mcp-box

Your AI agent's tools run with your permissions. When an MCP server can call run_command or write files, a buggy prompt or a malicious instruction can reach your home directory, your SSH keys, and your cloud credentials. mcp-box puts every MCP server inside a locked-down Docker sandbox with a read-only root filesystem, explicit writable mounts, dropped Linux capabilities, and per-profile network policy. Only the folders you mount are writable; the rest of your host stays out of reach.

mcp-box is the boringly reliable way to run MCP servers safely: inspectable, reproducible, and default-deny by design.

mcp-box is a single, portable Go binary that launches turnkey, immutable, strictly-isolated container sandboxes for Model Context Protocol (MCP) servers. Nix is used for deterministic image builds when present, but it is entirely optional. Without Nix, mcp-box pulls the same pinned OCI image definitions from GHCR and runs them under the same Docker isolation model. Docker is the only hard requirement.

Same server, same hardening, same sandbox boundary — only the image source changes.

Security Audit Checks

You can audit the sandbox boundary directly by overriding the container command with -- bash -c '...'. These checks prove the boundary itself, not just the MCP server running inside it. The hardening (read-only root, no network, dropped capabilities, host UID mapping) applies to any process in the box:

1. Read-only filesystem — writes outside /workspace are rejected:

   ./mcp-box run shell -- bash -c 'touch /etc/naughty'
   # => touch: cannot touch '/etc/naughty': Read-only file system   (exit 1)
   

2. Network isolation — with the default --network none, DNS/egress fail:

   ./mcp-box run shell -- bash -c 'curl -I https://google.com'
   # => curl: (6) Could not resolve host: google.com               (exit 6)
   

3. No privilege escalation — sudo is not even present in the image:

   ./mcp-box run shell -- bash -c 'sudo -l'
   # => bash: line 1: sudo: command not found                       (exit 127)
   

4. Host UID/GID mapping — the process is you, not root:

   ./mcp-box run shell -- bash -c 'id'
   # => uid=<your uid> gid=<your gid> ...   (files in /workspace are owned by you)
   

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Key Features

1. Strict Sandboxing:
   • Immutable Root (--read-only): The entire root filesystem is mounted read-only.
   • Transient State (--tmpfs): Writable spaces (/tmp and /run) exist solely in RAM and disappear once the container stops.
   • Zero Capabilities (--cap-drop=ALL): The running processes have no special Linux kernel capabilities.
   • No Privilege Escalation (no-new-privileges:true): Prevents elevation to root inside the sandbox.
   • Per-Profile Network Policy:
     • Most sandboxes default to --network none.
     • Networked profiles are explicit and opt-in, so a server only gets egress when its job genuinely requires it.
     • The fetch profile is the exception by design: it uses a controlled network mode for web access, while still keeping the container boundary intact.
   • Scoped Workspaces: Only specifically mounted host directories (--workspace) are visible to the server at /workspace.
2. Correct File Ownership:
   • Containers run mapped to your host UID/GID (-u $(id -u):$(id -g)), ensuring that files written to mounted workspaces are owned by you (not root) and don't trigger host-side permission errors.
3. Painless Integration:
   • Built-in configuration generator (mcp-box config <server>) prints out paste-ready JSON snippets to plug directly into claude_desktop_config.json or OpenClaw configurations.
4. Zero-Dependency Nix Autonomy:
   • If Nix is installed, running a sandbox automatically triggers a local rebuild and load of the OCI image.
   • If Nix is absent, mcp-box automatically detects this and falls back to pulling pre-built, identical, and secure OCI images directly from the GitHub Container Registry (ghcr.io/lowcache), making Nix entirely optional for the end-user.

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CLI Surface

mcp-box is designed to be inspectable, scriptable, and auditable.

• mcp-box run <server> — launch a sandboxed MCP server
• mcp-box stop <id> — stop a running sandbox
• mcp-box ps — list active sandboxes
• mcp-box inspect <id> — show the full sandbox spec and policy
• mcp-box logs <id> — read audit logs
• mcp-box logs --follow — stream live events
• mcp-box config <server> — print paste-ready client config
• mcp-box build <server> — rebuild or refresh an OCI image

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Quickstart

# 1. Get the binary (Docker is the only dependency)
curl -sSL https://github.com/lowcache/mcp-box/releases/latest/download/mcp-box-linux-amd64 -o mcp-box
chmod +x mcp-box && mv mcp-box ~/.local/bin/

# 2. Prove the sandbox holds — this write MUST fail
mcp-box run shell --workspace /tmp/demo -- bash -c 'touch /etc/naughty'
# => touch: cannot touch '/etc/naughty': Read-only file system

# 3. Wire it into your AI client (paste the output into claude_desktop_config.json)
mcp-box config sqlite

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Pre-Packaged Sandboxes

Server Name	Language	Included Utilities	Network Mode	Primary Purpose
sqlite	Python	sqlite3 CLI, fastmcp SDK	none	High-performance, isolated database querying.
shell	Python	bash, ripgrep, fd, git, curl, jq, sqlite, tar	none	Safe, sandboxed script running and file operations.
filesystem	Node.js	ripgrep, fd, git	none	Scoped filesystem read/write and code searching.
fetch	Node.js	curl	bridge	Safe, isolated web fetching and scraping.

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Policy, Compose, and Auditability

mcp-box is not just a runner — it is a trust layer for MCP execution.

Planned core platform features:

• Policy profiles for common trust levels: readonly, dev, trusted, internet-off, internet-on
• Compose-style specs for multi-server setups
• Append-only audit logs for sandbox lifecycle and denied actions
• ps / inspect / logs so every sandbox can be checked after launch
• Pinned image digests for reproducible, reviewable execution

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Dependencies

Depending on your installation path, mcp-box has distinct dependency requirements:

• Runtime Boundary (All Users):
  • Docker Engine (Must be active and running locally on the host system).
• Local Image Building (Source Flow with Nix):
  • Nix (with experimental flakes and nix-command enabled).
• CLI Compilation (Source Flow with Go):
  • Go compiler v1.22 or higher (only required if building the executable from source without using Nix).
• Zero-Dependency Fallback Flow:
  • None. The pre-compiled CLI binary runs standalone and automatically pulls the pre-built, multi-arch OCI images straight from GHCR into your local Docker daemon.

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Architecture

graph TD
    subgraph Host [Host Environment]
        Agent[AI Agent / Claude Desktop] <-->|stdio piping| CLI[mcp-box CLI]
        CLI -->|Checks for Nix| NixDetect{Nix Installed?}
        
        NixDetect -->|Yes: Source Flow| NixBuild[nix build .#server]
        NixBuild -->|Stream tarball| DockerLoad[docker load]
        
        NixDetect -->|No: Registry Flow| DockerPull[docker pull ghcr.io]
        DockerPull -->|Tag locally| DockerLoad
        
        DockerLoad -->|Loads image| Docker[Docker Engine]
    end
    
    subgraph Sandbox [Docker Sandbox]
        Server[MCP Server]
        Tools[Isolated Tools: git, rg, sqlite3, curl]
        Workspace[Mounted Workspace: /workspace]
    end
    
    Docker -->|spawns with strict isolation| Sandbox
    CLI <-->|stdio piping| Server

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Installation

Depending on your host environment, you can install and run mcp-box with three different avenues:

Option A: Pre-built Go Binary (No Nix / Docker-only)

For systems that only have Docker installed:

1. Download the compiled CLI binary (pick the asset matching your OS/arch — linux/darwin, amd64/arm64):

   curl -sSL https://github.com/lowcache/mcp-box/releases/latest/download/mcp-box-linux-amd64 -o mcp-box
   chmod +x mcp-box
   

2. Move to PATH (Optional):

   mv mcp-box ~/.local/bin/  # Or another folder in your PATH
   

On first execution, mcp-box will automatically detect the absence of Nix and pull the pre-built OCI images from ghcr.io/lowcache into your local Docker daemon.

Option B: From Source via Nix Flake (Nix/NixOS)

For systems running Nix/NixOS:

• Run directly without installing:

  nix run github:lowcache/mcp-box -- list
  

• Install to your user profile:

  nix profile install github:lowcache/mcp-box
  

• Declarative Installation (NixOS / Home Manager): Add the flake input and package to your configuration:

  # flake.nix inputs:
  inputs.mcp-box.url = "github:lowcache/mcp-box";
  
  # In systemPackages or home.packages:
  inputs.mcp-box.packages.${pkgs.system}.default
  

On first execution, mcp-box will build the Go binary and OCI images purely from source and load them directly into your local Docker daemon.

Option C: Compile from Source (Go Compiler)

If you want to compile the CLI binary manually without Nix:

1. Clone the repository:

   git clone https://github.com/lowcache/mcp-box.git
   cd mcp-box
   

2. Compile the binary (the Go module lives in src/go):

   cd src/go
   go build -o ../../mcp-box .
   

You can now run ./mcp-box directly, which will pull OCI layers from the registry or build locally using Nix based on your host environment.

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Usage Guide

1. Show Help & Supported Servers

./mcp-box help
./mcp-box list

2. Run a Sandbox Interactively

You can launch any server interactively to test its behavior and tools:

./mcp-box run sqlite --workspace /tmp/sandbox-db -- --db /workspace/test.db

3. Build/Force-Update an OCI Image

If you want to manually rebuild or force-update a Nix-built image:

./mcp-box build sqlite

4. Integration with AI Clients

mcp-box config <server> prints a ready-to-paste mcpServers JSON block (with an absolute path to the binary). The same block works for both Claude Desktop and Claude Code.

./mcp-box config sqlite

Claude Code (CLI)

Claude Code does not use claude_desktop_config.json. Add the server with one command — note the nested -- (the first separates claude's flags from the subprocess; the second is consumed by mcp-box to forward server args):

# user scope (available in every project); drop -s user for the current project only
claude mcp add -s user mcp-box-sqlite -- \
  mcp-box run sqlite --workspace /abs/path/to/workspace -- --db /workspace/db.sqlite

Or, to commit a shareable config to your repo, drop the mcp-box config JSON block straight into a project-root .mcp.json — it uses the exact {"mcpServers": …} shape that config emits.

Verify with claude mcp list.

Claude Desktop

Paste the config block into claude_desktop_config.json. Its location is OS-specific:

OS	Path
macOS	~/Library/Application Support/Claude/claude_desktop_config.json
Windows	%APPDATA%\Claude\claude_desktop_config.json
Linux	~/.config/Claude/claude_desktop_config.json

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