900 (WXGA+) at 25 Hz
  • 1200x800 (WXGA) at 25 Hz
  • 1024x768 (XGA) at 60 Hz
When specifying a viewport:
  • Width and Height are required and must match one of the supported configurations exactly
  • Refresh Rate is optional - if omitted, it will be automatically determined from the width and height combination
The server will reject any viewport configuration that doesn’t exactly match one of the supported combinations listed above.

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Changing viewport after browser creation

You can change the viewport of a browser after it has been created using the update browser endpoint. 
// Create a browser with default viewport
const kernelBrowser = await kernel.browsers.create();

// Later, change the viewport
await kernel.browsers.update(kernelBrowser.session_id, {
  viewport: {
    width: 1024,
    height: 768
  }
});
There are important limitations when changing the viewport:
  • Headful browsers: You cannot resize the viewport while live view is active or while a replay is actively recording.
  • Headless browsers: Changing the viewport triggers a Chromium restart, which may disrupt active CDP connections.

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Considerations

  • The viewport configuration is set when the browser is created and applies to the initial browser window
  • Higher resolutions (like 2560x1440) may impact the performance and responsiveness of live view sessions
  • The viewport size affects how websites render, especially those with responsive designs
  • Screenshots taken from the browser will match the configured viewport dimensions
  • In headless mode, the viewport width and height still apply for rendering and screenshots, but the refresh_rate parameter is ignored