Browser Viewport Presets

The embedded browser pane can be resized to specific viewport dimensions for responsive testing. This is a real viewport resize through CDP — CSS media queries fire at the new width, window.innerWidth reports the new size, and screenshots capture at the simulated dimensions.

The earlier workaround (wrapping the page in a 375px iframe) is gone — viewport presets replace it.

CLI

codemux browser viewport <preset>
codemux browser viewport <WxH>
codemux browser viewport reset
codemux browser viewport-presets   # list available presets

Examples:

codemux browser viewport mobile           # iPhone-sized
codemux browser viewport tablet           # iPad-sized
codemux browser viewport desktop          # 1280x800
codemux browser viewport desktop-large    # 1920x1080
codemux browser viewport 375x812          # custom dimensions
codemux browser viewport reset            # snap back to the actual pane size

The preset names are deliberately size buckets, not device models. Specific device names ("iPhone 17 Pro Max") go stale every hardware refresh; mobile and tablet don't.

MCP

Agents can hit the same surface via MCP:

• browser_viewport — apply a preset, custom WxH, or reset
• browser_viewport_presets — list all available presets with their dimensions

When To Use It

• CSS / Tailwind breakpoints — change the viewport and see whether your md: / lg: rules fire.
• Touch UIs — simulate a touch-screen layout in the same pane you already have open.
• Screenshot regressions — capture screenshots at exact dimensions for visual diffing.
• Responsive bug reproduction — quickly switch between mobile and desktop without opening DevTools.

Behavior

• The resize applies a real CDP Emulation.setDeviceMetricsOverride plus an actual viewport resize, so device-pixel-ratio (DPR) updates too.
• The browser pane keeps showing the same screenshot stream; the rendered page now matches the chosen dimensions.
• reset restores the viewport to the actual rendered pane size — the natural state.

Related

• Browser — the embedded browser pane
• Browser Agent Commands — full CLI reference for browser control