fence

Fence sandboxes them without containers. It restricts network access and filesystem access at the OS level, so a rogue script can't phone home or delete your files.

What's free: Everything. Apache 2.0 license. Single Go binary, no dependencies, no account.

The value proposition is simplicity. Docker gives you isolation but requires the Docker daemon, images, and significant overhead. Fence is one binary. Run `fence <command>` and it executes with network and filesystem restrictions. That's it. For AI agent sandboxing, where you're running LLM-generated code and need guardrails, this is exactly the right weight class.

The catch: it's brand new and early-stage. Linux only (uses kernel namespaces and seccomp). No macOS or Windows support. The security model is narrower than a full container. It restricts network and filesystem but doesn't provide complete process isolation. For high-security use cases, you still want containers or VMs. For 'don't let this script access the internet or my home directory,' it's perfect.