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'), kept here in types/. To read one running from source:

claude --plugin-dir mods/diff

Testing

A mod's tests are in its tests/ folder, and run with

claude plugin test mods/diff

A test gets the engine's own $ and a plugin's on. Each call on $ is one the engine makes, through every hook of the mod loaded as it ships. The hooks the test registers with on sit beneath the mod, where the rest of the world would be, and nothing is beneath them: a call they leave unanswered throws, naming its event.

A test file is named for what it covers under hooks/ (register.test.ts beside hooks/register.ts, git.test.ts beside hooks/git/), and holds its imports, the tier the mod loads in, and one describe titled with that name; what several tests share sits under tests/fixtures/, one export a file.

import { describe, expect, mock, test, tier } from 'claude-code/testing'

tier('builtin')

describe('register', () => {
  test('outside a git repository /diff says so, opens nothing', async ($, on) => {
    const opened: string[] = []
    mock.clock(on)
    on('session.start', ($, e) => ({ cwd: e.cwd }))
    on('command.register', ($, e) => ({ value: { command: e.name } }))
    on('process.run', () => ({
      value: { exitCode: 128, stdout: '', stderr: 'fatal: not a git repository' },
    }))
    on('ui.open', ($, e, next) => {
      opened.push(e.id)
      return next(e)
    })

    await $.session.start({ surface: 'terminal', isInteractive: true, cwd: '/work' })
    const { text } = await $.command.run({
      command: 'diff',
      args: '',
      origin: { kind: 'composer' },
    })

    expect(text).toContain("isn't in a git repository")
    expect(opened).toEqual([])
  })
})

The kit's mock answers the world beneath the mod from memory, noun by noun, each member a plain function over on registering hooks where the test calls it: mock.env(on, variables) for $.env, mock.store(on, entries) for $.store, and mock.clock(on) for $.clock, whose advance(ms) resolves every wait the mod asked for ($.clock.sleep, after, every) as the clock crosses it. To see what a dispatch does before it answers, start it unawaited, await clock.settle(), then look: the clock stays where it was. $.ui.press({ plugin, key }) presses a Button the test rendered, as a click in the terminal does.

tsc -p mods/tsconfig.json typechecks every mod's hooks and tests against types/ and each mod's own types/ contract.

Composing mods: noun contracts

A mod that adds a noun to $ in the engine.create fold owns that noun's types, and keeps them in one place: its types/index.d.ts, a declaration file with no imports that exports the types the noun is made of, each named for the noun, and declares the noun on EngineInterface in claude-code (telemetry/types/index.d.ts exports Telemetry, TelemetryLogEntry, TelemetryMarkEntry and the rest, and declares $.telemetry).

The contract is the only declaration of the noun. The mod's own hooks import its types from the folder (import type { Telemetry } from '../types'), and the value its engine.create hook returns is checked against EngineInterface['telemetry'], so the implementation cannot drift from what callers read.
A mod that calls another's noun reads the same file and never copies it: mods/tsconfig.json includes */types/**/*.d.ts, so $.telemetry.log(…) in diff types against telemetry's contract as it stands, and a helper that must name one of its types imports it from that folder by path.
A test of a mod that calls another's noun seats a provider for it, an inline plugin whose engine.create hook adds the noun, and answers the calls the way it answers the engine's: on('telemetry.log', ($, e) => ({ value: undefined })) runs above the provider's own method, its e typed by the contract. With no provider loaded the $ build refuses the hook, naming the noun nobody provides.

A plugin outside this repository that depends on a mod's noun points its tsconfig include at that mod's types/ folder for now; once the engine writes the contracts of the plugins a session has installed, /plugin-types will put them beside claude-code.d.ts and the include goes away.

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.