Browser Runtime

The browser runtime runs your project’s Node.js environment inside your browser tab. There’s nothing to provision: the sandbox boots when you open the project, installs dependencies, and starts your framework’s dev server with hot reload. It’s the default runtime and it’s free on every plan.

What it includes

• Node.js and npm — install packages with npm install from the terminal, or let the agent do it.
• Your dev server — started automatically on port 3000 and shown in the preview.
• A terminal — open as many bash tabs as you need from the bottom panel.
• Terminal agents — Claude Code, OpenCode and Pi. Codex needs the server runtime.
• Extra tools — ffmpeg and ffprobe for audio and video, and pdftotext and pdfinfo for PDFs, so agents can process media and documents.

Good to know

• Agent history stays in your browser. Claude Code and OpenCode keep each project’s conversations in the browser, and you can reopen them with /resume or /sessions.
• The sandbox lives in the tab. Closing the tab stops the dev server and the agents. Your saved versions and unsaved changes are kept for when you return.
• No preview on phones. Phones can’t run the browser sandbox’s preview. Switch the project to the server runtime to preview it on mobile.
• Network requests from the sandbox, like curl, go through Frontend’s proxy.

When to switch to a server

If a package needs native Linux binaries, or you want Codex, switch to the server runtime in Settings → Advanced → Runtime. See Browser vs Server for a comparison.