Where Judge0 wins
Judge0 strengths
- Open source — AGPL/MIT-style. You can self-host, audit, and fork.
- 60+ languages — Long tail covered: Pascal, COBOL, Fortran, Prolog, Octave, Lua, ...
- Brand maturity in competitive programming — Powers a lot of well-known online judges. The team has a decade of experience tuning it.
- Submission/job model — A clean tokenized model that maps cleanly onto coding-judge workflows.
- Free self-hosted use — Run it on your own hardware at zero per-call cost (you pay infrastructure).
SandboxAPI strengths
- Modern core runtimes — Python 3.12, Node 22, TypeScript 5.6.3 via esbuild, Go 1.22, Java 21, GCC 12 for C/C++, and Bash 5.2.
- gVisor isolation — Production requires
runsc, which mediates application syscalls through a user-space kernel and adds a boundary beyond the container runtime. - Stateful sessions — Files and installed packages persist across calls, which is the key code-interpreter primitive for agents.
- Package install — pip and npm inside a session today, backed by cached, sandboxed mirrors.
- RapidAPI-backed MCP — Four stateless tools for Claude Code, Codex, Cursor, VS Code, and Antigravity with Bearer authentication; client-specific setup is documented.
- Simpler API — Sync calls return inline, streaming uses SSE, and longer jobs use async polling.
- Built for AI agents first — Every roadmap decision starts with: does an LLM agent need this?
Feature matrix
| Feature | Judge0 | SandboxAPI |
|---|---|---|
| Languages | 60+ | 8 (modern, current) |
| Python version | 3.8 / 3.11 | 3.12 |
| Node version | 12 / 18 | 22 |
| Go version | 1.20 / 1.22 | 1.22 |
| Isolation | isolate (Linux namespaces) | gVisor (runsc) + isolate |
| Stateful sessions | — | ✓ |
| Package install | — | ✓ pip/npm in sessions (Pro+) |
| SSE streaming output | — | ✓ |
| Async execution | callback URL | ✓ polling |
| Multi-file submissions | ✓ (additional_files) | Use session files today; ZIP upload is roadmap |
| Output verification | ✓ (expected_output) | Use your grading harness today; built-in verdicts are roadmap |
| Stdin support | ✓ | ✓ |
| Batch execution | ✓ (submissions/batch) | ✓ (up to 200 accepted items; bounded workers) |
| MCP server | — | ✓ (4 stateless tools) |
| Self-hostable | ✓ | — (not currently supported) |
| Pricing model | RapidAPI tiers / self-host | Four RapidAPI plans |
| Free plan | 50/day on RapidAPI | 500 requests / RapidAPI subscription period |
Migrating from Judge0
The wire formats differ — here's a field-level mapping for the most common payload fields. Most migrations are a half-hour exercise.
| Judge0 field | SandboxAPI field | Notes |
|---|---|---|
source_code | code | Both accept up to 1MB. Drop any base64 wrapping. |
language_id (integer) | language (string) | e.g. 71 → "python3", 63 → "javascript", 62 → "java" |
stdin | stdin | Identical. |
expected_output | — | Run a small grading harness in your code today; built-in verdict fields are roadmap. |
cpu_time_limit | timeout | Same broad idea: execution seconds, capped by plan. |
wall_time_limit | — | SandboxAPI exposes a single timeout field. |
memory_limit (KB) | — | Fixed 512 MiB for stateless runs; sessions accept memory_limit up to 1 GiB. |
compiler_options | — | Use the language defaults today. Custom compile flags are roadmap. |
command_line_arguments | — | Pass input through stdin or embed a small wrapper in code. |
additional_files | Session filesystem | Upload/write files across session calls today; single-call ZIP upload is roadmap. |
callback_url | — | Async jobs return job_id and poll_url. |
token (in response) | id (sync) / job_id (async) | Sync calls return inline. Async jobs are polled at /v1/executions/{job_id}. |
When to pick which
This is where most comparison pages get lazy. Here's the honest answer for three concrete personas.
The AI agent developer
You're building a code-interpreter agent on top of OpenAI / Anthropic / local models. You need stateful sessions, package install, and modern runtimes. You don't care about Pascal.
Pick SandboxAPIThe online judge developer
You're building a competitive-programming platform. You need 30+ languages including the long tail. You care about deterministic execution timing for grading. You want the option to self-host as you scale.
Pick Judge0The education platform developer
You're building "run code in the browser" for students. You need a modern core language set, current versions, streaming output, and a tight integration story. You'll scale to thousands of students per assignment.
SandboxAPI is the better fitBoth are good products. Judge0 covers the breadth of competitive-programming languages; SandboxAPI covers the depth of modern runtimes and AI-agent workflows. Pick the one whose strengths match your workload.