Svelte config

Drop a svelte.config.js at the root of your app to customize the Svelte compiler. Mochi reads it on startup (and during mochi build) and merges its compilerOptions into the framework defaults.

// svelte.config.js
export default {
  compilerOptions: {
    experimental: {
      async: true,
    },
  },
};

The file is optional. If it is missing, Mochi logs a warning and uses its defaults — the most important one being experimental.async: true, which is what allows await at the top level of .svelte components and is currently a Svelte 5 opt-in.

Custom config path

By default Mochi looks for ./svelte.config.js at the current working directory. Point it elsewhere with svelteConfigPath on Mochi.serve() or build() — relative paths resolve against process.cwd(), absolute paths are used as-is:

await Mochi.serve({
  svelteConfigPath: './configs/svelte.staging.config.js',
  routes,
});

Merge order

Framework defaults
Your compilerOptions — overlaid on top of the defaults. You can override most fields.

Framework-owned fields

A small set of compilerOptions are managed by Mochi and cannot be overridden, because they are part of the framework’s contract with the compiler:

Field	Why
`generate`	Set to `'server'` for SSR builds, `'client'` for hydration
`filename`	Always the actual file path
`dev`	Tied to the `development` flag passed to `Mochi.serve()`

Everything else is yours to set.

Where it applies

The merged options are used everywhere Mochi invokes the Svelte compiler:

SSR compilation of .svelte files
SSR compilation of .svelte.js / .svelte.ts rune modules
Client-side island bundles (both .svelte and .svelte.[jt]s)
mdsvex .md / .svx files (server target)

Limitations

Only compilerOptions is read today. SvelteKit-style keys like preprocess, extensions, and kit are not supported.