diff --git a/.changeset/six-chicken-kneel.md b/.changeset/six-chicken-kneel.md
new file mode 100644
index 0000000000..f7fcbd14c6
--- /dev/null
+++ b/.changeset/six-chicken-kneel.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: resolve legacy component props equality for mutations
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/utils.js b/packages/svelte/src/compiler/phases/3-transform/client/utils.js
index 6137cfaecc..e77ee236ef 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/utils.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/utils.js
@@ -454,7 +454,6 @@ export function serialize_proxy_reassignment(value, proxy_reference, state) {
 		? b.call(
 				'$.proxy',
 				value,
-				b.true,
 				b.null,
 				typeof proxy_reference === 'string'
 					? b.id(proxy_reference)
diff --git a/packages/svelte/src/internal/client/proxy.js b/packages/svelte/src/internal/client/proxy.js
index 78d835dec4..f5900ba957 100644
--- a/packages/svelte/src/internal/client/proxy.js
+++ b/packages/svelte/src/internal/client/proxy.js
@@ -11,7 +11,7 @@ import {
 	object_prototype
 } from './utils.js';
 import { check_ownership, widen_ownership } from './dev/ownership.js';
-import { mutable_source, source, set } from './reactivity/sources.js';
+import { source, set } from './reactivity/sources.js';
 import { STATE_FROZEN_SYMBOL, STATE_SYMBOL } from './constants.js';
 import { UNINITIALIZED } from '../../constants.js';
 import * as e from './errors.js';
@@ -19,12 +19,11 @@ import * as e from './errors.js';
 /**
  * @template T
  * @param {T} value
- * @param {boolean} [immutable]
  * @param {import('#client').ProxyMetadata | null} [parent]
  * @param {import('#client').Source<T>} [prev] dev mode only
  * @returns {import('#client').ProxyStateObject<T> | T}
  */
-export function proxy(value, immutable = true, parent = null, prev) {
+export function proxy(value, parent = null, prev) {
 	if (
 		typeof value === 'object' &&
 		value != null &&
@@ -59,7 +58,6 @@ export function proxy(value, immutable = true, parent = null, prev) {
 					s: new Map(),
 					v: source(0),
 					a: is_array(value),
-					i: immutable,
 					p: proxy,
 					t: value
 				}),
@@ -169,7 +167,7 @@ const state_proxy_handler = {
 			const metadata = target[STATE_SYMBOL];
 
 			const s = metadata.s.get(prop);
-			if (s !== undefined) set(s, proxy(descriptor.value, metadata.i, metadata));
+			if (s !== undefined) set(s, proxy(descriptor.value, metadata));
 		}
 
 		return Reflect.defineProperty(target, prop, descriptor);
@@ -215,7 +213,7 @@ const state_proxy_handler = {
 
 		// create a source, but only if it's an own property and not a prototype property
 		if (s === undefined && (!(prop in target) || get_descriptor(target, prop)?.writable)) {
-			s = (metadata.i ? source : mutable_source)(proxy(target[prop], metadata.i, metadata));
+			s = source(proxy(target[prop], metadata));
 			metadata.s.set(prop, s);
 		}
 
@@ -256,9 +254,7 @@ const state_proxy_handler = {
 			(current_effect !== null && (!has || get_descriptor(target, prop)?.writable))
 		) {
 			if (s === undefined) {
-				s = (metadata.i ? source : mutable_source)(
-					has ? proxy(target[prop], metadata.i, metadata) : UNINITIALIZED
-				);
+				s = source(has ? proxy(target[prop], metadata) : UNINITIALIZED);
 				metadata.s.set(prop, s);
 			}
 			const value = get(s);
@@ -283,7 +279,7 @@ const state_proxy_handler = {
 			s = metadata.s.get(prop);
 		}
 		if (s !== undefined) {
-			set(s, proxy(value, metadata.i, metadata));
+			set(s, proxy(value, metadata));
 		}
 		const is_array = metadata.a;
 		const not_has = !(prop in target);
diff --git a/packages/svelte/src/internal/client/types.d.ts b/packages/svelte/src/internal/client/types.d.ts
index 0d2d5a29c2..78b1d8d6fd 100644
--- a/packages/svelte/src/internal/client/types.d.ts
+++ b/packages/svelte/src/internal/client/types.d.ts
@@ -180,8 +180,6 @@ export interface ProxyMetadata<T = Record<string | symbol, any>> {
 	v: Source<number>;
 	/** `true` if the proxified object is an array */
 	a: boolean;
-	/** Immutable: Whether to use a source or mutable source under the hood */
-	i: boolean;
 	/** The associated proxy */
 	p: ProxyStateObject<T>;
 	/** The original target this proxy was created for */
diff --git a/packages/svelte/src/legacy/legacy-client.js b/packages/svelte/src/legacy/legacy-client.js
index b5f499f37d..35108b83a9 100644
--- a/packages/svelte/src/legacy/legacy-client.js
+++ b/packages/svelte/src/legacy/legacy-client.js
@@ -1,8 +1,9 @@
 /** @import { ComponentConstructorOptions, ComponentType, SvelteComponent, Component } from 'svelte' */
-import { proxy } from '../internal/client/proxy.js';
+import { mutable_source, get, set } from 'svelte/internal/client';
 import { user_pre_effect } from '../internal/client/reactivity/effects.js';
 import { hydrate, mount, unmount } from '../internal/client/render.js';
 import { define_property } from '../internal/client/utils.js';
+import { safe_not_equal } from '../internal/client/reactivity/equality.js';
 
 /**
  * Takes the same options as a Svelte 4 component and the component function and returns a Svelte 4 compatible component.
@@ -69,10 +70,51 @@ class Svelte4Component {
 	 * }} options
 	 */
 	constructor(options) {
-		// Using proxy state here isn't completely mirroring the Svelte 4 behavior, because mutations to a property
-		// cause fine-grained updates to only the places where that property is used, and not the entire property.
-		// Reactive statements and actions (the things where this matters) are handling this properly regardless, so it should be fine in practise.
-		const props = proxy({ ...(options.props || {}), $$events: {} }, false);
+		var sources = new Map();
+		var add_source = (/** @type {string | symbol} */ key) => {
+			var s = mutable_source(0);
+			sources.set(key, s);
+			return s;
+		};
+		// Replicate coarse-grained props through a proxy that has a version source for
+		// each property, which is increment on updates to the property itself. Do not
+		// use our $state proxy because that one has fine-grained reactivity.
+		const props = new Proxy(
+			{ ...(options.props || {}), $$events: {} },
+			{
+				get(target, prop, receiver) {
+					var value = Reflect.get(target, prop, receiver);
+					var s = sources.get(prop);
+					if (s === undefined) {
+						s = add_source(prop);
+					}
+					get(s);
+					return value;
+				},
+				has(target, prop) {
+					var value = Reflect.has(target, prop);
+					var s = sources.get(prop);
+					if (s !== undefined) {
+						get(s);
+					}
+					return value;
+				},
+				set(target, prop, value) {
+					var s = sources.get(prop);
+					// @ts-ignore
+					var prev_value = target[prop];
+					if (s === undefined) {
+						s = add_source(prop);
+					} else if (safe_not_equal(prev_value, value)) {
+						// Increment version
+						set(s, s.v + 1);
+					}
+					// @ts-ignore
+					target[prop] = value;
+					return true;
+				}
+			}
+		);
 		this.#instance = (options.hydrate ? hydrate : mount)(options.component, {
 			target: options.target,
 			props,
diff --git a/packages/svelte/tests/runtime-legacy/samples/component-props-mutated/_config.js b/packages/svelte/tests/runtime-legacy/samples/component-props-mutated/_config.js
new file mode 100644
index 0000000000..91d0aaa0d3
--- /dev/null
+++ b/packages/svelte/tests/runtime-legacy/samples/component-props-mutated/_config.js
@@ -0,0 +1,24 @@
+import { test } from '../../test';
+
+const data = {
+	message: 'hello'
+};
+
+export default test({
+	get props() {
+		data.message = 'hello';
+
+		return {
+			data
+		};
+	},
+
+	html: '<p>hello</p>',
+
+	async test({ assert, component, target }) {
+		data.message = 'goodbye';
+		await component.$set({ data });
+
+		assert.htmlEqual(target.innerHTML, '<p>goodbye</p>');
+	}
+});
diff --git a/packages/svelte/tests/runtime-legacy/samples/component-props-mutated/main.svelte b/packages/svelte/tests/runtime-legacy/samples/component-props-mutated/main.svelte
new file mode 100644
index 0000000000..30195a0836
--- /dev/null
+++ b/packages/svelte/tests/runtime-legacy/samples/component-props-mutated/main.svelte
@@ -0,0 +1,5 @@
+<script>
+	export let data;
+</script>
+
+<p>{data.message}</p>