vercel-sandbox

Vercel Sandbox

Vercel Sandbox runs untrusted or AI-generated code inside an ephemeral Firecracker microVM. You get a real Linux VM with a filesystem and network — created on demand over an API, and stopped (or snapshotted) when you're done. Reach for it when code you don't fully trust needs to run: AI agent tool calls, code generation, user submissions, builds, or experiments.

Do not use in-process sandboxes like vm2 (known escapes) or child_process/eval for untrusted code. Those share your process; a Sandbox is a separate VM.

Install

pnpm add @vercel/sandbox   # or npm i / yarn add / bun add

There is also a Python SDK (vercel package, vercel.sandbox) and a sandbox CLI. This skill shows the JS SDK unless noted.

Minimal example

The core loop is create → run → stop. For one-off work, stop in a finally so a thrown error can't leak a running VM (you're billed while it runs). stop() is safe to call more than once.

import { Sandbox } from "@vercel/sandbox";

const sandbox = await Sandbox.create();
try {
  const result = await sandbox.runCommand("python3", ["-c", "print(2 + 2)"]);
  console.log(await result.stdout()); // "4\n"
  console.log(result.exitCode);       // 0
} finally {
  await sandbox.stop();
}

Sandbox.create() with no arguments boots the default image (vercel/sandbox/universal, Ubuntu with Node.js 24, Python 3.14 as python3, and common tools), 2 vCPUs, and a 5-minute timeout.

Authentication

• On Vercel (Functions, Cron, builds): the SDK authenticates automatically via the deployment's OIDC token. No config.
• Local dev: run vercel link then vercel env pull to get a VERCEL_OIDC_TOKEN in .env.local (valid ~12h; re-pull when it expires).
• External / CI (no OIDC available): set VERCEL_TOKEN, VERCEL_TEAM_ID, VERCEL_PROJECT_ID. The SDK picks these up automatically.

This is auth for the process calling the SDK. It is separate from any credential you want available inside the VM — the sandbox does not automatically carry your VERCEL_OIDC_TOKEN (see Running AI agents).

Creating a sandbox

Common Sandbox.create() options (all optional):

Running commands

runCommand runs a binary directly — there is no shell, so pipes, redirects, &&, and globs do not work unless you invoke a shell yourself. It resolves with the finished command regardless of exit code (it does not throw on a non-zero exit); check result.exitCode. It only rejects on an actual failure to run — e.g. the session timing out mid-command.

Commands run as a non-root user (ubuntu, in the sudo group) by default; pass sudo: true for root.

const r = await sandbox.runCommand("npm", ["install"]);
if (r.exitCode !== 0) throw new Error(await r.stderr()); // non-zero does NOT throw

// Needs a shell for the redirect / pipe. Use absolute paths (see below).
await sandbox.runCommand("bash", ["-c", "echo hi > /vercel/sandbox/out.txt && cat /vercel/sandbox/out.txt"]);

// Root for one command (sudo is object-form only)
await sandbox.runCommand({ cmd: "apt-get", args: ["update"], sudo: true });

// Long-running process: detached (object form only) returns immediately
const server = await sandbox.runCommand({ cmd: "npm", args: ["run", "dev"], detached: true });

runCommand returns a finished command with await result.stdout(), await result.stderr(), and result.exitCode (or a live Command when detached: true). The object form also takes cwd, env, and stdout/stderr (a Writable to stream into). sudo and detached are object-form only.

Working directory: the file methods below are rooted at /vercel/sandbox, but do not assume a command's default working directory is the same. Whenever a command reads or writes files you created with writeFiles/readFileToBuffer, use absolute paths under /vercel/sandbox (or pass an explicit cwd) so both sides point at the same place.

Files

File-method paths are relative to /vercel/sandbox unless absolute. content must be a Buffer.

await sandbox.writeFiles([
  { path: "input.txt", content: Buffer.from("line one\nline two\n") },
  { path: "run.sh", content: Buffer.from("#!/bin/bash\necho hi"), mode: 0o755 },
]);

// readFileToBuffer returns a Buffer, or null if the file is missing — guard it.
// (readFile returns a ReadableStream; neither returns a string, so convert yourself.)
const buf = await sandbox.readFileToBuffer({ path: "input.txt" });
const text = buf?.toString("utf8") ?? "";

await sandbox.mkDir("src/generated");

To pull source in at create time, use source: a git repo ({ type: "git", url, username, password, depth?, revision? } — username/password authenticate a private repo), a tarball ({ type: "tarball", url }), or a snapshot ({ type: "snapshot", snapshotId }).

Installing system packages

The default image is Ubuntu — use apt-get, and run apt-get update first (package lists ship empty, so install fails without it). This needs sudo, and there is no shell, so run it through bash -c and check the exit code:

const install = await sandbox.runCommand({
  cmd: "bash",
  args: ["-c", "apt-get update && apt-get install -y ffmpeg"],
  sudo: true,
});
if (install.exitCode !== 0) throw new Error(await install.stderr());

For a different base, use a managed image (vercel/sandbox/arch uses pacman/yay) or build a custom image so packages are baked in and there's nothing to install at runtime.

Ports and preview URLs

Expose ports at create time (up to 15), start a server listening on 0.0.0.0 (not 127.0.0.1, or it's unreachable — the host flag is framework-specific), then read its public URL. detached returns when the process spawns, not when it's listening, so poll for readiness before using the URL:

const sandbox = await Sandbox.create({ ports: [3000] });
// Bind 0.0.0.0 — e.g. Next/Vite: `run dev -- --host 0.0.0.0`; node http: listen("0.0.0.0")
await sandbox.runCommand({ cmd: "npm", args: ["run", "dev", "--", "--host", "0.0.0.0"], detached: true });

// Wait until the port actually answers inside the VM
for (let i = 0; i < 30; i++) {
  const ping = await sandbox.runCommand("bash", ["-c", "curl -sf http://localhost:3000 >/dev/null && echo up || true"]);
  if ((await ping.stdout()).includes("up")) break;
  await new Promise((r) => setTimeout(r, 1000));
}
const url = sandbox.domain(3000); // public HTTPS URL for port 3000

The URL is served by the running session. If the sandbox is stopped, nothing is listening until you resume it and restart the server — so for a durable preview keep the session alive (extendTimeout) rather than relying on the URL between sessions. Traffic to and from exposed ports is billable (requests and responses both count).

Lifecycle and persistence

Persistence is the default. When a persistent sandbox stops, its filesystem is snapshotted automatically; a later call resumes it into a fresh session. Only the filesystem is saved — running processes do not survive a stop/resume, so restart long-running servers on resume (see below).

• Sandbox vs session: a sandbox is a long-lived entity identified by name; a session is one VM boot. The max session duration caps each session, not the sandbox — resuming starts a new session with a fresh timeout, so a persistent sandbox's total lifetime is effectively unbounded.
• Retrieve / resume: Sandbox.get({ name }) returns the handle immediately and auto-resumes on the next call that needs a running VM (pass resume: false to disable). getOrCreate does not resume before returning by default; pass resume: true to resume and await onResume immediately. stop() and update() never auto-resume. Use getOrCreate when the sandbox may not exist yet, get when you know it does.
• getOrCreate accepts the same create options as create (ports, persistent, resources, networkPolicy, env, …). They apply only when it creates the sandbox; if the named sandbox already exists it's returned with its existing config (use sandbox.update({ … }) to change it).
• Hooks are per call, and fire on mutually exclusive events: onCreate runs once, the first time getOrCreate creates the sandbox; onResume runs on a resume. So to start a service exactly once per session, start it in both onCreate (first boot) and onResume (later boots). Hooks are arguments to this getOrCreate call, not stored on the sandbox — a different process resuming via Sandbox.get won't run them, so restart what it needs itself.
• A detached server returns as soon as the process spawns, not when it's listening — so after starting it (in onCreate for the first boot and onResume for later ones) poll until the port answers before treating domain(port) as live (see Ports).

const startDev = (s) =>
  s.runCommand({ cmd: "npm", args: ["run", "dev"], detached: true, cwd: "/vercel/sandbox" });

const sandbox = await Sandbox.getOrCreate({
  name: "agent-ws",
  ports: [3000],
  onCreate: async (s) => {          // once, on first creation
    await s.runCommand({ cmd: "git", args: ["clone", repoUrl, "."], cwd: "/vercel/sandbox" });
    await s.runCommand({ cmd: "npm", args: ["install"], cwd: "/vercel/sandbox" });
    await startDev(s);              // up on first boot, before domain() is read
  },
  onResume: async (s) => startDev(s), // every later resume
});
// Poll for the server to be listening (see Ports) before using the URL.
const url = sandbox.domain(3000);
// Don't stop this sandbox in a finally — stopping kills the dev server and the URL.
// If this process might find the sandbox already existing (not freshly created), pass
// `resume: true` above and start the server yourself — the hooks only fire on create/this call.

A separate later process reconnects by name and resumes on the first command. It won't run the hooks above, so restart anything it needs:

const sandbox = await Sandbox.get({ name: "agent-ws" });
const test = await sandbox.runCommand({ cmd: "npm", args: ["test"], cwd: "/vercel/sandbox" }); // resumes, then runs

Opt out for one-off work: Sandbox.create({ persistent: false }) — the filesystem is discarded on stop and you accrue no snapshot-storage cost. Recommended for scratch/CI tasks.

Snapshots

A snapshot is a saved full-filesystem image you can boot new sandboxes from — the way to skip repeated dependency installs (create-from-snapshot is much faster than installing from scratch).

const running = await Sandbox.create({ image: "vercel/sandbox/node:24" });
await running.runCommand({ cmd: "bash", args: ["-c", "apt-get update && apt-get install -y ffmpeg"], sudo: true });
const snap = await running.snapshot(); // sandbox stops automatically after; do NOT call stop()

const fast = await Sandbox.create({ source: { type: "snapshot", snapshotId: snap.snapshotId } });

Snapshots expire 30 days after last use by default. Control retention with snapshotExpiration (ms; 0 = never) and keepLastSnapshots: { count: 1 } (keep only the latest — keeps storage flat). Persistent sandboxes create these automatically on stop.

Images

Pass image to control the environment. Managed images live under vercel/sandbox:

Custom images (bake in your own tools) go through Vercel Container Registry: vercel vcr build docker . my-repo:latest --push, then image: "my-repo:latest". Team-scoped (team/project/repo:tag) and public images work too. Note: Sandbox does not run a Dockerfile ENTRYPOINT/CMD — start processes yourself with runCommand after create. Pin a digest (image@sha256:...) for reproducibility.

Drives (beta)

A drive is persistent storage you mount into a sandbox as a directory; unlike a snapshot (a full-filesystem copy per sandbox), a drive is one directory many sandboxes share and keep updating across runs. Good for agent workspaces, dependency caches, and shared data.

import { Sandbox, Drive } from "@vercel/sandbox";

const drive = await Drive.getOrCreate({ name: "workspace-cache" });
const sandbox = await Sandbox.create({ mounts: { "/data": drive } }); // read-write

// Concurrent read-only access via a drive snapshot
const reader = await Sandbox.create({ mounts: { "/data": drive.snapshot() } });

Up to 4 drives per run. Default size 1 TiB (1 GiB on Hobby), max 16 TiB. A drive lives in one region; a sandbox mounting it must run in that region and can't use failover regions. Only one sandbox at a time can mount a drive read-write; use drive.snapshot() for shared reads.

Network policy and credential brokering

The egress firewall is Sandbox's key security control for untrusted code. Set networkPolicy at create or via sandbox.update({ networkPolicy }):

• "allow-all" (default) — all egress allowed.
• "deny-all" — blocks all egress, including DNS. Start here for untrusted code.
• Rule object — an allow list restricts egress to only the listed domains (everything else is denied); add subnets.allow/subnets.deny for IP ranges (deny wins). Domain matching is SNI-based, so it only applies to TLS traffic — pair with deny-all/subnet rules if non-TLS egress must be blocked too.

Credential brokering: a transform rule injects a secret header on egress to an allowed domain, so code inside the VM can call an authenticated API without the secret ever entering the sandbox. Because the allow list denies everything else, the box can reach only that one domain:

const sandbox = await Sandbox.create({
  networkPolicy: {
    allow: [{
      domain: "api.example.com",
      transform: { headers: { authorization: `Bearer ${process.env.API_SECRET}` } },
    }],
  },
});
// Inside the VM: fetch("https://api.example.com/…") is authenticated by the
// firewall; the VM never holds API_SECRET and can't reach any other TLS host.
// Domain rules are SNI-based (TLS only) — add `subnets: { deny: [...] }` to also
// block non-TLS / raw-IP egress if the code is fully untrusted.

Running AI agents in a sandbox

To run AI-generated code, or a coding agent (Claude Code, Codex) that edits and executes code, put it in a sandbox. Two ways to give it model access, by trust level:

Trusted agent — inject the OIDC token, call the AI Gateway directly. The AI Gateway accepts a Vercel OIDC token as a bearer credential, so no model API key is needed. The sandbox does not automatically have your VERCEL_OIDC_TOKEN, so pass it in via env:

const sandbox = await Sandbox.create({
  env: { VERCEL_OIDC_TOKEN: process.env.VERCEL_OIDC_TOKEN! },
});
// Inside the VM, hit the gateway (OpenAI-compatible at /v1, Anthropic-compatible at root):
//   curl https://ai-gateway.vercel.sh/v1/chat/completions \
//     -H "Authorization: Bearer $VERCEL_OIDC_TOKEN" -H "Content-Type: application/json" \
//     -d '{"model":"anthropic/claude-sonnet-5","messages":[{"role":"user","content":"hi"}]}'
// The AI SDK auto-resolves VERCEL_OIDC_TOKEN when AI_GATEWAY_API_KEY isn't set. Model ids
// come from the AI Gateway model catalog (provider/model, e.g. "anthropic/claude-sonnet-5").

Untrusted code — broker the credential, keep it out of the VM. For code you don't trust, don't put the token in the VM at all. Allow only the gateway and inject the auth header at the firewall so the box holds no credential and can reach no other TLS host (add subnets.deny to block non-TLS egress too):

const sandbox = await Sandbox.create({
  networkPolicy: {
    allow: [{
      domain: "ai-gateway.vercel.sh",
      transform: { headers: { authorization: `Bearer ${process.env.VERCEL_OIDC_TOKEN}` } },
    }],
  },
});
// Agent code calls https://ai-gateway.vercel.sh with no token present in the VM.

The OIDC token is ~12h; for longer sessions re-inject on resume or scope work to the token's life.

Multi-agent isolation

Run several agents in one sandbox, each as its own Linux user with a private home directory (JS SDK only; image must include /bin/bash):

const alice = await sandbox.createUser("alice"); // /home/alice
await alice.runCommand("whoami"); // runs as alice
const root = sandbox.asUser("root");

Files in one user's home are unreadable by another. Share a workspace with sandbox.createGroup("team") (dir at /shared/team) and addUserToGroup.

CLI

The sandbox CLI (also vercel sandbox) mirrors the SDK, Docker-style:

sandbox create --name my-box              # create (persistent; --non-persistent to opt out)
sandbox exec my-box -- npm test           # run a command in a named sandbox (resumes if stopped)
sandbox run -- node --version             # create an ephemeral box, run once
sandbox connect my-box                    # interactive shell (aliases: ssh, shell)
sandbox copy ./local my-box:/remote       # copy files (alias: cp)
sandbox list                              # list sandboxes (alias: ls)
sandbox drives get-or-create cache        # create a drive
sandbox stop my-box

Limits and cost

• Session duration: Hobby 45 min, Pro/Ent 24h (per session; resume for longer).
• Resources: vcpus 1 or even up to 4/8/32 (Hobby/Pro/Ent), 2 GB RAM per vCPU, 15 ports, 64 GB disk.
• Concurrency: Hobby 10, Pro/Ent 10,000 concurrent sandboxes.
• Network: data your sandbox downloads (npm, git, datasets) is free; data it sends out and all exposed-port traffic is billable.
• Isolation: each sandbox is a separate Firecracker microVM, so a crash, fork bomb, or disk-fill is contained to that VM. There are no per-process CPU/PID quotas inside the VM beyond the vCPU and 64 GB disk limits — cap risk with a short timeout and deny-all for untrusted code.
• Save money: call stop() when done with one-off work, right-size vCPUs (down to 1), use persistent: false for scratch runs, and a smaller image or keepLastSnapshots: { count: 1 } to cut snapshot storage.

Best-practice checklist

• For one-off work, stop() in a finally (safe to call more than once). Exception: a long-lived sandbox serving an exposed port — don't stop it, or the URL goes dead; leave it running (it persists/resumes).
• runCommand has no shell (wrap pipes/redirects/&& in bash -c) and does not throw on non-zero exit (check result.exitCode); it rejects if the session times out mid-command.
• Use absolute /vercel/sandbox paths (or an explicit cwd) when a command reads/writes files you created with writeFiles.
• apt-get update before apt-get install; commands are non-root by default (sudo: true for root); sudo/detached are object-form only.
• Start per-session services (dev servers) in both onCreate and onResume — only the filesystem survives a stop.
• For untrusted code: networkPolicy: "deny-all" (or a tight allow-list), a short timeout (e.g. 30_000), vcpus: 1, and persistent: false.
• Exposed servers must bind 0.0.0.0, not 127.0.0.1.
• Pin a custom image by digest for reproducible boots; ENTRYPOINT/CMD don't run.