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 FRAMEWORK_COMPILER_DEFAULTS.

Both ESM (export default) and CJS (module.exports) are supported. In dev, the file is watched — edits trigger a reload of compilerOptions without restarting the server.

Framework defaults

Two fields are seeded before your config is merged in:

Either can be overridden by setting it in your own compilerOptions.

svelteConfigPath

Pass svelteConfigPath to Mochi.serve() or build() to load the config from somewhere other than ./svelte.config.js. 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

Three compilerOptions are forced by Mochi at every compile call site and cannot be overridden — they are part of the framework’s contract with the compiler:

Every other field — runes, css, accessors, cssHash, discloseVersion, experimental.*, etc. — is yours to set.

Do NOT set generate or filename in your config; the merge step overwrites them on every compile.

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)

What is not read

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

• preprocess — register preprocessors via the compile:preprocessors filter on the extensions API instead, not in svelte.config.js.
• extensions — Mochi’s accepted extensions are fixed (.svelte, .svelte.[jt]s, .md, .svx). Adding .svx to extensions here has no effect; it is already wired into the mdsvex loader.
• kit — SvelteKit-only; ignored.

Do NOT put a preprocess array in svelte.config.js and expect it to run; instead, expose preprocessors through a Mochi extension and the compile:preprocessors filter.