Vivari

Vivari is an open-source WebContainer. It runs Node, Bun and Python projects 100% inside the browser: a virtual filesystem, a Node-compatible runtime, a process model, and virtual networking, all in Web Workers with no server doing the work.

It is not a shim over hand-written core modules. Vivari runs Node's own lib/ JavaScript, the real npm, yarn and pnpm, and real CPython.

The name

Vivari (vih-VAH-ree) comes from the Latin vivarium, a self-contained enclosure for living things. That's exactly what it is for a running Node app: a sealed environment in the browser where a whole project lives and runs.

It ships in two forms:

• The Studio: a full in-browser IDE built on Vivari.
• The SDK: @vivari/core (framework-agnostic) and @vivari/react (components and hooks for embedding a VM, its preview, its files and its processes) that you can embed in your own app.

Try it now

The fastest way to understand Vivari is to open the Studio and run a template: each shell command becomes its own worker/process, and an in-VM dev server is previewed live through a Service Worker.

Why it exists

Node's APIs (fs.readFileSync, require(), …) are synchronous. Browsers don't let you block on async work, except on a Web Worker thread, where Atomics.wait() can genuinely park execution. That single primitive makes a synchronous Node runtime possible in the browser. See How it works for the full story.

What you get

Capability	What it means
Virtual filesystem	A POSIX-ish VFS in Rust/Wasm, persisted to OPFS
Node runtime	Node's own lib/, synchronous require, node_modules resolution, a REPL
Process model	A kernel + PID table + shell; execSync, pipes, signals
Virtual networking	http.createServer().listen() previewed live in an iframe
Package managers	npm / yarn / pnpm / corepack with content-addressed caches
Frameworks	Next.js 16 (App Router + RSC), Vite (React, Vue, Svelte 5, Solid, Qwik, Preact, Lit), Astro, Angular, and the servers (Express, NestJS, Fastify, Hono, …)
Bun	bun install / run / test, Bun.serve, Bun.build, bun:sqlite, bun repl. An API-compatible shim, not the native binary
Python	Real CPython 3.14 via Pyodide, with pip and a REPL; the templates around it (pytest, the notebook, the scientific stack, the web frameworks) are experimental
Databases	SQLite via bun:sqlite; the sql.js, Python sqlite3/SQLAlchemy and PGlite (Postgres) templates are experimental
Debugger	Breakpoints, stepping and evaluation in guest Node processes over the Chrome DevTools Protocol (in the Studio)

Experimental does not mean a sketch: it runs, and you can go and try it. It means the thing is not yet held in place by a check that would catch it breaking. Most of these carry a per-template flag, whose bar for graduation is specific: a green spike of the template's own (scripts/spike-<name>.mjs). Nuxt, SvelteKit, TanStack Router, PGlite, sql.js and every Python template except the bare python starter are experimental today.

Genuinely open

Vivari is MIT-licensed. Unlike a proprietary WebContainer API, there is no commercial license and no usage fee: embed it, fork it, ship it.

Next steps

• Getting started: install and boot your first instance.
• Core API: the @vivari/core reference.
• React: the @vivari/react components and hooks.
• Deployment: host the landing, docs, and studio on Cloudflare Pages.