AgentDreamer : Agentic 3D Motion Reasoning for Text-Controllable Physics Simulation

We propose AgentDreamer, an agent-guided optimization framework that aligns text instructions with simulated motion through explicit 3D motion planning. Given a text instruction and scene geometry, an off-the-shelf agent translates the instruction into a structured semantic motion prior, specifying what should move, in which direction, and in what temporal order. The agent represents this prior using parametric functions over a series of temporal chunks, yielding dense 3D trajectories. We then optimize the parameters of a differentiable MPM simulator such that its rollout best matches the prior while remaining constrained by the scene dynamics. This allows the agent to provide semantic guidance from language, while the simulator grounds it in physically feasible motion. We further introduce Motion-IG, an information-theoretic metric for text-motion alignment based on masked text recovery. By comparing the likelihood of recovering masked motion-related content with and without the simulated motion, Motion-IG measures the information contributed by the motion beyond that available from the remaining text and scene context. Experiments show that AgentDreamer improves text alignment over video-based guidance, producing simulations that more closely follow the requested physical behavior.