Hey everyone 👋 I've been fascinated by system dynamics for a while — stocks, flows, feedback loops — but every tool I tried (Vensim, Stella, InsightMaker) shows you the result as a line chart. You have to read the system instead of watching it happen. After seeing what napkin.ai did for turning text into visual diagrams, I started wondering what the equivalent would look like for simulation instead of just diagrams — something you could actually watch behave, not just plot. That turned into Nirai. Instead of charts, the simulation renders as real water flowing through tanks and pipes in the browser. Things like the bullwhip effect — oscillations that amplify as they move through a supply chain — are hard to catch in a line chart and hard to miss once you've watched the water surge and lag in real time. You can also export the whole animated simulation as an MP4, which was the actual reason I started building this — I wanted something I could drop into a lecture slide or a client deck instead of a screenshot of a graph. Under the hood: the solver is Rust compiled to WebAssembly running fully client-side (RK4 integration, fasteval2 for the formula engine — trig functions, lookup tables, random/stochastic functions), so sliders update the simulation instantly with no server round-trip. There's also an AI assistant that can propose actual stocks and flows onto your canvas, not just chat — it uses your own API key (OpenAI/Gemini/OpenRouter), so nothing goes through our servers and there's no usage cap. It's free to try — 3 models, up to 1,000 simulation steps, watermarked export. Pro is a one-time $99 (no subscription) for unlimited models and clean exports. I built this solo, so I'd genuinely love feedback — especially from anyone who's used Vensim/Stella/InsightMaker before and can tell me what's missing, or anyone in game design/education/climate work who can think of a model worth trying it on. Happy to answer anything about the WASM/Rust side too if that's of interest.

Nirai

Watch your system dynamics model flow, not just chart

Watch your system dynamics model flow, not just chart