py-sandboxrt — Python Sandbox Runtime

A Python port of the Anthropic Sandbox Runtime, providing lightweight OS-level sandboxing for arbitrary processes without requiring a container.

py-sandboxrt uses native OS sandboxing primitives (sandbox-exec on macOS, bubblewrap on Linux) and proxy-based network filtering. It can be used to sandbox the behaviour of agents, local MCP servers, bash commands, and arbitrary processes.

Installation

Install from PyPI using pip:

pip install py-sandboxrt

Or using uv:

uv add py-sandboxrt

Development Installation

Clone the repository and install in development mode:

git clone https://github.com/saolalab/py-sandboxrt.git
cd py-sandboxrt
pip install -e ".[dev]"

Or with uv:

uv sync --dev

Quick Start

CLI

# Run a command in the sandbox
srt echo "hello world"

# With debug logging
srt --debug curl https://example.com

# Custom settings file
srt --settings /path/to/srt-settings.json npm install

# Command string mode
srt -c "curl https://example.com && echo done"

Library

import asyncio
from srt import SandboxManager, SandboxRuntimeConfig
from srt.config import NetworkConfig, FilesystemConfig

config = SandboxRuntimeConfig(
    network=NetworkConfig(
        allowed_domains=["example.com", "api.github.com"],
        denied_domains=[],
    ),
    filesystem=FilesystemConfig(
        deny_read=["~/.ssh"],
        allow_write=[".", "/tmp"],
        deny_write=[".env"],
    ),
)


async def main():
    mgr = SandboxManager()
    await mgr.initialize(config)

    sandboxed_cmd = await mgr.wrap_with_sandbox("curl https://example.com")

    import subprocess
    proc = subprocess.Popen(sandboxed_cmd, shell=True)
    proc.wait()

    mgr.cleanup_after_command()
    await mgr.reset()


asyncio.run(main())

Configuration

Settings File

By default, py-srt reads ~/.srt-settings.json. Override with --settings:

srt --settings /path/to/config.json <command>

The config file uses the same JSON format as the TypeScript version (camelCase keys are automatically converted):

{
  "network": {
    "allowedDomains": [
      "github.com",
      "*.github.com",
      "npmjs.org",
      "*.npmjs.org"
    ],
    "deniedDomains": ["malicious.com"]
  },
  "filesystem": {
    "denyRead": ["~/.ssh"],
    "allowWrite": [".", "src/", "test/", "/tmp"],
    "denyWrite": [".env", "config/production.json"]
  }
}

Network Configuration (allow-only pattern)

All network access is denied by default.

Key	Description
allowedDomains	Domains permitted to connect to (supports *.example.com wildcards)
deniedDomains	Domains explicitly blocked (checked first, overrides allow)
allowUnixSockets	macOS only: specific Unix socket paths to allow
allowAllUnixSockets	Allow all Unix sockets on both platforms
allowLocalBinding	Allow binding to local ports (default: false)

Filesystem Configuration

Key	Pattern	Description
denyRead	deny-only	Paths to block reading (default: allow all reads)
allowWrite	allow-only	Paths to allow writing (default: deny all writes)
denyWrite	deny-within-allow	Paths to block writing even within allowed paths

Mandatory Deny Paths (Auto-Protected)

Certain sensitive files are always blocked from writes, even within allowed paths:

• Shell configs: .bashrc, .bash_profile, .zshrc, .zprofile, .profile
• Git configs: .gitconfig, .gitmodules
• Other: .ripgreprc, .mcp.json
• Directories: .vscode/, .idea/, .claude/commands/, .claude/agents/, .git/hooks/

How It Works

Dual Isolation Model

• macOS: Uses sandbox-exec with dynamically generated Seatbelt profiles
• Linux: Uses bubblewrap for containerized filesystem and network namespace isolation

Network Filtering

HTTP and SOCKS5 proxy servers run on the host, filtering all traffic against domain allowlists:

1. HTTP/HTTPS — intercepted by an HTTP proxy that validates CONNECT tunnels and forwards requests
2. Other TCP — handled by a SOCKS5 proxy for SSH, database connections, etc.
3. Linux bridge — socat relays between Unix sockets (inside sandbox) and TCP ports (host proxies)

Filesystem Restrictions

• Read (deny-only): All reads allowed by default; deny specific paths
• Write (allow-only): All writes denied by default; explicitly allow paths
• macOS uses Seatbelt regex/glob matching; Linux uses bubblewrap bind mounts

Architecture

src/srt/
├── __init__.py           # Public API exports
├── cli.py                # CLI entrypoint (srt command)
├── config.py             # Pydantic configuration models
├── sandbox_manager.py    # Main orchestrator
├── http_proxy.py         # HTTP/HTTPS proxy with domain filtering
├── socks_proxy.py        # SOCKS5 proxy with domain filtering
├── macos_sandbox.py      # macOS sandbox-exec Seatbelt profiles
├── linux_sandbox.py      # Linux bubblewrap sandboxing
├── sandbox_utils.py      # Shared utilities (paths, globs, env vars)
├── violation_store.py    # In-memory violation tracking
├── platform_utils.py     # Platform detection
└── debug.py              # Debug logging

Platform Support

Platform	Mechanism	Dependencies
macOS	sandbox-exec + Seatbelt profiles	ripgrep
Linux	bubblewrap + network namespaces	bubblewrap, socat, ripgrep
Windows	Not supported	—

Installing Dependencies

macOS:

brew install ripgrep

Linux (Debian/Ubuntu):

sudo apt-get install bubblewrap socat ripgrep

Debug Logging

Set the SRT_DEBUG environment variable to enable verbose logging to stderr:

SRT_DEBUG=1 srt curl https://example.com
# or
srt --debug curl https://example.com

License

Apache-2.0 — see LICENSE for details.

Credits

Python port of anthropic-experimental/sandbox-runtime.