plugin-authoring

WHERE TO WRITE IT. Write each mod in its own child folder, /Users/asgeirtj/.claude/dev-mods/8ef18cad-66ed-48be-8b40-80138958eb42/<mod-name>/, three files written directly:

• .claude-plugin/plugin.json: { "name": "<mod-name>", "version": "0.1.0", "description": "<one line>" }
• hooks/hooks.json: { "modules": ["./register.tsx"] }, one path, relative to that file
• hooks/register.tsx (or .ts): the hooks module, export const register: Register = (on, options) => { ... }, the type imported from 'claude-code'

A mod that keeps values in $.state has a fourth file, types/index.d.ts: its type contract, declaring each value in interface PluginState under the mod's name, named in plugin.json as "types": "./types/index.d.ts". The module imports its value types from '../types', and claude plugin validate holds every $.state key the module names to that contract.

WHERE THE TYPES ARE. The engine writes them; there is no command to run. Before the mod has loaded: /private/tmp/claude-501/bundled-skills/2.1.301/ace383c272560b2deb3fdf4f15a983db/plugin-authoring/types/claude-code.d.ts, this build's API and built-in tools, written as this skill loaded (this process's own folder: after a restart the next load of this skill writes and names a new one). Once the engine has loaded the mod (from this mods folder, or a --plugin-dir folder it hot-reloads), <mod folder>/.claude-plugin/types/ holds the same for the editor: claude-code/index.d.ts, the API; claude-code-tools/index.d.ts, so e.tool === "Bash" narrows e; claude-code-mcp/index.d.ts, the MCP tools connected when the mod last reloaded; each dependencies plugin's contract; and a tsconfig.json the mod's own extends, so tsc -p <mod folder> type-checks it. The API file carries every event's input and result, every noun and method on $ with its doc comment and an example, and every element's props, in about 14,000 lines: grep it for the name at hand ('tool.call', Pane: {, export type ToolCallResult) and read the declaration it lands on.

WHAT HAPPENS WHEN THE TURN ENDS. Loading this skill through the Skill tool or its slash command starts the engine's watch on the folder that holds the mod folders. The first file written under it makes the engine ask the person, once, right then, while the turn goes on: "Enable hot reloading for this session?", with How does this work? first, then Enable for this session and Not now. That question is the switch, and the person alone answers it: no permission mode, rule or hook does. On Enable for this session the folder joins the session's plugin folders and the mod loads when the turn ends, whole, and each later edit reloads it when the turn that made the edit ends. The answer reaches you as a notice at the start of your next turn, one of: enabled, with what the load came to; declined (the mods are written and load the next time this session starts; the person can ask for the question again); still open (a new prompt from the person takes the question down, and the engine asks again when that turn ends); or off, with the reason (nobody could be asked, as under claude -p; the question got no pick three times; an organization's policy; an untrusted workspace). A process that restarted loads an enabled folder again by itself; otherwise its watch starts the next time this skill loads, and a manifest already in the folder raises the question when that turn ends.

A reload is a fresh load of the module: register runs again and session.start fires again. Values in $.state (the session's) and $.store (across sessions) are the host's and stay; the module's own variables start over.

THIS SESSION is drawn in a terminal. A pane the person opened seats at any width; one opened unasked seats from 144 columns, or 110 as $.ui.open's doc says, and waits below that. Where hot reloading is off as nobody could be asked, claude --plugin-dir <mod folder> starts a session with the mod loaded.

A mod in one paragraph

on(event, matcher?, hook) adds a hook, and every hook is ($, e, next): $ is the engine interface, each call spelled noun then method; e is the event's input, a plain frozen value; next(e) runs the plugins beneath and then the engine's own behaviour, resolving to the event's result. A hook that returns without next answers for itself; next({ ...e, x }) rewrites what the rest sees. The module runs in an environment of its own, with no DOM and no Node: $ reaches everything outside it. JSX compiles against the global h, and the elements come from the drawing surface's own table, const { Box, Text, Button } = $.ui.resolve(e), where e.surface is terminal, desktop, vscode or mobile.

From the ask to the shape

Each example, in /private/tmp/claude-501/bundled-skills/2.1.301/ace383c272560b2deb3fdf4f15a983db/plugin-authoring/examples/, is one complete hooks module; with the two JSON files above, and its contract where it has one, it is a mod that loads, validates and type-checks on this build.

The person asks for	What it is	Shown in
a pane, panel, sidebar, live view	$.ui.open({ id, title }), drawn by a ui.render hook on { component: 'Pane', requestId: id }; it resolves { isPlaced }, whether a surface draws it now	pane.tsx, its contract pane-state.d.ts
a band or row above the prompt	a ui.render hook on { component: 'AbovePrompt' } returning a tree, or next(e) with nothing to show	band.tsx, its contract band-state.d.ts
a status line entry	$.ui.status(text) from any hook; undefined clears it	tool-call.ts
a toast	$.ui.toast(text) from any hook	band.tsx
block, rewrite or react to a tool call	on('tool.call', { tool }, hook): return { deny }, call next({ ...e, ... }), or await next(e) and act on the result	tool-call.ts
change or react to a prompt	on('prompt.submit', hook): next({ ...e, text })	band.tsx
change the system prompt	on('prompt.compose', hook): answer next(e)'s { sections } with one added last (scope: 'session'), replaced or dropped	its doc in the types
play a sound	$.audio.play({ asset }) from any hook, the asset a file of the mod's	its doc in the types
a slash command	$.command.register({ name, description }) in session.start, answered by a command.run hook returning { text }	pane.tsx
values a drawing reads	atom(ref, initial), read($, atom) while drawing, update($, atom, fn) from a handler or another event; the write redraws the readers; each value declared in the contract	pane-state.d.ts
work on a timer, a tool the model calls, a subagent type, model calls, files, processes	$.clock, $.tool, $.agent, $.model, $.fs, $.process	/private/tmp/claude-501/bundled-skills/2.1.301/ace383c272560b2deb3fdf4f15a983db/plugin-authoring/reference.md

Checking it and reading what the engine refused

claude plugin validate <mod folder> reads the manifest and the module's source the way the engine will, and reports what the module hooks and calls and all the engine would refuse. Type-check the mod: with its types laid, tsc -p <mod folder>; else (the first write, claude -p, an SDK host), with the tsconfig.json from the types file's header, kept outside the mod folder, its include naming that file and the mod's hooks. Write at least one *.test.ts for the behaviour asked for and run claude plugin test <mod folder>. A finished mod is at its full path, loaded or not by the hot-reloading answer: enabled, it is loaded when the turn ends, with nothing to run; any other answer, it is not loaded.

While the session hot-reloads a plugin folder the transcript carries one dim line naming the plugin, the event and the reason when a hook fails or a module does not load, and says when a hook's tree did not validate and the engine drew its own instead: <plugin>: ui.render (<Component>) refused: <reason>; the engine drew its own, <plugin> being the one plugin whose hook could have drawn the tree, hooks otherwise. In any other session those lines go to the debug log alone, as <plugin>: <line>. The debug log (claude --debug) carries a line for every occurrence and every result the engine refused; in every session such a tree has a line there beginning ui.render (<Component>): a hook returned a tree that does not validate, followed by the reason.

/private/tmp/claude-501/bundled-skills/2.1.301/ace383c272560b2deb3fdf4f15a983db/plugin-authoring/reference.md is the long form: the full event list, streaming events, ui.render in depth, $.state contracts, --plugin-dir and CLAUDE_CODE_PLUGIN_DIRS while developing, userConfig options, what a test holds, and Sharing a mod, the GitHub road, done when the person asks, or says yes where this skill offers it: .claude-plugin/marketplace.json in its repository and, in the README, /plugin install <mod> --marketplace <owner>/<repo>.