# Sandbox

> Fence the loop

- Skill: `jakeselby/sandbox` (Agent Skill)
- Install (CLI): `npx skillmds@latest add jakeselby/sandbox`
- Raw SKILL.md: https://api.skillmd.com/api/skills/jakeselby/sandbox/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: JakeSelby (https://skillmd.com/u/jakeselby)
- Updated: 2026-09-22
- Page: https://skillmd.com/skills/jakeselby/sandbox

---


# Fence the loop

A permission mode decides whether a call runs. A sandbox decides what a command can reach once it
is running, and the OS enforces that on every child process. An unattended loop needs the second
kind: nobody is at the prompt to answer for the first.

## Runtime scope

The configuration and container example below are Claude Code-specific. Do not copy those
settings into Codex. For Codex, use its native sandbox and approval controls as documented in
[the configuration reference](https://learn.chatgpt.com/docs/config-file/config-reference).
A read-only sandbox constrains filesystem writes; approval policy is a separate control.
Native hooks are not a replacement for OS confinement. Custom Codex role defaults can be
superseded by the parent turn's permissions; see `docs/runtime-controls.md` before delegating
work that requires a hard boundary. Client qualification remains in the compatibility catalog.

## The Claude Code sandbox

It "runs on macOS, Linux, and WSL2. Native Windows is not supported"; Linux and WSL2 need
`bubblewrap` and `socat` installed first. Put this in `~/.claude/settings.json` to cover every
project; the `/sandbox` panel writes `enabled` to `.claude/settings.local.json` for one project.

```json
{
  "sandbox": {
    "enabled": true,
    "failIfUnavailable": true,
    "allowUnsandboxedCommands": false,
    "network": { "allowedDomains": [], "strictAllowlist": true },
    "filesystem": { "denyRead": ["~/.ssh", "~/.aws", "~/.config/gh"] }
  }
}
```

`strictAllowlist` over an empty `allowedDomains` is network off: Claude Code then "denies sandboxed
commands access to any host outside the allowlist instead of prompting". Only user, managed and
`--settings` settings set it; a repository's own file cannot. The deny entries are load-bearing —
the default read policy covers the whole disk, and "this default still allows reading credential
files such as `~/.aws/credentials` and `~/.ssh/`." Add `sandbox.credentials.envVars` entries with
`"mode": "deny"` to unset tokens for sandboxed commands too. `failIfUnavailable` makes a missing
dependency a hard stop rather than a silent unsandboxed fallback, and `allowUnsandboxedCommands:
false` removes the retry-outside escape hatch. Subagents inherit the session's sandbox; commands
you type at the `!` prompt do not.

For one session, writing no file: `claude --settings '{"sandbox":{"enabled":true}}'`. Confirm with
`/sandbox`: the **Config** tab shows the resolved settings, and a **Dependencies** tab appearing
means a package is missing. A prompt titled "Bash command (unsandboxed)" is the signal a command
left the boundary. Keys and defaults: https://code.claude.com/docs/en/sandboxing

## A container

Harder boundary, coarser tooling. Mount the worktree and nothing else, stay non-root, and let the
container be the isolation — do not nest the built-in sandbox inside it.

```bash
docker run --rm -it --network none \
  --user "$(id -u):$(id -g)" \
  -v "$PWD:/work" -w /work \
  -v "$HOME/.claude:/config:ro" -e CLAUDE_CONFIG_DIR=/config -e ANTHROPIC_API_KEY \
  <image-with-the-cli> claude --dangerously-skip-permissions -p "<the loop prompt>"
```

`podman` substitutes unchanged. What breaks, in order: `--network none` cuts the model API too, so
as written this runs only against a local model — for a loop that must reach the API, allow that
one host and nothing else and the shape holds. Then web search and fetch, every MCP server reached
over the network, and every package install; a read-only config mount blocks session state and
credential writes, so auth arrives by environment variable. Losing all of it is the point when the
task parses input you did not write: a path the loop lacks cannot be talked into opening.

## Which one

Built-in sandbox for daily work: a settings change, every tool still works, the OS still enforces
the boundary. Container for a loop that runs while you sleep, a repo whose build scripts you have
not read, or anything handling untrusted content. Neither isolates branches — run inside a
worktree as well, per the `worktree-per-agent` skill.