Attempted to instantiate App with `new App`, which is no longer valid in Svelte 5 — use mount()

// before (Svelte 3/4) main.js
import App from './App.svelte'
const app = new App({ target: document.getElementById('app'), props: { foo: 'bar' } })
app.$destroy()

// after (Svelte 5)
import { mount, unmount } from 'svelte'
import App from './App.svelte'
const app = mount(App, { target: document.getElementById('app'), props: { foo: 'bar' } })
unmount(app, { outro: true })

mount is not synchronous: onMount has not run when it returns. Call flushSync() from svelte after it if you need that. Replace $set with $state props, and replace $on with callback props.

For third-party code you cannot change, add a compatibility setting (it adds overhead):

// svelte.config.js
export default { compilerOptions: { compatibility: { componentApi: 4 } } }

With SvelteKit, upgrade SvelteKit to a version that supports Svelte 5.