> ## Documentation Index
> Fetch the complete documentation index at: https://docs.tester.army/llms.txt
> Use this file to discover all available pages before exploring further.

# Viewport Size

> Run web tests at different browser viewport sizes to check responsive layouts on desktop, mobile, and tablet.

Use test settings to run a web test at a different browser viewport size. The viewport is fixed before the run starts; the agent cannot resize the browser, page, screen, window, or viewport from inside a test step.

## Supported Viewport Sizes

Web tests run at one of these presets. The API and MCP tools select a preset by name:

| Preset name | Size (CSS pixels) |
| - | - |
| `desktop` | 1280 x 800 |
| `desktop-hd` | 1920 x 1080 |
| `mobile` | 390 x 844 |
| `tablet` | 768 x 1024 |

Pick `desktop-hd` when the UI under test is dense or cramped at 1280 x 800 - for example wide data grids or multi-column dashboards - so the agent sees it at its intended size.

Other sizes are not supported: the browser infrastructure runs at a fixed set of display resolutions, so the API rejects anything but these preset names.

## Change Viewport Size

1. Open the project that contains the test.
2. Open the test you want to run at a different size.
3. Open **Test settings**.
4. Select **Viewport Size**.
5. Save the test.
6. Run the test again.

The selected viewport applies to future runs of that test. Existing runs keep the viewport they started with.

## Writing Test Steps

Do not add steps such as "resize the page to mobile" or "change the browser to 390px wide". Instead, configure the test viewport first, then write steps that describe the user behavior to verify at that size.

If a run fails with `VIEWPORT_RESIZE_UNSUPPORTED`, update the test settings and retry the run.


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