Same double-pendulum prompt, same host renderer, and two models picked opposite θ conventions. You can see it within seconds.

I ran the same double pendulum generation contract against Claude 3.5 Sonnet and DeepSeek V3 on OpenRouter, both under identical initial conditions (θ1 = π/2, θ2 = π/2, both angular velocities zero). The host renderer in public/workers/simulator-host.js reads info.theta1 and info.theta2 from whatever the model's getInfo returns, then draws both bobs using a fixed pivot at top center and a fixed scale derived from L1+L2. It does not care what convention the model used internally. It just plots the angle it receives.