Judge0 alternatives in 2026: code execution APIs compared

Quick picks

• Judging built in (test cases, verdicts, custom checkers): Sphere Engine Problems, or Judge0 for one expected output per submission.
• The most languages: JDoodle (114), OneCompiler (110+) or Judge0 CE (71 language versions).
• Self-hosting for free: Judge0 (GPL-3.0), Piston (MIT) or Codapi (Apache-2.0).
• A large free tier and the lowest cost per run: OnlineCompiler.io, which also answers Judge0's API, so a Judge0 client moves by changing its base URL.

At a glance

What 300,000 runs a month costs

A mid-sized course or assessment platform can easily run 10,000 submissions a day. At published prices, that's:

Daily quotas matter too: Judge0's hosted plans and JDoodle count runs per day, so a deadline spike can hit the cap even when the monthly total is fine.

Judge0

The most widely used open-source option, and the one most "build an online judge" tutorials use.

• Strengths: judging built in (send expected_output and get a verdict such as Accepted or Wrong Answer), batches of up to 20 submissions, 71 language versions on CE plus 30 on Extra CE, and limits you can raise per submission (up to 2 GB and 20 s of CPU on the hosted CE instance). You can self-host it under GPL-3.0.
• Trade-offs: hosted plans are daily quotas, from €27 a month for 2,000 runs a day, and sign-up goes through RapidAPI. Waiting for the result in one request (wait=true) is switched off on the official instance, so you submit and then poll or use a callback. Many CE language versions are old: Java tops out at JDK 17, Ruby at 2.7 and Haskell at GHC 8.8. The last open-source release is v1.13.1 from April 2024, which fixed three critical sandbox escapes.

Sphere Engine

• Strengths: the most complete judging of any option here. The Problems module stores test cases, returns a verdict for each one, and supports custom judges. Runs get 1.5 GB of RAM, and the Compilers module returns up to 1 MB per output stream.
• Trade-offs: prices are quote-only. Problems returns at most 5 kB of stdout per test case, each run gets one core, and versions lag (Python 3.12, GCC 13.2, Go 1.22).

JDoodle

• Strengths: 114 languages with current versions (Python 3.14, JDK 26, GCC 15), multi-file projects, a compile-only mode, WebSocket streaming, an embeddable IDE, and optional internet access on paid plans.
• Trade-offs: billing is in daily credits (20 free a day), and a WebSocket token, a multi-file upload or internet access each cost an extra credit. Per-run limits aren't published, and there's no judging.

Full comparison: JDoodle API alternative.

OneCompiler

• Strengths: 110+ languages, stdin up to 1,000,000 characters, and batch stdin: pass an array and the code runs once per input, which suits test cases.
• Trade-offs: the API page doesn't say how many credits a run costs or what the time, memory and output limits are. No judging.

Codapi

• Strengths: a small, simple sync API aimed at interactive docs and tutorials, open source under Apache-2.0, and an inexpensive cloud plan.
• Trade-offs: the cloud plan covers 5,000 runs a month and the vendor says pricing will probably change. No judging.

Piston

• Strengths: MIT-licensed, 84 languages, multi-file runs and command-line arguments, and per-request limits.
• Trade-offs: the public API at emkc.org has been whitelist-only since 15 February 2026, so for most projects it's self-host only. The public instance's versions are old (Python 3.10, Java 15). No judging. If you were on the public API, see moving off Piston.

OnlineCompiler.io

• Strengths: 100,000 free runs a month including commercial use, and $49 a month for 1,000,000. Current versions (Python 3.14, Java OpenJDK 25, Go 1.26, Rust 1.93 and more). 30 seconds and 2 CPUs per run. Every run gets a fresh gVisor sandbox (a user-space kernel) with no network. One key works with a sync endpoint, an async endpoint with callbacks, WebSocket, an embeddable editor, an MCP server, and Judge0- and Piston-compatible endpoints.
• Where it falls short: 12 languages. Output is kept up to 64 KB per stream (longer output is cut and flagged with truncated), so tests with very large expected output need another approach. Judging goes as far as Judge0's: one expected output per submission through the Judge0-compatible API, with no stored test cases or custom checkers (see the online judge guide). 512 MB of memory, one source file per run, and it's hosted only: there's no self-hosted version.

Moving from Judge0 to OnlineCompiler.io

Judge0 code keeps working: set its base URL to https://api.onlinecompiler.io/api/judge0 and send your OnlineCompiler.io key in X-Auth-Token. The usual Judge0 flow (submit, get a token, poll) runs unchanged:

// Judge0: submit, then poll until the status leaves In Queue (1) and Processing (2).
const b64 = (s) => Buffer.from(s).toString("base64");

const submit = await fetch(`${JUDGE0_URL}/submissions?base64_encoded=true`, {
  method: "POST",
  headers,
  body: JSON.stringify({
    language_id: 113, // Python 3.14.0
    source_code: b64(code),
    stdin: b64(input),
    expected_output: b64(expected),
  }),
});
const { token } = await submit.json();

let r;
do {
  await new Promise((resolve) => setTimeout(resolve, 1000));
  r = await (await fetch(`${JUDGE0_URL}/submissions/${token}?base64_encoded=true`, { headers })).json();
} while (r.status.id <= 2);

console.log(r.status.description); // "Accepted", "Wrong Answer", ...

Or use the native API, with one request per test case and the result in the response:

// OnlineCompiler.io: one request per test case, then compare the output yourself.
async function judge(code, compiler, test) {
  const res = await fetch("https://api.onlinecompiler.io/api/run-code-sync/", {
    method: "POST",
    headers: {
      Authorization: process.env.ONLINECOMPILER_API_KEY,
      "Content-Type": "application/json",
    },
    body: JSON.stringify({ compiler, code, input: test.input }),
  });
  const r = await res.json();

  if (r.exit_code === 124) return "Time Limit Exceeded";
  if (r.exit_code === 137) return "Memory Limit Exceeded";
  if (r.truncated) return "Output Limit Exceeded";
  if (r.status !== "success") return "Runtime or Compilation Error"; // details in r.error
  return r.output.trim() === test.expected.trim() ? "Accepted" : "Wrong Answer";
}

console.log(await judge(code, "python-3.14", { input: "21", expected: "42" }));

Language ids map like this. For what carries over, statuses, callbacks and batches, see the full Judge0 migration guide.

Frequently asked questions

Is Judge0 free?

The open-source Judge0 is free to self-host under the GPL-3.0 licence. The hosted plans on judge0.com start at 27 euros a month for 2,000 submissions a day, with sign-up through RapidAPI.

Is Judge0 secure?

Judge0 sandboxes code with isolate. Three critical sandbox-escape vulnerabilities (CVE-2024-28185, CVE-2024-28189 and CVE-2024-29021) were fixed in version 1.13.1 in April 2024, so self-hosted installs must run 1.13.1 or later.

Which Judge0 alternatives have built-in judging?

Sphere Engine Problems has the most complete judging: stored test cases, a verdict for each, and custom judges. Judge0 itself compares one expected output per submission. OnlineCompiler.io does the same through its Judge0-compatible API. JDoodle, OneCompiler, Codapi and Piston run code and return the output; you compare it yourself.

What is the cheapest hosted Judge0 alternative?

At published prices, OnlineCompiler.io: 100,000 runs a month are free, and Pro is $49 a month for 1,000,000. The free tier allows commercial use.

Should I use an AI sandbox such as E2B or Modal instead?

Those are built for AI agents that need long-lived machines with package installs, files and network access, and they bill by running time. For short, stateless runs against test cases, a code execution API is simpler and usually far cheaper.