Plain rolldown

tsdown runs on rolldown, and the plugin uses only standard build hooks, so it also works in a plain rolldown build. This setup was tested with rolldown 1.2.16. You set up the two things tsdown would otherwise give it:

• One output file per source file, so a component’s ./util.ts import finds ./util.js beside it: preserveModules with preserveModulesRoot: 'src'.
• Types for your .ts files: rolldown-plugin-dts, the plugin tsdown uses. This plugin only adds the .svelte.d.ts files.

import { defineConfig } from 'rolldown';
import { dts } from 'rolldown-plugin-dts';
import { svelte } from '@ogygia/tsdown-plugin-svelte';

export default defineConfig({
  input: ['src/index.ts', 'src/util.ts'],
  external: ['svelte', /\.svelte$/],
  plugins: [dts(), svelte()],
  output: {
    dir: 'dist',
    format: 'esm',
    preserveModules: true,
    preserveModulesRoot: 'src',
    entryFileNames: '[name].js'
  }
});

List every .ts file a component imports in input, so each one is built to its own file. Set the plugin’s out to output.dir if that is not dist.

The plugin ships components; it does not compile .svelte files into an app. For that, use @sveltejs/vite-plugin-svelte or rollup-plugin-svelte.