A mod is a Claude Code plugin whose behaviour lives in a hooks module: one register(on, options) entry that hooks the engine's events as functions ($, e, next). These three ship inside Claude Code; this folder is their source, published as it is built into the binary.

Mod	What it does	Seated
`sec-default`	Keeps an organization's classic hooks, prompt content, managed settings and tool policy out of reach of the plugins a person installs; adds no policy of its own.	Outermost, on a machine with managed settings or for a Team or Enterprise organization, unless managed `prependPlugins` says otherwise
`diff`	`/diff`: the session's uncommitted changes in a pane beside the transcript, file by file with their hunks, refreshed as Claude edits files and runs commands.	Built in
`telemetry`	Adds `$.telemetry` (`log`, `mark`) in the `engine.create` fold so a plugin can record an event as a first-party analytics row; sends nothing wherever Claude Code's analytics are off.	Built in

Each folder is a complete plugin: .claude-plugin/plugin.json, a hooks/hooks.json naming the module, and TypeScript under hooks/ typed against the declarations /plugin-types writes (import type … from 'claude-code'). To read one running from source:

claude --plugin-dir mods/diff

Early access: hooks modules load only where function hooks are enabled, and the API these mods are written against may change between releases without notice. They are not listed in this repository's marketplace; the copies that matter are the ones already in your Claude Code.