diff --git a/packages/svelte/CHANGELOG.md b/packages/svelte/CHANGELOG.md
index ad72bfabb03a..4f1be5e299fd 100644
--- a/packages/svelte/CHANGELOG.md
+++ b/packages/svelte/CHANGELOG.md
@@ -1,5 +1,13 @@
 # svelte
 
+## 5.35.3
+
+### Patch Changes
+
+- fix: account for mounting when `select_option` in `attribute_effect` ([#16309](https://github.com/sveltejs/svelte/pull/16309))
+
+- fix: do not proxify the value assigned to a derived ([#16302](https://github.com/sveltejs/svelte/pull/16302))
+
 ## 5.35.2
 
 ### Patch Changes
diff --git a/packages/svelte/package.json b/packages/svelte/package.json
index 872c21a1794e..30a8f8833af1 100644
--- a/packages/svelte/package.json
+++ b/packages/svelte/package.json
@@ -2,7 +2,7 @@
   "name": "svelte",
   "description": "Cybernetically enhanced web apps",
   "license": "MIT",
-  "version": "5.35.2",
+  "version": "5.35.3",
   "type": "module",
   "types": "./types/index.d.ts",
   "engines": {
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/visitors/AssignmentExpression.js b/packages/svelte/src/compiler/phases/3-transform/client/visitors/AssignmentExpression.js
index ce190814f8d8..7d64d60bca5a 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/visitors/AssignmentExpression.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/visitors/AssignmentExpression.js
@@ -137,6 +137,7 @@ function build_assignment(operator, left, right, context) {
 				binding.kind !== 'prop' &&
 				binding.kind !== 'bindable_prop' &&
 				binding.kind !== 'raw_state' &&
+				binding.kind !== 'derived' &&
 				binding.kind !== 'store_sub' &&
 				context.state.analysis.runes &&
 				should_proxy(right, context.state.scope) &&
diff --git a/packages/svelte/src/internal/client/dom/elements/attributes.js b/packages/svelte/src/internal/client/dom/elements/attributes.js
index 5db685cf3e90..66f484a59ace 100644
--- a/packages/svelte/src/internal/client/dom/elements/attributes.js
+++ b/packages/svelte/src/internal/client/dom/elements/attributes.js
@@ -491,7 +491,7 @@ export function attribute_effect(
 		var current = set_attributes(element, prev, next, css_hash, skip_warning);
 
 		if (inited && is_select && 'value' in next) {
-			select_option(/** @type {HTMLSelectElement} */ (element), next.value, false);
+			select_option(/** @type {HTMLSelectElement} */ (element), next.value);
 		}
 
 		for (let symbol of Object.getOwnPropertySymbols(effects)) {
@@ -516,7 +516,7 @@ export function attribute_effect(
 		var select = /** @type {HTMLSelectElement} */ (element);
 
 		effect(() => {
-			select_option(select, /** @type {Record<string | symbol, any>} */ (prev).value);
+			select_option(select, /** @type {Record<string | symbol, any>} */ (prev).value, true);
 			init_select(select);
 		});
 	}
diff --git a/packages/svelte/src/internal/client/dom/elements/bindings/select.js b/packages/svelte/src/internal/client/dom/elements/bindings/select.js
index 5e89686d8654..e39fb865cdf5 100644
--- a/packages/svelte/src/internal/client/dom/elements/bindings/select.js
+++ b/packages/svelte/src/internal/client/dom/elements/bindings/select.js
@@ -9,9 +9,9 @@ import * as w from '../../../warnings.js';
  * @template V
  * @param {HTMLSelectElement} select
  * @param {V} value
- * @param {boolean} [mounting]
+ * @param {boolean} mounting
  */
-export function select_option(select, value, mounting) {
+export function select_option(select, value, mounting = false) {
 	if (select.multiple) {
 		// If value is null or undefined, keep the selection as is
 		if (value == undefined) {
diff --git a/packages/svelte/src/internal/client/reactivity/sources.js b/packages/svelte/src/internal/client/reactivity/sources.js
index 0db353023257..a86ccfee4ff4 100644
--- a/packages/svelte/src/internal/client/reactivity/sources.js
+++ b/packages/svelte/src/internal/client/reactivity/sources.js
@@ -11,7 +11,7 @@ import {
 	untrack,
 	increment_write_version,
 	update_effect,
-	reaction_sources,
+	source_ownership,
 	check_dirtiness,
 	untracking,
 	is_destroying_effect,
@@ -140,7 +140,7 @@ export function set(source, value, should_proxy = false) {
 		(!untracking || (active_reaction.f & INSPECT_EFFECT) !== 0) &&
 		is_runes() &&
 		(active_reaction.f & (DERIVED | BLOCK_EFFECT | INSPECT_EFFECT)) !== 0 &&
-		!(reaction_sources?.[1].includes(source) && reaction_sources[0] === active_reaction)
+		!(source_ownership?.reaction === active_reaction && source_ownership.sources.includes(source))
 	) {
 		e.state_unsafe_mutation();
 	}
diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 8e6242447e73..fce6c78b56e4 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -86,17 +86,17 @@ export function set_active_effect(effect) {
 /**
  * When sources are created within a reaction, reading and writing
  * them within that reaction should not cause a re-run
- * @type {null | [active_reaction: Reaction, sources: Source[]]}
+ * @type {null | { reaction: Reaction, sources: Source[] }}
  */
-export let reaction_sources = null;
+export let source_ownership = null;
 
 /** @param {Value} value */
 export function push_reaction_value(value) {
 	if (active_reaction !== null && active_reaction.f & EFFECT_IS_UPDATING) {
-		if (reaction_sources === null) {
-			reaction_sources = [active_reaction, [value]];
+		if (source_ownership === null) {
+			source_ownership = { reaction: active_reaction, sources: [value] };
 		} else {
-			reaction_sources[1].push(value);
+			source_ownership.sources.push(value);
 		}
 	}
 }
@@ -235,7 +235,12 @@ function schedule_possible_effect_self_invalidation(signal, effect, root = true)
 	for (var i = 0; i < reactions.length; i++) {
 		var reaction = reactions[i];
 
-		if (reaction_sources?.[1].includes(signal) && reaction_sources[0] === active_reaction) continue;
+		if (
+			source_ownership?.reaction === active_reaction &&
+			source_ownership.sources.includes(signal)
+		) {
+			continue;
+		}
 
 		if ((reaction.f & DERIVED) !== 0) {
 			schedule_possible_effect_self_invalidation(/** @type {Derived} */ (reaction), effect, false);
@@ -257,7 +262,7 @@ export function update_reaction(reaction) {
 	var previous_untracked_writes = untracked_writes;
 	var previous_reaction = active_reaction;
 	var previous_skip_reaction = skip_reaction;
-	var previous_reaction_sources = reaction_sources;
+	var previous_reaction_sources = source_ownership;
 	var previous_component_context = component_context;
 	var previous_untracking = untracking;
 
@@ -270,7 +275,7 @@ export function update_reaction(reaction) {
 		(flags & UNOWNED) !== 0 && (untracking || !is_updating_effect || active_reaction === null);
 	active_reaction = (flags & (BRANCH_EFFECT | ROOT_EFFECT)) === 0 ? reaction : null;
 
-	reaction_sources = null;
+	source_ownership = null;
 	set_component_context(reaction.ctx);
 	untracking = false;
 	read_version++;
@@ -358,7 +363,7 @@ export function update_reaction(reaction) {
 		untracked_writes = previous_untracked_writes;
 		active_reaction = previous_reaction;
 		skip_reaction = previous_skip_reaction;
-		reaction_sources = previous_reaction_sources;
+		source_ownership = previous_reaction_sources;
 		set_component_context(previous_component_context);
 		untracking = previous_untracking;
 
@@ -739,7 +744,10 @@ export function get(signal) {
 
 	// Register the dependency on the current reaction signal.
 	if (active_reaction !== null && !untracking) {
-		if (!reaction_sources?.[1].includes(signal) || reaction_sources[0] !== active_reaction) {
+		if (
+			source_ownership?.reaction !== active_reaction ||
+			!source_ownership?.sources.includes(signal)
+		) {
 			var deps = active_reaction.deps;
 			if (signal.rv < read_version) {
 				signal.rv = read_version;
diff --git a/packages/svelte/src/version.js b/packages/svelte/src/version.js
index 47c354c7a21e..d98b62290805 100644
--- a/packages/svelte/src/version.js
+++ b/packages/svelte/src/version.js
@@ -4,5 +4,5 @@
  * The current version, as set in package.json.
  * @type {string}
  */
-export const VERSION = '5.35.2';
+export const VERSION = '5.35.3';
 export const PUBLIC_VERSION = '5';
diff --git a/packages/svelte/tests/runtime-runes/samples/select-spread-option-selected/_config.js b/packages/svelte/tests/runtime-runes/samples/select-spread-option-selected/_config.js
new file mode 100644
index 000000000000..50da3ac09567
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/select-spread-option-selected/_config.js
@@ -0,0 +1,9 @@
+import { ok, test } from '../../test';
+
+export default test({
+	async test({ assert, target, instance }) {
+		const select = target.querySelector('select');
+		ok(select);
+		assert.equal(select.selectedIndex, 1);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/select-spread-option-selected/main.svelte b/packages/svelte/tests/runtime-runes/samples/select-spread-option-selected/main.svelte
new file mode 100644
index 000000000000..e5fc6c553538
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/select-spread-option-selected/main.svelte
@@ -0,0 +1,8 @@
+<script>
+	let others = {onclick: ()=> {}}
+</script>
+
+<select {...others}>
+	<option>o1</option>
+	<option selected>o2</option>
+</select>
diff --git a/packages/svelte/tests/runtime-runes/samples/writable-derived-4/_config.js b/packages/svelte/tests/runtime-runes/samples/writable-derived-4/_config.js
new file mode 100644
index 000000000000..8d312ca73ae5
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/writable-derived-4/_config.js
@@ -0,0 +1,23 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	test({ assert, target }) {
+		const [btn1, btn2, btn3] = target.getElementsByTagName('button');
+		const [div] = target.getElementsByTagName('div');
+
+		flushSync(() => btn1.click());
+		assert.equal(div.textContent, '1');
+		flushSync(() => btn2.click());
+		assert.equal(div.textContent, '1');
+		flushSync(() => btn3.click());
+		assert.equal(div.textContent, '2');
+
+		flushSync(() => btn1.click());
+		assert.equal(div.textContent, '2');
+		flushSync(() => btn2.click());
+		assert.equal(div.textContent, '2');
+		flushSync(() => btn3.click());
+		assert.equal(div.textContent, '3');
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/writable-derived-4/main.svelte b/packages/svelte/tests/runtime-runes/samples/writable-derived-4/main.svelte
new file mode 100644
index 000000000000..a24f69ba772d
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/writable-derived-4/main.svelte
@@ -0,0 +1,9 @@
+<script>
+	let count = $derived({ value: 1 });
+</script>
+
+<button onclick={() => count.value++}>mutate</button>
+<button onclick={() => count = count}>assign self</button>
+<button onclick={() => count = {...count} }>assign copy</button>
+
+<div>{count.value}</div>

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