What works where

Calypso runs your code in two tiers. Tier 1 is the default and needs no setup. Tier 2 (Termux) is opt-in and unlocks a real local toolchain.

Why two tiers?

Android sandboxes every app, and since Android 10 (API 29) apps cannot execute downloaded binaries (the “W^X” rule). That’s an OS-level limitation, not a Calypso one — it’s the reason a real toolchain needs Termux (a separate app with its own permitted execution environment).

The comparison

Capability	Tier 1 — On-device sandbox	Tier 2 — Termux
Setup required	None	One-time (see setup)
Run Python	✅ Pyodide (WASM)	✅ Real CPython
Run JavaScript	✅ Sandboxed WebView	✅ Real Node.js
Import pure-Python packages	✅ via micropip	✅ via pip
Import JS packages	✅ via esm.sh (ES modules)	✅ via npm
npm install (real node_modules)	❌	✅
Native / compiled packages	❌	✅
Node require, fs, process	❌	✅
Run any shell command	❌	✅ Run Command in Termux
Interactive terminal (REPLs, prompts)	❌	✅ Embedded Terminal
Preview a local dev server	✅ Web Preview	✅ Web Preview
Create React / Vue / Svelte / Angular projects	❌	✅ New Project
IntelliSense (autocomplete, hover, diagnostics, go-to-def)	⚠️ Basic (keywords + snippets)	✅ Full (language servers)
git CLI	❌ (use Calypso’s built-in Git)	✅

Tier 1 — on-device sandbox (default)

This is the path for casual use and quick iteration:

• Python via Pyodide; JavaScript in a sandboxed WebView.
• Package imports without install: JS bare imports are rewritten to https://esm.sh/<pkg> and run as ES modules; Python uses micropip to fetch pure-Python wheels on first import.
• No filesystem, no shell, no native code.

When you hit a ceiling (a native package, Node’s require/fs/process, or an esm.sh failure), Calypso shows a hint pointing you toward Termux.

Tier 2 — Termux (opt-in)

For real projects — npm install, native modules, a full shell. After a one-time setup, the Run menu gives you several ways to use it:

• New Project — scaffold React, Vue, Svelte, or Angular projects through Termux, install dependencies, and open the project in Calypso.
• Run Active File in Termux — runs the open file with the real toolchain and streams output back into Calypso’s Output panel.
• Run Command in Termux — type any shell command (e.g. npm install axios) and stream its output, without leaving the editor. No more wrapping commands in an install.sh.
• Terminal — a full interactive terminal embedded in Calypso (real stdin, Ctrl-C, colours). Docked beside the editor on tablets, full-screen on phones. See Terminal & web preview.
• Web Preview — open a dev server you started in the Terminal (e.g. localhost:3000) right inside Calypso. See Terminal & web preview.

Termux also powers IntelliSense — installing the language servers (one tap on the setup screen) gives real autocomplete, hover, diagnostics and go-to-definition, all running on-device.

Termux-backed projects live in Termux’s workspace (~/CalypsoProjects), and Calypso opens them through the Termux integration.

Calypso-created web projects are portable. They are ordinary framework projects, so you can push them to GitHub, clone them on a desktop, edit them in VS Code, and pull the changes back into Calypso.

Working on a phone?

You’ll usually move faster with Run Active File in Termux and Run Command in Termux — they do most of what a terminal does, with bigger text and no on-screen-keyboard fiddliness. Reach for the embedded Terminal when you need a genuinely interactive session (REPLs, prompts, watching live output); it’s at its best on a tablet or with a hardware keyboard.

Tip

Termux is always an explicit choice, never the default. If you never need native packages or a shell, you never have to install it.