tau-sim

LLM-assisted MuJoCo simulation IDE — runs in your browser, locally on your laptop, or as a hosted demo. Edit MJCF / Python on the left, watch episodes render live in the center, and ask any model (OpenRouter / OpenAI / Gemini) to propose changes that appear as reviewable diffs.

Built by Tau Intelligence.

Try it

One command, anywhere with Python ≥ 3.10

uvx tau-sim     # or: pipx run tau-sim

That installs everything (FastAPI, MuJoCo, OSMesa-friendly bindings) in an isolated environment and opens http://127.0.0.1:7860 in your browser. Project files are stored under ~/.tau/projects/.

If you've already got uv, you can also do:

uv tool install tau-sim
tau --help

From source

cd backend
python -m venv .venv && source .venv/bin/activate
pip install -e .
cp .env.example .env       # (optional) configure default LLM provider
tau

Frontend dev server

cd frontend
npm install
npm run dev      # http://localhost:5173 (proxies /api and /ws to :8000)

When developing the frontend separately, run the backend via uvicorn app.main:app --reload --port 8000 so the Vite proxy targets the right port.

Architecture

┌──────────────────────────┐         ┌──────────────────────────────┐
│ React + Vite + Monaco    │  HTTP   │ FastAPI                      │
│  - Editor (MJCF / Py)    │ ──────▶ │  /api/projects   (CRUD)      │
│  - Viewer (frames)       │         │  /api/chat       (LLM diff)  │
│  - Chat + Diff review    │ ◀────── │  /ws/sim/{id}    (frames)    │
│  - BYOK settings         │   WS    └──────────────┬───────────────┘
└──────────────────────────┘                        │
                                          ┌─────────▼────────┐
                                          │ subprocess sand- │
                                          │ box (cloud only) │
                                          └─────────┬────────┘
                                                    │
                                            ┌───────▼────────┐
                                            │ MuJoCo runner  │
                                            │ (offscreen GL) │
                                            └────────────────┘

Deploy as a single container

The repo ships a multi-stage Dockerfile that builds the React app, installs MuJoCo + OSMesa, and serves the SPA + API + WS from one FastAPI process on $PORT (default 7860).

docker build -t tau-sim .
docker run --rm -p 7860:7860 tau-sim
# open http://localhost:7860

The container defaults to multi-tenant mode: each visitor gets a cookie session, projects are stored under /tmp/projects_data/<session>/, and user env code runs inside a subprocess sandbox with CPU + wall-clock limits. For local single-user runs (your laptop), the tau CLI sets TAU_SINGLE_USER=1 automatically and disables the sandbox for speed.

Hugging Face Spaces (free)

1. Create a new Space → SDK Docker.
2. Push this repo. Rename SPACE_README.md to README.md at the Space root (or merge its YAML header into the existing README before pushing).
3. The Space builds the Docker image and exposes the app on its *.hf.space URL.

No API keys are baked into the image — users add their own via the in-app Settings panel (stored only in their browser).

Configuration (env vars)

Security note

env.py is executed by the backend whenever you press Run. The default Docker deployment runs it in a subprocess with rlimits — that protects CPU / disk / wall-clock but not arbitrary syscalls or network. For real public exposure also run the container with --memory, --pids-limit, and a no-egress network policy.

LLM-proposed diffs are never auto-applied; the user must click Apply.