Brick Dance: Choreographing LEGO Builds from Motion Specifications

Large language models write working software, yet they cannot reliably build working machines. The hard part of mechanical engineering is rarely a single part: it is an assembly of many parts whose connections must produce the motion a specification demands. We study this gap in LEGO, since its parts are fixed, expert-designed assemblies are plentiful, and, as we show, a machine can check whether an assembly holds together and moves as asked. Prior work on LEGO treats a build as a static shape, examining only whether its bricks connect and stand. We treat it as a mechanism, whose parts must move as specified: a door that opens 140° and carries twenty-five pieces with it. Such requirements are much closer to real mechanical engineering. To evaluate them, we build a decompiler that recovers a LEGO model's connections, its rigid bodies, and how far each joint turns before it collides, from its raw geometry. It reads 1,464 human-built models, and from any of them it derives a functional requirement specification without human authoring, which is what Brick Dance grades against, to our knowledge the first LEGO benchmark to evaluate motion and editing. Finally, we propose Brick Dance Copilot, a coding agent that uses our decompiler as tools and feedback: holding the model and the budget fixed, it submits a valid assembly on 89% of runs against 47% for an unmodified coding agent, and the gap is almost entirely physical.