Julia 1 — run it in a Firecracker microVM on ls-judge. Real toolchain, no wrapper.
Clicking "Run now" opens the playground with this exact code and a matching test case — passes on first Run.
n = parse(Int, readline())
println(n * 2)
21
42
Every submission is compiled and executed inside its own Firecracker microVM — the same isolation primitive AWS Lambda uses. No shared container, no shared kernel with other users' code. The Julia 1toolchain shipped with the microVM is the real thing (not a browser-side emulation): syscalls behave the way they do in production, and the compile / runtime behaviour matches what you would see on your own machine with that version installed.
Feed stdin from the Tests panel, watch verdicts stream per test case over a WebSocket. Free to use, no sign-up, works from any browser.