Aexus Answers

Fix: Headless Chrome Viewport & Resize Issues

Alex Johnson
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Fix: Headless Chrome Viewport & Resize Issues

Hey everyone, let's talk about a pretty annoying bug that pops up when you're trying to use the --viewport option or the resize_page tool in Chrome's headless mode. If you've been wrestling with this, you're not alone! This article is all about the issues, the root cause, and some potential fixes. We'll also discuss the workarounds to keep your automated testing and web scraping projects running smoothly. Let's dive in!

The Problem: --viewport and resize_page Fail in Headless Mode

So, what's the deal? When you're running Chrome in headless mode (meaning without a visible browser window), and you try to set the viewport size using the --viewport option or the resize_page tool, you're likely to run into trouble. You'll get a protocol error that looks something like this: "Protocol error (Browser.setContentsSize): 'Browser.setContentsSize' wasn't found." This error basically means that Chrome can't find the command it needs to resize the browser window. This is a major bummer because controlling the viewport size is crucial for tasks like taking screenshots at specific resolutions or testing responsive web designs. Let's break down the scenario a bit more:

Environment Breakdown

  • chrome-devtools-mcp version: 0.5.1 (The latest version as of this report)
  • Chrome version: 139.0.7258.138
  • Operating System: Ubuntu 22.04.5 LTS
  • Configuration: --headless=true --isolated=true --viewport=1280x720

Steps to Reproduce

To see this error yourself, you can follow these steps:

  1. Configuration: Set up your .mcp.json file to include the --viewport option. For example:

    {
  "chrome-devtools": {
    "command": "npx",
    "args": [
      "chrome-devtools-mcp@latest",
      "--channel=stable",
      "--headless=true",
      "--isolated=true",
      "--viewport=1280x720"
    ]
  }
}

  2. Restart MCP Server: Make sure to restart the MCP server after making any configuration changes.

  3. Try to Use a Tool: Attempt to use any tool that relies on viewport adjustments, like list_pages, navigate_page, or resize_page.

Expected Behavior

Ideally, here's what should happen when you run these commands:

  • The --viewport option should set the initial viewport size to the dimensions you specify (e.g., 1280x720).
  • The resize_page tool should allow you to dynamically change the page's viewport size.
  • All other tools should continue to function as expected, without any errors.

Actual Behavior

Instead of the expected behavior, you'll encounter an error. You will receive this error message when you attempt to use the tools:

  • All tools fail with the error: Protocol error (Browser.setContentsSize): 'Browser.setContentsSize' wasn't found

Root Cause: The Browser.setContentsSize Mystery

Now, let's get to the heart of the problem: why does this happen? After some digging into the source code, we can pinpoint the root cause. The issue lies in how Chrome DevTools Protocol (CDP) handles viewport resizing in headless mode. The --viewport initialization and the resize_page tool both use an internal API call page.resize(). This page.resize() function ultimately calls Browser.setContentsSize from the Chrome DevTools Protocol. The real kicker? Browser.setContentsSize is marked as Experimental in CDP and appears to be unavailable in headless mode. That explains why the command isn't found, leading to the protocol error. This method is designed to resize the browser window, but in headless mode, where there is no visible window to resize, it simply doesn't work.

Technical Deep Dive

  • src/browser.ts: This file is where the viewport is initialized using page.resize(). This sets the initial size of the browser window, but, as we know, it triggers the problematic Browser.setContentsSize call.
  • src/tools/pages.ts: This is where the resize_page tool is defined, which also uses page.resize(). So, when you try to use this tool, it's calling the same internal API, leading to the same error.

Suggested Solutions: Bypassing the Browser.setContentsSize Roadblock

So, how do we fix this? There are a couple of potential solutions:

Option 1: Embrace Emulation.setDeviceMetricsOverride

The more robust and widely accepted approach is to use Emulation.setDeviceMetricsOverride. This is the standard CDP method for setting the viewport size, and the best part is that it works perfectly fine in headless mode! It's what tools like Puppeteer and Playwright use for viewport control. Here's how it would look in code:

await page._client().send('Emulation.setDeviceMetricsOverride', {
  width: options.viewport.width,
  height: options.viewport.height,
  deviceScaleFactor: 1,
  mobile: false,
});

This method allows you to specify the desired width, height, and other device-related settings. Using this method will help ensure that your scripts can accurately set and control the viewport size in headless Chrome, avoiding the errors.

Option 2: Document the Limitation (And Offer a Workaround)

If switching to Emulation.setDeviceMetricsOverride isn't feasible right away, then documenting the limitation is a necessary step. The documentation should clearly state that the --viewport and resize_page tools don't function in headless mode. It should also provide a workaround or an alternative approach. This could involve:

  • Directly using Emulation.setDeviceMetricsOverride
  • Suggesting alternative tools that handle viewport resizing
  • Providing example code for setting the viewport using Emulation.setDeviceMetricsOverride

Additional Context: Why This Matters

Without a working --viewport option, the default viewport in headless mode is often set to something like 780x493. This default size can be a major problem for several reasons. It makes it difficult to capture screenshots at standard desktop resolutions (like 1280x720 or 1920x1080). If you're working on responsive web design, this limitation can also hinder your testing efforts. Because the default size isn't representative of common screen sizes, you can't easily ensure that your web pages look and function correctly on all devices.

Summary

In summary, the --viewport option and the resize_page tool fail in headless Chrome due to the use of Browser.setContentsSize, an experimental and unavailable method in headless mode. The best solution is to switch to Emulation.setDeviceMetricsOverride. If that's not immediately possible, clear documentation of the limitations is crucial. By addressing this issue, we can ensure that headless Chrome tools function reliably for tasks like automated testing, web scraping, and more.

Further Reading: For more in-depth information, you can refer to the official Chrome DevTools Protocol documentation, specifically the section on Browser.setContentsSize.

External Links:

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