Svelte New Added 10 September 2026

SvelteKit: Data returned from `load` is not serializable

Data from a server load function is serialised so the browser can hydrate with it, and the serialiser handles plain objects, arrays, Date, Map, Set and a few more, but not class instances. ORM rows are the usual offender: a Prisma Decimal, a Mongo ObjectId or a Mongoose document looks like a plain object in the terminal and is not one.

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

Quick fix

// +page.server.js: map the row to a plain object before returning it
export async function load({ params }) {
  const doc = await Order.findById(params.id);
  return {
    order: {
      id: doc._id.toString(),
      total: Number(doc.total),      // Decimal -> number
      placedAt: doc.placedAt         // Date is fine
    }
  };
}

// Mongoose has a helper for this
const order = doc.toObject({ flattenObjectIds: true });

// Prisma Decimal and BigInt both need converting
total: order.total.toNumber()

// Keeping a class on the client: send the fields, rebuild in a universal load
// +page.js
export async function load({ data }) {
  return { order: new Order(data.order) };
}

How to diagnose Svelte errors

Svelte errors mostly arise at the compiler's boundaries: the $ store prefix only works on real stores in component context, and SvelteKit's load functions run in two different environments with different capabilities. A 500 from a load function usually means server-only code (a database client, a secret, fs) reached a universal load that also runs in the browser.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

Check whether the file is a universal load (+page.js) or a server-only load (+page.server.js). Anything touching secrets or the filesystem belongs in the latter.
Read the full server-side stack trace in the terminal, not just the browser's 500 page: SvelteKit hides details in production for security.
For $ prefix errors, confirm the value is an actual store with a subscribe method and that you are inside a component, not a plain module.
Use $env/static/private and $env/dynamic/private for secrets so the compiler enforces the server-only boundary for you.
Reproduce with vite build && vite preview; dev-mode behaviour differs meaningfully from the production build.

Tools worth reaching for

vite build && vite preview
SvelteKit terminal logs
svelte-check
$env/static/private

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

svelte.dev

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