MuJoCo 4.0 Measured: 2,959× Better Integration, One Silent Actuator Change, and a Load Error That Never Says Why

Wed, 11 Nov 2026 00:00:00 +0000

MuJoCo 4.0.0 landed on 6 November 2026. The release notes are unusually long: 61 numbered items across ten sections, four of which are headed Breaking changes. The announcement thread offers a short upgrade section that opens with five words that decide whether your week is fine or ruined: “Most models load unchanged.”

That sentence is true for most models. It was not true for the model I tried first, and it was not written nearly as loosely as it reads.

This is an audit by measurement, not by summary. I installed mujoco==3.17.0 and mujoco==4.0.0 in two separate Python 3.14 environments on the same Linux box, ran the same models in both, and recorded what changed. Every number below is from a run on this machine between 8 and 11 November 2026, not from a vendor benchmark.

Breaking Changes on SoloSoft

MuJoCo 4.0 Measured: 2,959× Better Integration, One Silent Actuator Change, and a Load Error That Never Says Why