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 runtimes — Python 3.12, Node 22, .NET 9. Not Python 3.8 and Node 12.
- gVisor isolation — User-space kernel intercepts every syscall before reaching the host.
- Stateful sessions — Variables, files, packages persist across calls. The only credible code-interpreter primitive.
- Package install — pip / npm / gem / cargo inside a session. Cached, sandboxed, fast.
- MCP-native — 11 tools exposed for Claude Desktop, Cursor, VS Code.
- Simpler API — No tokens to poll for sync calls. Streaming SSE. Async with signed webhooks.
- Built for AI agents first — Every roadmap decision starts with: does an LLM agent need this?
Feature matrix
| Feature | Judge0 | SandboxAPI |
|---|---|---|
| Languages | 60+ | 12 (modern, current) |
| Python version | 3.8 / 3.11 | 3.12 |
| Node version | 12 / 18 | 22 |
| .NET version | 6 | 9 |
| Isolation | isolate (Linux namespaces) | gVisor (runsc) + isolate |
| Stateful sessions | — | ✓ |
| Package install (pip/npm/gem/cargo) | — | ✓ (Pro+) |
| SSE streaming output | — | ✓ |
| Async + signed webhooks | callback URL (unsigned) | ✓ (HMAC-SHA256) |
| Multi-file submissions | ✓ (additional_files) | ✓ (Phase 2) |
| Output verification | ✓ (expected_output) | ✓ (expected_output) |
| Stdin support | ✓ | ✓ |
| Batch execution | ✓ (submissions/batch) | ✓ (up to 200) |
| MCP server | — | ✓ (11 tools) |
| Self-hostable | ✓ | — (Phase 3 plan) |
| Pricing model | RapidAPI tiers / self-host | RapidAPI + direct API + Stripe |
| Free tier | 50/day on RapidAPI | 500/month |
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 | expected_output | Identical. Returns status: "wrong_answer" on mismatch. |
cpu_time_limit | timeout | Same semantics — CPU seconds, capped by plan. |
wall_time_limit | wall_time_limit | Identical. |
memory_limit (KB) | — | Memory is enforced per-tier; not per-request configurable. |
compiler_options | compiler_options | Identical, allowlisted. |
command_line_arguments | command_line_arguments | Identical. |
additional_files | additional_files | Identical — base64-encoded ZIP. |
callback_url | callback_url | SandboxAPI signs all callbacks with HMAC-SHA256 (X-SandboxAPI-Signature). |
token (in response) | id (sync) / token (async) | Sync calls return inline. Async returns a token, polled at /v1/executions/{token}. |
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 maybe 4–6 languages, current versions, multi-file submissions, output verification, 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.