I Let a Coding Agent Render Android UI Without an Emulator. A Screenshot Wasn't Enough.
When a coding agent changes Android XML, it is fairly confident about what it can read: it finds the layout, understands the constraints, and produces a diff. But there was almost always one more manual step afterwards. I would open the screen and decide whether the button, the text, or the card had actually ended up in the right place. I wanted to remove that repeated switch. I was not trying to…
A coding agent was able to render Android UI layouts without the need for an emulator, though screenshots alone proved insufficient. The agent changes XML, understands the layout constraints, and generates a diff. The goal was to eliminate the manual step of checking whether UI elements appeared in the correct place. Initially, a screenshot was used to verify the rendering, but it was not enough.
The MCP server, built with Kotlin/JVM and Robolectric, was created to automate the rendering process. The agent supplied a layout and data, and received the rendered result back. The screenshot was helpful, but the tool was incomplete. The agent could not accurately measure distances, check if elements went beyond screen boundaries, or identify specific Views with IDs.
The issue was that the image alone did not provide verifiable data. The solution was to render the layout, create a session, retrieve the View tree, and inspect a specific node by its ID. This allowed the agent to receive precise measurements and data, such as the position of a button relative to its container. This loop enabled the agent to move from visual inspection to measurable facts, facilitating continued changes and fixes.
The project was limited to XML/View projects and did not replace an emulator or device, as Robolectric does not guarantee a pixel-perfect match. The next steps involve capturing more detailed information, including prompts, tool calls, render IDs, XML differences, measurements, and verdicts. The coding agent played a crucial role in developing and testing this loop, ensuring it met the defined limits and performed verifiable criteria.
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