How to scan and transform "app.html" through Vite Custom Plugin?

[Git002]

I’m trying to write a custom Vite plugin for SvelteKit that modifies HTML when running npm run dev — basically to inject some scripts and metadata automatically.

The problem is: Vite never seems to detect or pass app.html through the plugin system. I’ve tried using transform(), transformIndexHtml(), and other plugin hooks. The plugin triggers just fine for .svelte files and even for CSS like app.css, but app.html is not getting passed.

My vite.config.ts setup:

import { sveltekit } from '@sveltejs/kit/vite';
import { defineConfig } from 'vite';
import htmlPlugin from './src/lib/myVitePlugin'; // testing my custom plugin

export default defineConfig({
  plugins: [htmlPlugin(), sveltekit()]
});

I expected transformIndexHtml or any other vite hook to catch app.html the way it does with index.html or any other .svelte files normally. But nothing happens — it seems like SvelteKit bypasses that path completely.

So my questions:

• Is app.html intentionally skipped from Vite’s HTML transform pipeline?
• If so, what’s the “right” way to programmatically inject global scripts/metadata in dev? I ddon't want the user to directly make changes in app.html and commit to it.

Would appreciate any clarity on this, and what the recommended approach is if I want to achieve it.

[Git002]

I did found a way, to basically inject <svelte:head> if not exists, or if it does, then add content into the root +layout.svelte file through a custom svelte pre-processor. Even though this fulfills my requirement, I still couldn't wrap my head as to why transform function couldn't get app.html only, and what's any other way to access that file from Vite Plugin...

[paulovieira]

I’m trying to write a custom Vite plugin for SvelteKit that modifies HTML when running npm run dev — basically to inject some scripts and metadata automatically.

Try the transformPageChunk option for resolve (in a server hook): https://svelte.dev/docs/kit/@sveltejs-kit#ResolveOptions

See these examples:

• https://svelte.dev/docs/kit/accessibility#The-lang-attribute
• https://svelte.dev/docs/kit/seo#Manual-setup-AMP