According to the article, a mod can respond to prompts, tool calls, and interface-render events; observe them; change their rendering; or handle an event instead of the default behavior.
The article says Claude Code ships six built-in mods, including agents-md, diff, plugin-authoring, sec-default, telemetry, and you-should-know. The last runs a side agent that surfaces information the main agent may have missed and is off by default. In the author's example, it flagged pre-existing uncommitted work across roughly 45 files.
The author also describes a custom Next Steps Supervisor mod that runs after a turn editing files, running commands, or making at least eight tool calls, then presents suggested next actions as clickable buttons. A separate open-source plugin, ModSmith, is presented as containing built-in mods and a template for composing new ones.
The source is an analysis and practitioner account. It does not independently establish the stability of the Mods API, the security properties of third-party plugins, broad availability, or performance and adoption data.