@erichll/pi-sandbox

pi-sandbox is a Linux and macOS process sandbox for Pi backed by Anthropic's @anthropic-ai/sandbox-runtime. It protects main-agent Bash commands and can also run complete process-backed subagent trees inside independent sandboxes.

Security model

• Linux uses bubblewrap mount/network namespaces and seccomp.
• macOS uses generated Seatbelt profiles.
• Filesystem access is a static policy: reads outside the allowed regions and writes outside the workspace fail closed. Runtime filesystem denials are not converted into dynamic grants because Sandbox Runtime does not expose a trustworthy filesystem ask callback.
• Network policy is evaluated as static deny, then static allow, then dynamic review. Each unmatched public destination is sent to pi-auto-review as a canonical hostname and concrete port. An approval applies only to that connection.
• Every Bash command and persistent subagent session owns a separate broker process and Sandbox Runtime manager. Concurrent workers therefore keep independent policy, proxy, approval, and cleanup lifecycles.
• Each sandboxed command is given a private, writable temp directory: a fresh pi-sandbox-tmp-* subdirectory of the host temp tree that is added to the sandbox's read/write allowlists, with the command's TMPDIR, TMP, and TEMP pointed at it. Only that isolated directory is writable — the shared host temp root (e.g. /tmp itself) stays read-only to the sandbox — and it is removed when the command exits. If the host temp directory is unavailable (e.g. a read-only /tmp) the command degrades to the previous denied-temp behaviour rather than failing.
• Project and global Pi security configuration, the trusted extensions tree, the installed package, Git hooks, and other Sandbox Runtime mandatory paths remain write-protected.
• Common workspace secrets are write-denied by default: root-level .env variants, secrets/ / .secrets/, plus a shallow scan for nested .env*, *.pem, *.key, *.p12, and *.pfx. Template files such as .env.example stay writable. On macOS, additional globs also block nested creates; Linux Sandbox Runtime only enforces literal paths, so nested creates of new secret files remain a residual risk outside the scanned set.
• Optional host IPC execution is disabled by default. When enabled, every complete host command still requires a one-shot pi-auto-review or human approval and runs outside the OS sandbox.

Windows is not supported by this Pi adapter.

Linux requirements

Install the native helpers before loading the extension:

sudo apt-get install bubblewrap socat ripgrep

Unprivileged user namespaces must be available. On Ubuntu 24.04+, either configure an AppArmor profile for bubblewrap or disable kernel.apparmor_restrict_unprivileged_userns.

macOS uses built-in Seatbelt support but still requires ripgrep.

Subagent provider

The provider is selected only from the trusted global file ~/.pi/agent/extensions/pi-sandbox/config.json:

If that file is missing, pi-sandbox still loads the legacy path ~/.pi/agent/pi-sandbox.json when present. Prefer the extension-local path for new installs.

{
  "subagents": {
    "provider": "builtin",
    "externalWorkerIsolation": "off"
  }
}

Supported modes:

• builtin (default): register the process-backed subagent tool and sandbox each complete worker process tree.
• pi-subagents: let the external extension own orchestration. Main-agent and inherited Bash execution remains sandboxed, but the external worker process itself is not wrapped while externalWorkerIsolation is off.
• off: protect Bash only.

pi-subagents 0.50.0 capability boundary

pi-sandbox verifies the following combination in its test suite and release gate. This version boundary is pinned exactly (see the package devDependencies) and additionally exercised against the latest published pi-subagents on a nightly CI schedule. The PI_SUBAGENT_PI_BINARY seam (externalWorkerIsolation: "enforce") is re-checked deterministically by the gate:external-isolation probe on the pinned and latest dependency.

Capability	builtin	pi-subagents 0.50.0
Outer worker sandbox	Yes	Opt-in (externalWorkerIsolation: "enforce")
Bash sandbox	Yes	Yes
Persistent follow-up	Yes	Yes
workflowScript, missions, schedules	No	Yes
permission-system parent forwarding	Not required	Verified by compatibility gate

Upgrading across a capability boundary (for example 0.49.0 → 0.50.0) requires re-running npm run gate:external-isolation and, before a release, the model-backed npm run gate:pi-subagents. See docs/compat-notes.md for the recorded compatibility seams and how each is verified on upgrade.

For the external provider, off leaves the worker process outside the outer Sandbox Runtime boundary. Do not treat inherited Bash sandboxing as complete worker isolation unless the trusted global configuration explicitly enables enforce.

Note on the external provider's public execution surface: pi-subagents 0.43.0 removed top-level agent/task direct execution. Every public subagent launch — including a minimal single child — is expressed as a workflowScript such as return runs.run('main', { agent, task }). The compatibility gate's "direct single child" probe uses that single-run form.

The configuration parser rejects malformed JSON, unknown fields, and unknown providers instead of silently weakening isolation.

Network domain policy

Persistent domain authorization is accepted only from the trusted global ~/.pi/agent/extensions/pi-sandbox/config.json (or its legacy trusted fallback when the new file is absent). Project configuration cannot override it.

{
  "network": {
    "allowedDomains": [
      "github.com",
      "*.github.com",
      "registry.npmjs.org:443"
    ],
    "deniedDomains": [
      "uploads.github.com",
      "*:22"
    ]
  }
}

Entries support exact domains, strict-subdomain wildcards such as *.example.com, and an optional :port restriction. A bare * (optionally with a port) is accepted only in deniedDomains. Values are trimmed and deduplicated, and malformed or unknown configuration fails closed.

The precedence is deterministic:

1. A matching deniedDomains entry rejects the connection.
2. Otherwise, a matching allowedDomains entry permits it without review.
3. Otherwise, the canonical public hostname and port enter the existing one-shot auto-review or human approval flow.

Both arrays default to empty. That preserves the prior behavior: no persistent network authorization, with every eligible public connection reviewed once. The same trusted arrays are copied into main-agent Bash, built-in subagents, and external pi-subagents workers when outer isolation is enforced. Missing or malformed external-worker policy transport hard-denies network access.

Domain allowlists are not a complete data-loss-prevention boundary. A multi-tenant or user-uploadable destination such as github.com can itself be an exfiltration channel; grant only the narrow domains and ports you intend. Real package-manager traffic may require additional hosts—for example, PyPI downloads commonly use files.pythonhosted.org. pi-sandbox never infers or silently adds those domains.

To opt in to outer worker isolation for the external provider, use:

{
  "subagents": {
    "provider": "pi-subagents",
    "externalWorkerIsolation": "enforce"
  }
}

enforce installs a session-scoped PI_SUBAGENT_PI_BINARY wrapper which starts the real Pi worker under a dedicated Sandbox Runtime broker. Bootstrap failure is terminal for that worker; it never falls back to host execution. The wrapper is preserved for nested child launches. This is opt-in while the permission-forwarding supervisor and managed-worktree gate continue to mature.

Managed worktrees receive their own writable checkout plus the smallest known Git metadata paths as read-only access so git status works. The main repository .git is never made writable by this policy; Git mutation from an outer-sandboxed external worker is therefore intentionally unsupported for now.

Optional host IPC fallback

Some host services expose only a Unix socket that the OS sandbox cannot use. The trusted global configuration can enable an approval-driven local Bash backend for those commands:

{
  "hostIPC": {
    "mode": "ask",
    "preflightCommandPrefixes": ["tmux", "/usr/bin/tmux"],
    "retryOnUnixSocketError": true
  }
}

mode is off by default and only accepts off or ask. Prefixes are trimmed, deduplicated, and must be non-empty. A prefix matches only at the start of the trimmed command and must end at whitespace or the end of the command. Prefixes select commands for review; they never authorize execution. The reviewer and one-shot grant are bound to the full command and working directory.

With retryOnUnixSocketError enabled, commands not selected for preflight still run in Sandbox Runtime first. A single host retry is considered only after a nonzero exit whose stderr identifies both Operation not permitted and a socket/connect/IPC operation. The original output is retained, and the approval warns that the first attempt may already have had partial side effects. Successful, timed-out, or aborted commands are never retried.

Host forwarding is intentionally unavailable inside built-in subagents in this version.

Additional trusted read paths

The trusted global configuration may append absolute paths to the sandbox's default read allowlist. This is useful for executables installed below the otherwise-denied home directory:

{
  "subagents": {
    "provider": "builtin"
  },
  "filesystem": {
    "additionalAllowRead": [
      "/home/user/.local/bin/rtk"
    ]
  }
}

filesystem.additionalAllowRead must be an array of absolute paths. These paths extend the defaults; they do not replace the workspace, Node.js, or Sandbox Runtime read permissions. Keep entries as narrow as possible and prefer an exact executable path over allowing an entire bin directory.

Persistent built-in sessions support start, follow_up, wait, status, stop, and nested handoff operations. At most four sessions are live at once, and nesting depth is capped at three.

Installation

Load pi-auto-review first so its broker is available:

pi install npm:@erichll/pi-auto-review
pi install npm:@erichll/pi-sandbox

Do not install security packages project-locally. Development loading from this repository requires PI_AUTO_REVIEW_ALLOW_UNTRUSTED_DEV=1.

Development

From the monorepo root:

npm install
npm run check
npm test
npm run gate:pi-subagents

gate:pi-subagents is a release/manual integration check, not a default CI test. Set PI_SUBAGENTS_GATE_MODEL to a configured test model and provide its normal Pi credential environment. The command creates a dedicated temporary agent directory, never reads or updates your production Pi config, and prints SKIP when those prerequisites are absent.

It allows 10 minutes for the direct-child phase and 15 minutes for the multi-stage workflow, and exits only after the parent reports returned child results. Override those bounds with positive millisecond values in PI_SUBAGENTS_GATE_DIRECT_TIMEOUT_MS and PI_SUBAGENTS_GATE_WORKFLOW_TIMEOUT_MS when needed for a dedicated provider.

The test suite covers real Linux Sandbox Runtime enforcement when its native dependencies are installed, plus deterministic broker, network approval, process cleanup, concurrency, and cross-platform orchestration tests.

Upstream

Sandbox Runtime is an Apache-2.0 licensed research preview: https://github.com/anthropic-experimental/sandbox-runtime.