sveltejs/kit · error · Error

called uneval before init_transport

Error message

called uneval before init_transport

What it means

uneval is a stub that SvelteKit replaces via init_transport when the server rendering pipeline initializes the devalue transport. If code calls uneval before that initialization, the stub throws in dev (and throws a blank error in prod). It signals an out-of-order or out-of-context call of an internal serialization hook.

Source

Thrown at packages/kit/src/runtime/app/internal/transport.js:7

/** @import { Transport } from '@sveltejs/kit/hooks' */
import * as devalue from 'devalue';
import { DEV } from 'esm-env';

/** @type {(thing: any) => string} */
export let uneval = () => {
	throw new Error(DEV ? 'called uneval before init_transport' : '');
};

/** @type {(data: any) => string} */
export let stringify = () => {
	throw new Error(DEV ? 'called stringify before init_transport' : '');
};

/** @type {(data: string) => any} */
export let parse = () => {
	throw new Error(DEV ? 'called parse before init_transport' : '');
};

/** @type {Record<string, (data: any) => any>} */
export let encoders = {};

/** @type {Record<string, (data: any) => any>} */
export let decoders = {};

View on GitHub (pinned to 03f1687fe6)

Solutions

1. Call uneval only within the request lifecycle managed by SvelteKit (e.g. inside load or server hooks), not in server bootstrap code
2. Use `devalue.stringify`/`uneval` from the devalue package directly if you need standalone serialization
3. If you maintain an adapter, ensure init_transport is invoked before rendering
4. Update @sveltejs/kit; this internal API changed with the transport refactor

Example fix

// before
import { uneval } from '@sveltejs/kit/internal'; // outside render
// after
import { stringify } from 'devalue';
const s = stringify(data);

import { uneval } from 'your-transport-module';
if (typeof uneval.__initialized !== 'boolean') {
  throw new Error('transport not initialized: call init_transport first');
}

try {
  const js = uneval(data);
} catch (e) {
  if (e.message === 'called uneval before init_transport') {
    // serialize outside kit with devalue instead
  } else throw e;
}

When it happens

Trigger: Calling the exported uneval from @sveltejs/kit (or reaching this module) before init_transport has run — typically custom server code or a plugin invoking server-side serialization outside a request render handled by SvelteKit.

Common situations: Calling transport/uneval utilities from adapter hooks or custom server entry points before kit initializes, importing internals directly, or tests invoking uneval without setting up kit's render pipeline.

Related errors

• called stringify before init_transport
• called parse before init_transport
• Data returned from action inside ${route_id} is not serializ
• Data returned from action inside ${route_id} is not serializ
• Invalid data for Set reviver

AI-assisted analysis of sveltejs/kit@03f1687fe6 (2026-09-02). Data as JSON: /api/errors/e1301de006a7f5be. Report an issue: GitHub.