Anthropic Sandbox Runtime (srt)

A lightweight sandboxing tool for enforcing filesystem and network restrictions on arbitrary processes at the OS level, without requiring a container.

srt 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.

Beta Research Preview

The Sandbox Runtime is a research preview developed for Claude Code to enable safer AI agents. It's being made available as an early open source preview to help the broader ecosystem build more secure agentic systems. As this is an early research preview, APIs and configuration formats may evolve. We welcome feedback and contributions to make AI agents safer by default!

Installation

npm install @anthropic-ai/sandbox-runtime

Basic Usage

# Network restrictions
$ srt "curl anthropic.com"
Running: curl anthropic.com
<html>...</html>  # Request succeeds

$ srt "curl example.com"
Running: curl example.com
Connection blocked by network allowlist  # Request blocked

# Filesystem restrictions
$ srt "cat README.md"
Running: cat README.md
# Anthropic Sandb...  # Current directory access allowed

$ srt "cat ~/.ssh/id_rsa"
Running: cat ~/.ssh/id_rsa
cat: /Users/ollie/.ssh/id_rsa: Operation not permitted  # Specific file blocked

Overview

This package provides a standalone sandbox implementation that can be used as both a CLI tool and a library. It's designed with a secure-by-default philosophy tailored for common developer use cases: processes start with minimal access, and you explicitly poke only the holes you need.

Key capabilities:

Network restrictions: Control which hosts/domains can be accessed via HTTP/HTTPS and other protocols
Filesystem restrictions: Control which files/directories can be read/written (defaulting to allowing writes to the current working directory)
Unix socket restrictions: Control access to local IPC sockets
Violation monitoring: On macOS, tap into the system's sandbox violation log store for real-time alerts

Example Use Case: Sandboxing MCP Servers

A key use case is sandboxing Model Context Protocol (MCP) servers to restrict their capabilities. For example, to sandbox the filesystem MCP server:

Without sandboxing (.mcp.json):

{
  "mcpServers": {
    "filesystem": {
      "command": "npx",
      "args": ["-y", "@modelcontextprotocol/server-filesystem"]
    }
  }
}

With sandboxing (.mcp.json):

{
  "mcpServers": {
    "filesystem": {
      "command": "srt",
      "args": ["npx", "-y", "@modelcontextprotocol/server-filesystem"]
    }
  }
}

Then configure restrictions in ~/.claude/settings.json:

{
  "permissions": {
    "deny": [
      "Edit(~/sensitive-folder)"
    ]
  }
}

Now the MCP server will be blocked from writing to the denied path:

> Write a file to ~/sensitive-folder
✗ Error: EPERM: operation not permitted, open '/Users/ollie/sensitive-folder/test.txt'

How It Works

The sandbox uses OS-level primitives to enforce restrictions that apply to the entire process tree:

macOS: Uses sandbox-exec with dynamically generated Seatbelt profiles
Linux: Uses bubblewrap for containerization with network namespace isolation

Dual Isolation Model

Both filesystem and network isolation are required for effective sandboxing. Without network isolation, a compromised process could exfiltrate SSH keys or other sensitive files. Without filesystem isolation, a process could escape the sandbox and gain unrestricted network access.

Filesystem Isolation enforces read and write restrictions:

Read: By default, read access is allowed everywhere. You can deny specific paths (e.g., Read(~/.ssh))
Write: By default, write access is only allowed in the current working directory. You can allow additional paths (e.g., Edit(/tmp)), and deny within allowed paths (e.g., deny Edit(./secret) even though . is allowed)

Network Isolation routes all traffic through proxy servers running on the host:

Linux: Requests are routed via the filesystem over a Unix domain socket. The network namespace of the sandboxed process is removed entirely, so all network traffic must go through the proxies running on the host (listening on Unix sockets that are bind-mounted into the sandbox)
macOS: The Seatbelt profile allows communication only to a specific localhost port. The proxies listen on this port, creating a controlled channel for all network access

Both HTTP/HTTPS (via HTTP proxy) and other TCP traffic (via SOCKS5 proxy) are mediated by these proxies, which enforce your domain allowlists and denylists.

For more details on sandboxing in Claude Code, see:

Architecture

src/
├── index.ts                  # Library exports
├── cli.ts                    # CLI entrypoint (srt command)
├── utils/                    # Shared utilities
│   ├── debug.ts             # Debug logging
│   ├── settings.ts          # Settings reader (permissions + sandbox config)
│   ├── platform.ts          # Platform detection
│   └── exec.ts              # Command execution utilities
└── sandbox/                  # Sandbox implementation
    ├── sandbox-manager.ts    # Main sandbox manager
    ├── sandbox-schemas.ts    # Zod schemas for validation
    ├── sandbox-violation-store.ts # Violation tracking
    ├── sandbox-utils.ts      # Shared sandbox utilities
    ├── http-proxy.ts         # HTTP/HTTPS proxy for network filtering
    ├── socks-proxy.ts        # SOCKS5 proxy for network filtering
    ├── linux-sandbox-utils.ts # Linux bubblewrap sandboxing
    └── macos-sandbox-utils.ts # macOS sandbox-exec sandboxing

Usage

As a CLI tool

The srt command (Anthropic Sandbox Runtime) wraps any command with security boundaries:

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

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

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

As a library

import { SandboxManager } from '@anthropic-ai/sandbox-runtime'
import { spawn } from 'child_process'

// Initialize the sandbox (starts proxy servers, etc.)
await SandboxManager.initialize()

// Wrap a command with sandbox restrictions
const sandboxedCommand = await SandboxManager.wrapWithSandbox('curl https://example.com')

// Execute the sandboxed command
const child = spawn(sandboxedCommand, { shell: true, stdio: 'inherit' })

// Handle exit
child.on('exit', (code) => {
  console.log(`Command exited with code ${code}`)
})

// Cleanup when done (optional, happens automatically on process exit)
await SandboxManager.reset()

Available exports

// Main sandbox manager
export { SandboxManager } from '@anthropic-ai/sandbox-runtime'

// Violation tracking
export { SandboxViolationStore } from '@anthropic-ai/sandbox-runtime'

// TypeScript types
export type {
  SandboxAskCallback,
  IgnoreViolationsConfig,
  FsReadRestrictionConfig,
  FsWriteRestrictionConfig,
  NetworkRestrictionConfig,
} from '@anthropic-ai/sandbox-runtime'

Configuration

NOTE: Configuration format is provisional and subject to change. The current configuration system is borrowed from Claude Code's permission model and uses Claude Code settings files. This format may evolve as the sandbox runtime matures as a standalone tool.

Settings File Locations

Settings are loaded and merged in priority order from:

User settings: ~/.claude/settings.json
Project settings: $CWD/.claude/settings.json
Local settings: $CWD/.claude/settings.local.json
Policy settings: Platform-specific managed settings
- macOS: /Library/Application Support/ClaudeCode/managed-settings.json
- Linux: /etc/claude-code/managed-settings.json
Flag settings: Custom path via --settings flag

Complete Configuration Example

{
  "sandbox": {
    "enabled": true,
    "network": {
      "allowUnixSockets": ["/var/run/docker.sock"],
      "allowLocalBinding": false,
      "httpProxyPort": 8888,
      "socksProxyPort": 1080
    }
  },
  "permissions": {
    "allow": [
      "WebFetch(domain:github.com)",
      "WebFetch(domain:lfs.github.com)",
      "WebFetch(domain:api.github.com)",
      "WebFetch(domain:npmjs.org)",
      "Edit(src/)",
      "Edit(test/)",
      "Read(.)"
    ],
    "deny": [
      "Edit(.env)",
      "Edit(config/production.json)",
      "Read(~/.ssh)",
      "WebFetch(domain:malicious.com)"
    ]
  }
}

Configuration Options

Sandbox Settings

sandbox.enabled - Enable/disable sandboxing (boolean)
sandbox.network.allowUnixSockets - Unix sockets that can be accessed (array of paths)
sandbox.network.allowLocalBinding - Allow binding to local ports (boolean)
sandbox.network.httpProxyPort - Port for your own HTTP proxy (default: uses built-in proxy)
sandbox.network.socksProxyPort - Port for your own SOCKS5 proxy (default: uses built-in proxy)

Permission Rules

Permission rules provide fine-grained control and use Claude Code's permission syntax:

WebFetch(domain:example.com) - Allow/deny network access to a domain
Edit(path) - Allow/deny file write access
Read(path) - Allow/deny file read access

Path Syntax (macOS):

Paths support git-style glob patterns on macOS, similar to .gitignore syntax:

* - Matches any characters except / (e.g., *.ts matches foo.ts but not foo/bar.ts)
** - Matches any characters including / (e.g., src/**/*.ts matches all .ts files in src/)
? - Matches any single character except / (e.g., file?.txt matches file1.txt)
[abc] - Matches any character in the set (e.g., file[0-9].txt matches file3.txt)

Examples:

Edit(src/) - Allow write to entire src/ directory
Edit(src/**/*.ts) - Allow write to all .ts files in src/ and subdirectories
Read(~/.ssh) - Deny read to SSH directory
Edit(.env) - Deny write to .env file (even if current directory is allowed)

Path Syntax (Linux):

Linux currently does not support glob matching. Use literal paths only:

Edit(src/) - Allow write to src/ directory
Read(/home/user/.ssh) - Deny read to SSH directory

All platforms:

Paths can be absolute (e.g., /home/user/.ssh) or relative to the current working directory (e.g., ./src)
~ expands to the user's home directory

Common Configuration Recipes

Allow GitHub access (all necessary endpoints):

{
  "permissions": {
    "allow": [
      "WebFetch(domain:github.com)",
      "WebFetch(domain:lfs.github.com)",
      "WebFetch(domain:api.github.com)"
    ]
  }
}

Restrict to specific directories:

{
  "permissions": {
    "allow": [
      "Edit(src/)",
      "Edit(test/)",
      "Read(.)"
    ],
    "deny": [
      "Edit(.env)",
      "Edit(secrets/)",
      "Read(~/.ssh)"
    ]
  }
}

Common Issues and Tips

Running Jest: Use --no-watchman flag to avoid sandbox violations:

srt "jest --no-watchman"

Watchman accesses files outside the sandbox boundaries, which will trigger permission errors. Disabling it allows Jest to run with the built-in file watcher instead.

Platform Support

macOS: Uses sandbox-exec with custom profiles (no additional dependencies)
Linux: Uses bubblewrap (bwrap) for containerization
Windows: Not yet supported

Platform-Specific Dependencies

Linux requires:

bubblewrap - Container runtime
- Ubuntu/Debian: apt-get install bubblewrap
- Fedora: dnf install bubblewrap
- Arch: pacman -S bubblewrap
socat - Socket relay for proxy bridging
- Ubuntu/Debian: apt-get install socat
- Fedora: dnf install socat
- Arch: pacman -S socat
ripgrep - Fast search tool for deny path detection
- Ubuntu/Debian: apt-get install ripgrep
- Fedora: dnf install ripgrep
- Arch: pacman -S ripgrep

macOS requires:

ripgrep - Fast search tool for deny path detection
- Install via Homebrew: brew install ripgrep
- Or download from: https://github.com/BurntSushi/ripgrep/releases

Development

# Install dependencies
npm install

# Build the project
npm run build

# Type checking
npm run typecheck

# Lint code
npm run lint

# Format code
npm run format

Implementation Details

Network Isolation Architecture

The sandbox runs HTTP and SOCKS5 proxy servers on the host machine that filter all network requests based on permission rules:

HTTP/HTTPS Traffic: An HTTP proxy server intercepts requests and validates them against allowed/denied domains
Other Network Traffic: A SOCKS5 proxy handles all other TCP connections (SSH, database connections, etc.)
Permission Enforcement: The proxies enforce the permissions rules from your configuration

Platform-specific proxy communication:

Linux: Requests are routed via the filesystem over Unix domain sockets (using socat for bridging). The network namespace is removed from the bubblewrap container, ensuring all network traffic must go through the proxies.
macOS: The Seatbelt profile allows communication only to specific localhost ports where the proxies listen. All other network access is blocked.

Filesystem Isolation

Filesystem restrictions are enforced at the OS level:

macOS: Uses sandbox-exec with dynamically generated Seatbelt profiles that specify allowed read/write paths
Linux: Uses bubblewrap with bind mounts, marking directories as read-only or read-write based on configuration

Default filesystem permissions:

Read: Allowed everywhere by default. You can deny specific paths using Read(path) deny rules
- Example: Deny Read(~/.ssh) to block access to SSH keys
Write: Only allowed in the current working directory by default. You can:
- Allow additional paths using Edit(path) allow rules (e.g., Edit(/tmp))
- Deny specific paths within allowed directories (e.g., deny Edit(.env) even though . is allowed)

This model lets you start with broad read access but tightly controlled write access, then refine both as needed.

Violation Detection and Monitoring

When a sandboxed process attempts to access a restricted resource:

Blocks the operation at the OS level (returns EPERM error)
Logs the violation (platform-specific mechanisms)
Notifies the user (in Claude Code, this triggers a permission prompt)

macOS: The sandbox runtime taps into macOS's system sandbox violation log store. This provides real-time notifications with detailed information about what was attempted and why it was blocked. This is the same mechanism Claude Code uses for violation detection.

# View sandbox violations in real-time
log stream --predicate 'process == "sandbox-exec"' --style syslog

Linux: Bubblewrap doesn't provide built-in violation reporting. Use strace to trace system calls and identify blocked operations:

# Trace all denied operations
strace -f srt <your-command> 2>&1 | grep EPERM

# Trace specific file operations
strace -f -e trace=open,openat,stat,access srt <your-command> 2>&1 | grep EPERM

# Trace network operations
strace -f -e trace=network srt <your-command> 2>&1 | grep EPERM

Advanced: Bring Your Own Proxy

For more sophisticated network filtering, you can configure the sandbox to use your own proxy instead of the built-in ones. This enables:

Traffic inspection: Use tools like mitmproxy to inspect and modify traffic
Custom filtering logic: Implement complex rules beyond simple domain allowlists
Audit logging: Log all network requests for compliance or debugging

Example with mitmproxy:

# Start mitmproxy with custom filtering script
mitmproxy -s custom_filter.py --listen-port 8888

# Configure sandbox to use your proxy
{
  "sandbox": {
    "network": {
      "httpProxyPort": 8888
    }
  }
}

Important security consideration: Even with domain allowlists, exfiltration vectors may exist. For example, allowing github.com lets a process push to any repository. With a custom MITM proxy and proper certificate setup, you can inspect and filter specific API calls to prevent this.

Security Limitations

Network Sandboxing Limitations: The network filtering system operates by restricting the domains that processes are allowed to connect to. It does not otherwise inspect the traffic passing through the proxy and users are responsible for ensuring they only allow trusted domains in their policy.

Users should be aware of potential risks that come from allowing broad domains like `github.com` that may allow for data exfiltration. Also, in some cases it may be possible to bypass the network filtering through [domain fronting](https://en.wikipedia.org/wiki/Domain_fronting).

Privilege Escalation via Unix Sockets: The allowUnixSockets configuration can inadvertently grant access to powerful system services that could lead to sandbox bypasses. For example, if it is used to allow access to /var/run/docker.sock this would effectively grant access to the host system through exploiting the docker socket. Users are encouraged to carefully consider any unix sockets that they allow through the sandbox.
Filesystem Permission Escalation: Overly broad filesystem write permissions can enable privilege escalation attacks. Allowing writes to directories containing executables in $PATH, system configuration directories, or user shell configuration files (.bashrc, .zshrc) can lead to code execution in different security contexts when other users or system processes access these files.
Linux Sandbox Strength: The Linux implementation provides strong filesystem and network isolation but includes an enableWeakerNestedSandbox mode that enables it to work inside of Docker environments without privileged namespaces. This option considerably weakens security and should only be used incases where additional isolation is otherwise enforced.

Known Limitations and Future Work

Linux proxy bypass: Currently uses environment variables (HTTP_PROXY, HTTPS_PROXY, ALL_PROXY) to direct traffic through proxies. This works for most applications but may be ignored by programs that don't respect these variables, leading to them being unable to connect to the internet.

Future improvements:

Proxychains support: Add support for proxychains with LD_PRELOAD on Linux to intercept network calls at a lower level, making bypass more difficult
Linux violation monitoring: Implement automatic strace-based violation detection for Linux, integrated with the violation store. Currently, Linux users must manually run strace to see violations, unlike macOS which has automatic violation monitoring via the system log store

Name		Name	Last commit message	Last commit date
Latest commit History 4 Commits
src		src
.gitignore		.gitignore
.prettierrc.json		.prettierrc.json
LICENSE		LICENSE
README.md		README.md
eslint.config.js		eslint.config.js
package-lock.json		package-lock.json
package.json		package.json
tsconfig.json		tsconfig.json

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Anthropic Sandbox Runtime (srt)

Installation

Basic Usage

Overview

Example Use Case: Sandboxing MCP Servers

How It Works

Dual Isolation Model

Architecture

Usage

As a CLI tool

As a library

Available exports

Configuration

Settings File Locations

Complete Configuration Example

Configuration Options

Sandbox Settings

Permission Rules

Common Configuration Recipes

Common Issues and Tips

Platform Support

Platform-Specific Dependencies

Development

Implementation Details

Network Isolation Architecture

Filesystem Isolation

Violation Detection and Monitoring

Advanced: Bring Your Own Proxy

Security Limitations

Known Limitations and Future Work

About

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Anthropic Sandbox Runtime (srt)

Installation

Basic Usage

Overview

Example Use Case: Sandboxing MCP Servers

How It Works

Dual Isolation Model

Architecture

Usage

As a CLI tool

As a library

Available exports

Configuration

Settings File Locations

Complete Configuration Example

Configuration Options

Sandbox Settings

Permission Rules

Common Configuration Recipes

Common Issues and Tips

Platform Support

Platform-Specific Dependencies

Development

Implementation Details

Network Isolation Architecture

Filesystem Isolation

Violation Detection and Monitoring

Advanced: Bring Your Own Proxy

Security Limitations

Known Limitations and Future Work

About

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