Write CX+AI. Press Run. It executes in this tab.

What runs here, and what does not

This is the real CX+AI register VM — the same one a native cx build uses — compiled to wasm32 and running inside your browser's sandbox. The rule is simple, and it is the only one you need: whatever CX+AI can do in a browser, it does. What a browser genuinely cannot do, CX+AI refuses by name — it never fakes a result.

• fwrite, fread and the rest of the file builtins work. They read and write a filesystem that lives inside this tab — real files as far as your program is concerned, and invisible to everything else. Nothing here can touch your disk, and nothing is uploaded: your source is compiled on our server, but it runs only in your browser. The tab's files vanish when you reload the page.
• system() stops with CX-E5037, naming the capability and refusing to invent a return value — a browser tab has no shell. Try the last example and read it.
• _C{} blocks stop with CX-E5003. A _C{} escape hatch is literal C compiled into a native binary, and there is no native binary here. Those programs need the downloadable compiler.
• The ai* builtins are native-only: they need a native HTTP client and a key, so they are not part of the register VM on any backend and the compile itself refuses with CX-E1013. That is the same answer a native cx --runvm gives.
• httpGet and httpPost work, over the browser's own fetch. Same names, same signatures, same code as a native build — your program does not know which target it is on. CORS is the browser's law, not ours: a tab can only read a site that allows it to, so a request to a server without the right Access-Control-Allow-Origin header is refused by the browser before it reaches us. CX+AI reports that as a named network error and an empty body — check httpStatus() and netError(). The browser deliberately does not say which of CORS, DNS or an unreachable host it was, so neither do we; inventing a confident reason would be worse than naming the possibilities.
• sshexec stops with CX-E5037, like system(). A tab cannot open an outbound SSH session, and no flag changes that — so it refuses by name rather than handing back an empty string. That is the line: what a browser can do, CX+AI does; what it genuinely cannot, CX+AI refuses out loud.
• Graphics programs are a different route — they compile to their own wasm module with raylib linked in, which this program-independent VM does not carry.

Everything else is the language as shipped. Every builtin you can call here is in the manuals and the builtins reference, and for the full toolchain — native builds, _C{}, files, networking, graphics, the module system — the releases are on GitHub.