sveltejs/svelte · error · Error

lifecycle_legacy_only

lifecycle_legacy_only

Error message

lifecycle_legacy_only
`${name}(...)` cannot be used in runes mode
https://svelte.dev/e/lifecycle_legacy_only

What it means

Thrown at index-client.js:206 and :229 inside beforeUpdate()/afterUpdate() when component_context.l is null, i.e. the component is in runes mode. These legacy lifecycle hooks rely on the pre-runes reactivity model and have no equivalent semantics under runes; Svelte 5 runes components must use $effect / $effect.pre instead.

Source

Thrown at packages/svelte/src/internal/client/errors.js:377

	} else {
		throw new Error(`https://svelte.dev/e/invalid_snippet`);
	}
}

/**
 * `%name%(...)` cannot be used in runes mode
 * @param {string} name
 * @returns {never}
 */
export function lifecycle_legacy_only(name) {
	if (DEV) {
		const error = new Error(`lifecycle_legacy_only\n\`${name}(...)\` cannot be used in runes mode\nhttps://svelte.dev/e/lifecycle_legacy_only`);

		error.name = 'Svelte error';

		throw error;
	} else {
		throw new Error(`https://svelte.dev/e/lifecycle_legacy_only`);
	}
}

/**
 * Cannot do `bind:%key%={undefined}` when `%key%` has a fallback value
 * @param {string} key
 * @returns {never}
 */
export function props_invalid_value(key) {
	if (DEV) {
		const error = new Error(`props_invalid_value\nCannot do \`bind:${key}={undefined}\` when \`${key}\` has a fallback value\nhttps://svelte.dev/e/props_invalid_value`);

		error.name = 'Svelte error';

		throw error;
	} else {
		throw new Error(`https://svelte.dev/e/props_invalid_value`);
	}

View on GitHub (pinned to 20b341f100)

Solutions

1. Replace beforeUpdate(fn) with $effect.pre(fn).
2. Replace afterUpdate(fn) with $effect(fn).
3. Remove the legacy import (beforeUpdate/afterUpdate) once migrated.

Example fix

// before
import { beforeUpdate, afterUpdate } from 'svelte';
beforeUpdate(() => { /* ... */ });
afterUpdate(() => { /* ... */ });

// after
$effect.pre(() => { /* runs before DOM update */ });
$effect(() => { /* runs after DOM update */ });

// Do not call beforeUpdate/afterUpdate in runes-mode components.
// Detect runes mode: if your component uses $state/$props/$effect, it is in runes mode.
// Replace beforeUpdate -> $effect.pre, afterUpdate -> $effect.

When it happens

Trigger: Importing and calling beforeUpdate(fn) or afterUpdate(fn) in a component that opts into runes mode (either explicitly via runes: true or implicitly because it uses runes like $state/$props). The guard checks component_context.l === null.

Common situations: Migrating a Svelte 4 component to Svelte 5 runes while leaving beforeUpdate/afterUpdate calls in place; copying legacy code into a runes component; library code that calls these hooks being used from a runes-mode app.

Related errors

• effect_orphan
• set_context_after_init
• migrating this component would require adding a `$${rune}` r
• can't migrate `${...}` to `$${rune}` because there's a varia
• effect_in_teardown

AI-assisted analysis of sveltejs/svelte@20b341f100 (2026-08-12). Data as JSON: /api/errors/f338b5df99b84929. Report an issue: GitHub.