sveltejs/svelte · error · Error

state_prototype_fixed

state_prototype_fixed

Error message

state_prototype_fixed
Cannot set prototype of `$state` object
https://svelte.dev/e/state_prototype_fixed

What it means

Thrown at internal/client/proxy.js:351 in the Proxy setPrototypeOf trap of a $state object. Svelte's reactivity and forking rely on the state object's prototype staying fixed; changing the prototype via Object.setPrototypeOf would invalidate the internal signal bookkeeping and Proxy invariants, so it is unconditionally blocked.

Source

Thrown at packages/svelte/src/internal/client/errors.js:476

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

/**
 * Cannot set prototype of `$state` object
 * @returns {never}
 */
export function state_prototype_fixed() {
	if (DEV) {
		const error = new Error(`state_prototype_fixed\nCannot set prototype of \`$state\` object\nhttps://svelte.dev/e/state_prototype_fixed`);

		error.name = 'Svelte error';

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

/**
 * Updating state inside `$derived(...)`, `$inspect(...)` or a template expression is forbidden. If the value should not be reactive, declare it without `$state`
 * @returns {never}
 */
export function state_unsafe_mutation() {
	if (DEV) {
		const error = new Error(`state_unsafe_mutation\nUpdating state inside \`$derived(...)\`, \`$inspect(...)\` or a template expression is forbidden. If the value should not be reactive, declare it without \`$state\`\nhttps://svelte.dev/e/state_unsafe_mutation`);

		error.name = 'Svelte error';

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

View on GitHub (pinned to 20b341f100)

Solutions

1. Create the object with the desired prototype first, then wrap it in $state: `const obj = Object.create(proto); const state = $state(obj)`.
2. Store methods separately (e.g. on a class instance kept in $state) rather than reassigning prototypes.
3. Use composition: keep data in $state and behaviour in plain functions that take the state as an argument.

Example fix

// before
const state = $state({});
Object.setPrototypeOf(state, methodsProto); // throws

// after
const obj = Object.create(methodsProto);
const state = $state(obj);

// Never call Object.setPrototypeOf on a $state object.
// Create the object with the prototype first, then wrap in $state:
// const obj = Object.create(proto); const state = $state(obj);

When it happens

Trigger: Calling Object.setPrototypeOf(stateObj, newProto) where stateObj is a $state-proxied object (e.g. `$state({})` or `$state(array)`). The trap at proxy.js:350-352 always throws.

Common situations: Assigning a class prototype onto a plain state object to add methods; mixing prototype-based inheritance with runes state; libraries that call setPrototypeOf for mixin patterns; attempted prototype pollution.

Related errors

• state_descriptors_fixed
• state_unsafe_mutation
• effect_in_unowned_derived
• effect_orphan
• effect_pending_outside_reaction

AI-assisted analysis of sveltejs/svelte@20b341f100 (2026-08-12). Data as JSON: /api/errors/c0f4b9a1dd4e4d98. Report an issue: GitHub.