@foldkit/vite-plugin

@foldkit/vite-plugin

Vite plugin for Foldkit: view identity branding for the differ, plus hot module reloading with model preservation.

Installation

npm install -D @foldkit/vite-plugin
# or
pnpm add -D @foldkit/vite-plugin
# or
yarn add -D @foldkit/vite-plugin

Usage

Add the plugin to your vite.config.ts:

import { defineConfig } from 'vite'

import { foldkit } from '@foldkit/vite-plugin'

export default defineConfig({
  plugins: [foldkit()],
})

View identity

Foldkit's differ tracks two independent kinds of identity: user keys, which match siblings in dynamic lists, and a framework-managed identity, which decides whether a matched position is still the same thing. When the producing view function changes, the differ replaces the node instead of patching it, so DOM state cannot bleed across an identity change. Branches rendered inline by one view function share that function's identity and patch in place, exactly as same-type elements do in React; extracting the branches into named view functions makes them identity boundaries.

This plugin supplies that identity. At build time, in dev and production alike, it wraps every function return in your application modules with a branding call that stamps returned vnodes with the function's id (module path plus function name), set-if-absent. Identity therefore attaches at view-function boundaries, and any branching syntax behaves the same: if/else, ternaries, Effect Match, switch statements, and pattern-matching libraries are all equivalent, because identity belongs to the function that produced the subtree, not to the branch that selected it.

Foldkit core modules are never instrumented, and functions that never return vnodes are wrapped inertly. Builds without this plugin fall back to positional matching plus keys, where branch points need hand-written keys.

Hot module reloading

When you save a file during development, the plugin:

• Preserves your application's current state (model)
• Triggers a full page reload
• Restores the preserved model after reload

This means you can make code changes without losing your application's state - forms stay filled, counters keep their values, game positions are maintained, etc.

How it works

The plugin uses Vite's WebSocket connection to communicate between the dev server and browser:

• On file change: The browser sends the current model to the Vite server for preservation
• On reload: The browser requests the preserved model from the server and initializes the Foldkit runtime with it

Model is preserved across hot reloads but cleared on manual browser refreshes, giving you control over when to reset your app.

DevTools overlay

When @foldkit/devtools is installed as a development dependency, the plugin mounts its overlay automatically during development and leaves it out of production builds. No application import or devTools.overlay field is needed.

To include the overlay in production, list @foldkit/devtools in regular dependencies and set devTools.show to 'Always'. Dependency placement controls whether Vite includes the overlay, and show controls whether the Foldkit runtime mounts it.

DevTools MCP relay

Pass devToolsMcpPort to enable the relay that exposes your running Foldkit app to AI agents via the @foldkit/devtools-mcp MCP server:

plugins: [foldkit({ devToolsMcpPort: 9988 })]

When set, the plugin opens a separate WebSocket server on the given port. The MCP server connects to it and forwards typed Request and Response frames between AI agents and your runtime. Without devToolsMcpPort (the default), the relay is not started and the plugin behaves exactly as before.

See the DevTools MCP documentation for setup, the available tools, and how dispatch validation works.

License

MIT