Chrome DevTools: Fix Browser.setContentsSize Not Found Error
If you're a web developer or QA tester working with Chrome DevTools to simulate mobile viewports, you might have encountered a frustrating error: Protocol error (Browser.setContentsSize): 'Browser.setContentsSize' wasn't found. This error typically surfaces when you attempt to use a command like resize_page with specific mobile dimensions, only to have your testing process abruptly halted. It's a common stumbling block, especially when leveraging tools like Chrome DevTools MCP or similar frameworks that interact with the Chrome Debugging Protocol. The root cause often lies in compatibility issues between the version of Chrome you're using and the specific DevTools Protocol commands expected by your testing tools. Understanding this error is the first step toward resolving it and ensuring your mobile testing workflows run smoothly.
Understanding the Browser.setContentsSize Error
The Browser.setContentsSize not found error message you're seeing directly indicates that the Chrome DevTools Protocol (CDP) command your tool is trying to execute, Browser.setContentsSize, is not recognized by the version of Chrome you're connected to. This is a crucial point because the CDP is constantly evolving. New commands are added, existing ones are modified, and some might even be deprecated or removed across different Chrome versions. Your testing framework, like Chrome DevTools MCP in this scenario, is programmed to use a specific set of CDP commands. When it encounters a Chrome version that either doesn't support Browser.setContentsSize or has replaced it with a different command, this error occurs. It's akin to trying to use an old remote control with a brand-new TV; the buttons just don't map to the expected functions anymore. The context provided mentions using this command for mobile viewports, which suggests that resizing the page's content dimensions is the intended operation. The fact that it's failing specifically when trying to mimic mobile sizes (e.g., width=414, height=896) is a strong clue. Mobile emulation often involves more than just changing the window size; it might require adjusting rendering, pixel density, and other properties that the setContentsSize command was intended to handle. The environment details, such as Chrome version being unknown (remote debugging) and running on Linux (WSL2), are important. Remote debugging can sometimes lead to unexpected version mismatches, and different operating systems, while less likely to cause protocol command issues directly, can influence the overall setup. The key takeaway here is that the interface between your testing tool and the Chrome browser has a mismatch in its communication language, specifically around the command for setting content dimensions for resizing.
Why Mobile Viewport Testing Fails with Browser.setContentsSize
When you're diving into mobile viewport testing, the goal is to accurately replicate how your website or application will look and behave on various mobile devices. This isn't just about changing the window dimensions; it involves emulating specific screen resolutions, pixel densities (like Retina displays), and sometimes even touch events and network conditions. The Browser.setContentsSize command, when it was available and supported, was likely part of the mechanism to achieve this precise content scaling within the browser's rendering engine. The error Protocol error (Browser.setContentsSize): 'Browser.setContentsSize' wasn't found points to a significant disconnect. It means the CDP version your resize_page function is trying to use has been superseded or changed. In recent versions of Chrome, the way the browser handles viewport resizing and content scaling might have shifted. For instance, instead of a direct setContentsSize command, the functionality might now be handled by other commands, possibly within the Page domain or a newly introduced Emulation domain, that are designed to offer more granular control. This could involve commands related to setViewport which often takes parameters for width, height, deviceScaleFactor, and mobile. The mobile boolean, in particular, is critical for enabling mobile-specific rendering and CSS media queries. The error could also arise if the resize_page function itself is using an outdated CDP client library that hasn't been updated to reflect these changes in the Chrome browser. So, even if your Chrome version does support the functionality, if the tool you're using to access that functionality is using outdated commands, you'll still hit this wall. The fact that the workaround mentioned is using emulate for CPU/network throttling but not viewport sizes further reinforces that the issue is specifically tied to the resizing aspect of mobile emulation. The emulate command in CDP often targets performance characteristics rather than visual layout, indicating a separate part of the emulation suite is working, but the visual viewport adjustment is broken due to this protocol command mismatch. Debugging this often requires looking at the Chrome DevTools Protocol Change Log to see when and how viewport manipulation commands have evolved.
Finding a Solution: Updated Commands and Workarounds
When faced with the Browser.setContentsSize not found error, the primary solution often involves updating your tools or adapting your code to use the current Chrome DevTools Protocol commands. Since Browser.setContentsSize appears to be deprecated or removed, you'll need to find its modern equivalent. The most common replacement for viewport manipulation, including mobile emulation, is found within the Page domain, specifically using the Page.setViewport command. This command allows you to set the width, height, deviceScaleFactor, and importantly, a mobile boolean. To use this effectively, you would replace your resize_page call with a sequence that targets Page.setViewport. For example, if your resize_page function was internally trying to call Browser.setContentsSize, you would modify it to instead send a command like: `{
