The box image carries skills for common project tasks. The entrypoint installs them into the agent's home at every box start, so they are available in every box and follow the box image when it is updated.

A skill of the same name in the box's merged agent set replaces the built-in one, which is how a project overrides a built-in skill with its own instructions.

Boxes runs the agent in an unprivileged account inside a container with a read-only root filesystem. There is no sudo, apt, or Docker daemon in a box.

The nix skill provides instructions for installing software using the nix a package manager that does not require root access.

The Nix store and user profile persist when a box stops or restarts, so installed tools remain available.

Nix fetches packages through the box egress proxy. A restrictive egress allowlist must permit cache.nixos.org and channels.nixos.org.

Skill instructions: nix

Services started by the agent are only available inside the box's own network. When a user want to review or test a running application, the share-app skill provides a public URL that forwards to the box's private network.

This uses the free Microsoft Dev Tunnels service, which needs the Dev Tunnels credential.

Anyone with the generated URL can open it, so the tunnel should be removed when sharing is finished.

Skill instructions: share-app

Some answers are easier to understand with a picture, for example how components connect or how a request moves through a system. The dashboard can show mermaid diagrams in the agent's answers.

The diagrams skill tells the agent when a diagram helps and when text is sufficient. It also gives rules to write the mermaid source: which diagram type to use for which purpose and how to accommodate small screens.

Before the agent sends a diagram, it can render the diagram to an image with the mermaid CLI and the Chromium of the box. Then it can find syntax errors and layout problems itself. The CLI is downloaded from the npm registry when the agent first uses it.

Skill instructions: diagrams

When doing web development, browser debugging is necessary. The box image includes the Playwright CLI and the playwright-cli skill to do this.

Playwright makes it possible to run a web service in a real headless browser, inspect the accessibility tree, interact with DOM elements, and verify behavior. Chromium is already installed. Firefox and WebKit can be downloaded when a project needs them.

Skill instructions: playwright-cli