diff --git a/.changeset/forty-peaches-unite.md b/.changeset/forty-peaches-unite.md
new file mode 100644
index 0000000000..b24993deb2
--- /dev/null
+++ b/.changeset/forty-peaches-unite.md
@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: ensure unstate() only deeply applies to plain objects and arrays
diff --git a/packages/svelte/src/internal/client/proxy/proxy.js b/packages/svelte/src/internal/client/proxy/proxy.js
index ae993267d6..d0bb285e10 100644
--- a/packages/svelte/src/internal/client/proxy/proxy.js
+++ b/packages/svelte/src/internal/client/proxy/proxy.js
@@ -11,11 +11,15 @@ import {
 	batch_inspect
 } from '../runtime.js';
 import {
+	array_prototype,
 	define_property,
 	get_descriptor,
 	get_descriptors,
+	get_prototype_of,
 	is_array,
-	object_keys
+	is_frozen,
+	object_keys,
+	object_prototype
 } from '../utils.js';
 
 /** @typedef {{ s: Map<string | symbol, import('../types.js').SourceSignal<any>>; v: import('../types.js').SourceSignal<number>; a: boolean, i: boolean, p: StateObject }} Metadata */
@@ -23,12 +27,6 @@ import {
 
 export const STATE_SYMBOL = Symbol('$state');
 export const READONLY_SYMBOL = Symbol('readonly');
-
-const object_prototype = Object.prototype;
-const array_prototype = Array.prototype;
-const get_prototype_of = Object.getPrototypeOf;
-const is_frozen = Object.isFrozen;
-
 /**
  * @template {StateObject} T
  * @param {T} value
diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 126692b1a1..97804e11ce 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -1,7 +1,16 @@
 import { DEV } from 'esm-env';
 import { subscribe_to_store } from '../../store/utils.js';
 import { EMPTY_FUNC, run_all } from '../common.js';
-import { get_descriptor, get_descriptors, is_array, is_frozen, object_freeze } from './utils.js';
+import {
+	array_prototype,
+	get_descriptor,
+	get_descriptors,
+	get_prototype_of,
+	is_array,
+	is_frozen,
+	object_freeze,
+	object_prototype
+} from './utils.js';
 import {
 	PROPS_IS_LAZY_INITIAL,
 	PROPS_IS_IMMUTABLE,
@@ -1975,19 +1984,23 @@ function deep_unstate(value, visited = new Map()) {
 			visited.set(value, unstated);
 			return unstated;
 		}
-
-		let contains_unstated = false;
-		/** @type {any} */
-		const nested_unstated = Array.isArray(value) ? [] : {};
-		for (let key in value) {
-			const result = deep_unstate(value[key], visited);
-			nested_unstated[key] = result;
-			if (result !== value[key]) {
-				contains_unstated = true;
+		const prototype = get_prototype_of(value);
+		// Only deeply unstate plain objects and arrays
+		if (prototype === object_prototype || prototype === array_prototype) {
+			let contains_unstated = false;
+			/** @type {any} */
+			const nested_unstated = Array.isArray(value) ? [] : {};
+			for (let key in value) {
+				const result = deep_unstate(value[key], visited);
+				nested_unstated[key] = result;
+				if (result !== value[key]) {
+					contains_unstated = true;
+				}
 			}
+			visited.set(value, contains_unstated ? nested_unstated : value);
+		} else {
+			visited.set(value, value);
 		}
-
-		visited.set(value, contains_unstated ? nested_unstated : value);
 	}
 
 	return visited.get(value) ?? value;
diff --git a/packages/svelte/src/internal/client/utils.js b/packages/svelte/src/internal/client/utils.js
index 7c1b01515e..1da6f2fb39 100644
--- a/packages/svelte/src/internal/client/utils.js
+++ b/packages/svelte/src/internal/client/utils.js
@@ -10,6 +10,9 @@ export var object_freeze = Object.freeze;
 export var define_property = Object.defineProperty;
 export var get_descriptor = Object.getOwnPropertyDescriptor;
 export var get_descriptors = Object.getOwnPropertyDescriptors;
+export var object_prototype = Object.prototype;
+export var array_prototype = Array.prototype;
+export var get_prototype_of = Object.getPrototypeOf;
 
 /**
  * @param {any} thing