Svelte config

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Svelte config

Mochi loads ./svelte.config.js from the cwd at startup and merges its compilerOptions into the framework defaults. Drop the file at the root of your app to customize the Svelte compiler.

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

The file is optional. If it is missing, Mochi logs [mochi] No Svelte config found at … — using framework defaults. and continues with the framework defaults.

Both ESM (export default) and CJS (module.exports) are supported. In dev, Mochi watches the file — an edit reloads compilerOptions without restarting the server.

Framework defaults

Two fields are seeded before your config merges in:

Field	Default	Why
`experimental.async`	`true`	Enables top-level `await` in `.svelte` components (Svelte 5 opt-in).
`discloseVersion`	`false`	Suppresses the `<!-- svelte v… -->` comment in SSR output.

Override either by setting it in your own compilerOptions.

svelteConfigPath

Pass svelteConfigPath to Mochi.serve() or build() to load the config from somewhere else. Relative paths resolve against process.cwd(). Absolute paths are used as-is.

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

Framework-owned fields

Mochi forces three compilerOptions at every compile call site:

Field	Forced to
`generate`	`'server'` for SSR builds, `'client'` for hydration bundles.
`filename`	The actual file path being compiled.
`dev`	The `Mochi.serve()` `development` flag — client target only. Server compiles do not force `dev`.

Every other field — runes, css, accessors, cssHash, discloseVersion, experimental.* — 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
mdsvex .md / .svx files (server target)

Under svelteCompiler: 'rsvelte', the function-valued cssHash and warningFilter are stripped: cssHash with a one-time warning, warningFilter silently, since Mochi never surfaces compiler warnings. See rsvelte.

What is not read

Only compilerOptions is honored. SvelteKit-style top-level keys are ignored:

preprocess — register preprocessors through the compile:preprocessors filter instead.
extensions — Mochi’s accepted extensions are fixed (.svelte, .svelte.[jt]s, .md, .svx).
kit — SvelteKit-only.