Common questions

No. Anonymous visitors get 100 executions per month per IP; that is enough to try every language on the catalogue. Signing up bumps you to 500 executions per month and unlocks submission history + higher rate limits.

Open the playground, pick a language from the sidebar, paste code into the editor and click Run. Test cases are optional — hit Run without them and the runner just executes stdin against your code and streams stdout back.

Yes — the playground reads ?runtime= and ?problem= from the URL, so /play?runtime=python-3.13&problem=two-sum drops the visitor directly into a solvable setup. Language pages on /lang/[slug] already generate these deep links.

65+ languages currently, spanning Python, C++, Java, Go, Rust, JavaScript / TypeScript, C#, Ruby, PHP, Kotlin, Swift, Haskell, OCaml, COBOL, and dozens more. The full catalogue is at /languages, with a dedicated page per language showing every runtime version.

Real toolchains. Every runtime image ships the vendor's own compiler / interpreter binary inside a Firecracker microVM. Python 3.13 on ls-judge is CPython 3.13; Java 21 is OpenJDK 21; Rust 1.87 is rustc 1.87. Syscall behaviour, stdlib coverage, and compile flags match a local install.

Yes. Add extra files in the playground sidebar and they land in the same workspace as your entry file — compiled together where the language supports it (C++ with #include, Java packages, Python module imports, Go package layout).

Runtime images are added via the admin surface; contributors interested in an official add can open a request on the source repository. Each new image needs a build recipe + a smoke test set.

Yes — the Free tier has no card requirement and no time limit. Every language, every runtime version, 500 executions per month, 60 requests per minute. Anonymous visitors get 100 executions per month per IP so you can try before signing up.

The API returns 402 Payment Required with an upgrade URL. On paid tiers we plan to consume from a credit pack after the monthly quota exhausts; on the Free tier you wait for the next reset (anchored per-user, not calendar).

Yes — every tier ships with a default RPM ceiling but individual users can request a higher one via the contact form on /pricing. We reply within a business day; the change is per-account so it does not affect anyone else.

Every submission gets its own Firecracker microVM — the same isolation primitive AWS Lambda uses. It has a private kernel, a private network namespace (network disabled by default), and no shared filesystem with any other submission. A segfault in one job cannot reach another.

No — network is disabled in every microVM. If a runtime needs package fetches (Rust with cargo, Node with npm) the packages are pre-installed in the runtime image; live fetches at execution time are not permitted.

Yes, tied to your account — signed-in submissions surface in /me/submissions and stay for the retention window of your plan (30 days on Free, 180 days on Starter, longer on higher tiers). Anonymous submissions are not linked to any identity and are dropped after the same retention window.