A coding agent can write the markup, wire the styles, and ship a page in seconds. It can even look at that page. What it cannot do is measure it against the design, pixel for pixel. Glancing at a screenshot, it will not catch that the heading sits a few pixels too low, that the color has cooled by a shade, that the font quietly failed to load. It builds by eye, and the page drifts from the design with nothing exact to catch it.
Design Diff gives it that measure. It lays the design over a screenshot of the live page at the exact same pixel size, then returns one number for what matched and the box around what did not. Call it, read the score, fix what the box points at, call it again. That loop runs until the page matches the design, and no one has to be watching.
It is built agent-first, a command and a library that speak plain JSON, so any agent or script can drive it. The same signal serves people just as well, from developers closing a handoff to design engineers building a system to designers who want proof rather than a promise.
Install
It runs on Bun and needs no global install, since bunx fetches and runs it in one step. The first run downloads Chromium once. Every run after that is instant.
