Preprocessing .svelte files causes error

[theetrain]

Not sure if duplicate of #653.

This impacts sveltejs/cli#384.

Problem

When configuring a new SvelteKit project with mdsvex, using .svelte as an extensions option, an error appears.

svelte.config.js

import { mdsvex } from 'mdsvex'
import adapter from '@sveltejs/adapter-auto'

/** @type {import('@sveltejs/kit').Config} */
const config = {
  kit: {
    adapter: adapter(),
  },

  preprocess: [
    mdsvex({
      extensions: ['.svelte', '.svx'],
    }),
  ],
  extensions: ['.svelte', '.svx'],
}

export default config

src/routes/+page.svelte (fresh project)

<h1>Welcome to SvelteKit</h1>
<p>Visit <a href="https://svelte.dev/docs/kit">svelte.dev/docs/kit</a> to read the documentation</p>

Runtime error:

[vite] Pre-transform error: .svelte-kit/generated/root.svelte:48:68 `</p>` attempted to close an element that was not open
https://svelte.dev/e/element_invalid_closing_tag
[vite] Error when evaluating SSR module /.svelte-kit/generated/root.js:
|- 

[vite] Error when evaluating SSR module /.svelte-kit/generated/server/internal.js:
|- 

[vite] Error when evaluating SSR module /node_modules/@sveltejs/kit/src/runtime/server/index.js:

Environment

  System:
    OS: macOS 15.2
    CPU: (8) x64 Intel(R) Core(TM) i7-4770HQ CPU @ 2.20GHz
    Memory: 396.71 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.11.0 - ~/.volta/tools/image/node/22.11.0/bin/node
    npm: 10.9.0 - ~/.volta/tools/image/node/22.11.0/bin/npm
    pnpm: 9.15.2 - ~/.volta/bin/pnpm
  Browsers:
    Chrome: 131.0.6778.205
    Safari: 18.2
  npmPackages:
    @sveltejs/adapter-auto: ^3.0.0 => 3.3.1 
    @sveltejs/kit: ^2.0.0 => 2.15.1 
    @sveltejs/vite-plugin-svelte: ^4.0.0 => 4.0.4 
    mdsvex: ^0.11.2 => 0.11.2 
    svelte: ^5.0.0 => 5.16.0 
    vite: ^5.4.11 => 5.4.11

Issue also appears with [email protected].

[manuel3108]

Renaming src/routes/+page.svelte to src/routes/+page.svx actually solves the error, without modifying anything else. So this seems to be related to the specific .svelte extension.