diff --git a/.prettierignore b/.prettierignore
index d5c124353c..72cd10aca8 100644
--- a/.prettierignore
+++ b/.prettierignore
@@ -7,6 +7,7 @@ packages/**/config/*.js
 
 # packages/svelte
 packages/svelte/messages/**/*.md
+packages/svelte/scripts/_bundle.js
 packages/svelte/src/compiler/errors.js
 packages/svelte/src/compiler/warnings.js
 packages/svelte/src/internal/client/errors.js
@@ -25,8 +26,7 @@ packages/svelte/tests/hydration/samples/*/_expected.html
 packages/svelte/tests/hydration/samples/*/_override.html
 packages/svelte/types
 packages/svelte/compiler/index.js
-playgrounds/sandbox/input/**.svelte
-playgrounds/sandbox/output
+playgrounds/sandbox/src/*
 
 # sites/svelte.dev
 sites/svelte.dev/static/svelte-app.json
diff --git a/documentation/docs/07-misc/03-typescript.md b/documentation/docs/07-misc/03-typescript.md
index fbf8817069..ff33885fb8 100644
--- a/documentation/docs/07-misc/03-typescript.md
+++ b/documentation/docs/07-misc/03-typescript.md
@@ -83,7 +83,7 @@ If you're using tools like Rollup or Webpack instead, install their respective S
 
 When using TypeScript, make sure your `tsconfig.json` is setup correctly.
 
-- Use a [`target`](https://www.typescriptlang.org/tsconfig/#target) of at least `ES2022`, or a `target` of at least `ES2015` alongside [`useDefineForClassFields`](https://www.typescriptlang.org/tsconfig/#useDefineForClassFields). This ensures that rune declarations on class fields are not messed with, which would break the Svelte compiler
+- Use a [`target`](https://www.typescriptlang.org/tsconfig/#target) of at least `ES2015` so classes are not compiled to functions
 - Set [`verbatimModuleSyntax`](https://www.typescriptlang.org/tsconfig/#verbatimModuleSyntax) to `true` so that imports are left as-is
 - Set [`isolatedModules`](https://www.typescriptlang.org/tsconfig/#isolatedModules) to `true` so that each file is looked at in isolation. TypeScript has a few features which require cross-file analysis and compilation, which the Svelte compiler and tooling like Vite don't do. 
 
diff --git a/eslint.config.js b/eslint.config.js
index d6c977a36a..d7044fc9f1 100644
--- a/eslint.config.js
+++ b/eslint.config.js
@@ -87,6 +87,7 @@ export default [
 			'**/*.d.ts',
 			'**/tests',
 			'packages/svelte/scripts/process-messages/templates/*.js',
+			'packages/svelte/scripts/_bundle.js',
 			'packages/svelte/src/compiler/errors.js',
 			'packages/svelte/src/internal/client/errors.js',
 			'packages/svelte/src/internal/client/warnings.js',
diff --git a/packages/svelte/CHANGELOG.md b/packages/svelte/CHANGELOG.md
index e254be754f..ef64091ca3 100644
--- a/packages/svelte/CHANGELOG.md
+++ b/packages/svelte/CHANGELOG.md
@@ -1,5 +1,11 @@
 # svelte
 
+## 5.34.9
+
+### Patch Changes
+
+- fix: ensure unowned deriveds can add themselves as reactions while connected ([#16249](https://github.com/sveltejs/svelte/pull/16249))
+
 ## 5.34.8
 
 ### Patch Changes
diff --git a/packages/svelte/package.json b/packages/svelte/package.json
index 6933ecafbe..2d88d2a051 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": "5.34.8",
+  "version": "5.34.9",
   "type": "module",
   "types": "./types/index.d.ts",
   "engines": {
diff --git a/packages/svelte/scripts/check-treeshakeability.js b/packages/svelte/scripts/check-treeshakeability.js
index 1501ee6954..e883496fe2 100644
--- a/packages/svelte/scripts/check-treeshakeability.js
+++ b/packages/svelte/scripts/check-treeshakeability.js
@@ -118,36 +118,40 @@ const bundle = await bundle_code(
 	).js.code
 );
 
-if (!bundle.includes('hydrate_node') && !bundle.includes('hydrate_next')) {
-	// eslint-disable-next-line no-console
-	console.error(`✅ Hydration code treeshakeable`);
-} else {
-	failed = true;
-	// eslint-disable-next-line no-console
-	console.error(`❌ Hydration code not treeshakeable`);
-}
+/**
+ * @param {string} case_name
+ * @param {string[]} strings
+ */
+function check_bundle(case_name, ...strings) {
+	for (const string of strings) {
+		const index = bundle.indexOf(string);
+		if (index >= 0) {
+			// eslint-disable-next-line no-console
+			console.error(`❌ ${case_name} not treeshakeable`);
+			failed = true;
 
-if (!bundle.includes('component_context.l')) {
-	// eslint-disable-next-line no-console
-	console.error(`✅ Legacy code treeshakeable`);
-} else {
-	failed = true;
+			let lines = bundle.slice(index - 500, index + 500).split('\n');
+			const target_line = lines.findIndex((line) => line.includes(string));
+			// mark the failed line
+			lines = lines
+				.map((line, i) => (i === target_line ? `> ${line}` : `| ${line}`))
+				.slice(target_line - 5, target_line + 6);
+			// eslint-disable-next-line no-console
+			console.error('The first failed line:\n' + lines.join('\n'));
+			return;
+		}
+	}
 	// eslint-disable-next-line no-console
-	console.error(`❌ Legacy code not treeshakeable`);
+	console.error(`✅ ${case_name} treeshakeable`);
 }
 
-if (!bundle.includes(`'CreatedAt'`)) {
-	// eslint-disable-next-line no-console
-	console.error(`✅ $inspect.trace code treeshakeable`);
-} else {
-	failed = true;
-	// eslint-disable-next-line no-console
-	console.error(`❌ $inspect.trace code not treeshakeable`);
-}
+check_bundle('Hydration code', 'hydrate_node', 'hydrate_next');
+check_bundle('Legacy code', 'component_context.l');
+check_bundle('$inspect.trace', `'CreatedAt'`);
 
 if (failed) {
 	// eslint-disable-next-line no-console
-	console.error(bundle);
+	console.error('Full bundle at', path.resolve('scripts/_bundle.js'));
 	fs.writeFileSync('scripts/_bundle.js', bundle);
 }
 
diff --git a/packages/svelte/src/compiler/index.js b/packages/svelte/src/compiler/index.js
index 11db091936..2aa9a820d2 100644
--- a/packages/svelte/src/compiler/index.js
+++ b/packages/svelte/src/compiler/index.js
@@ -3,7 +3,6 @@
 /** @import { AST } from './public.js' */
 import { walk as zimmerframe_walk } from 'zimmerframe';
 import { convert } from './legacy.js';
-import { parse as parse_acorn } from './phases/1-parse/acorn.js';
 import { parse as _parse } from './phases/1-parse/index.js';
 import { remove_typescript_nodes } from './phases/1-parse/remove_typescript_nodes.js';
 import { analyze_component, analyze_module } from './phases/2-analyze/index.js';
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/visitors/RegularElement.js b/packages/svelte/src/compiler/phases/3-transform/client/visitors/RegularElement.js
index 1aefff0db0..ae8680f594 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/visitors/RegularElement.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/visitors/RegularElement.js
@@ -22,13 +22,7 @@ import {
 	build_set_style
 } from './shared/element.js';
 import { process_children } from './shared/fragment.js';
-import {
-	build_render_statement,
-	build_template_chunk,
-	build_update_assignment,
-	get_expression_id,
-	memoize_expression
-} from './shared/utils.js';
+import { build_render_statement, build_template_chunk, get_expression_id } from './shared/utils.js';
 import { visit_event_attribute } from './shared/events.js';
 
 /**
@@ -200,16 +194,16 @@ export function RegularElement(node, context) {
 
 	const node_id = context.state.node;
 
+	/** If true, needs `__value` for inputs */
+	const needs_special_value_handling =
+		node.name === 'option' ||
+		node.name === 'select' ||
+		bindings.has('group') ||
+		bindings.has('checked');
+
 	if (has_spread) {
 		build_attribute_effect(attributes, class_directives, style_directives, context, node, node_id);
 	} else {
-		/** If true, needs `__value` for inputs */
-		const needs_special_value_handling =
-			node.name === 'option' ||
-			node.name === 'select' ||
-			bindings.has('group') ||
-			bindings.has('checked');
-
 		for (const attribute of /** @type {AST.Attribute[]} */ (attributes)) {
 			if (is_event_attribute(attribute)) {
 				visit_event_attribute(attribute, context);
@@ -217,7 +211,6 @@ export function RegularElement(node, context) {
 			}
 
 			if (needs_special_value_handling && attribute.name === 'value') {
-				build_element_special_value_attribute(node.name, node_id, attribute, context);
 				continue;
 			}
 
@@ -392,6 +385,15 @@ export function RegularElement(node, context) {
 		context.state.update.push(b.stmt(b.assignment('=', dir, dir)));
 	}
 
+	if (!has_spread && needs_special_value_handling) {
+		for (const attribute of /** @type {AST.Attribute[]} */ (attributes)) {
+			if (attribute.name === 'value') {
+				build_element_special_value_attribute(node.name, node_id, attribute, context);
+				break;
+			}
+		}
+	}
+
 	context.state.template.pop_element();
 }
 
@@ -622,12 +624,7 @@ function build_element_special_value_attribute(element, node_id, attribute, cont
 		element === 'select' && attribute.value !== true && !is_text_attribute(attribute);
 
 	const { value, has_state } = build_attribute_value(attribute.value, context, (value, metadata) =>
-		metadata.has_call
-			? // if is a select with value we will also invoke `init_select` which need a reference before the template effect so we memoize separately
-				is_select_with_value
-				? memoize_expression(state, value)
-				: get_expression_id(state.expressions, value)
-			: value
+		metadata.has_call ? get_expression_id(state.expressions, value) : value
 	);
 
 	const evaluated = context.state.scope.evaluate(value);
@@ -652,23 +649,21 @@ function build_element_special_value_attribute(element, node_id, attribute, cont
 			: inner_assignment
 	);
 
-	if (is_select_with_value) {
-		state.init.push(b.stmt(b.call('$.init_select', node_id, b.thunk(value))));
-	}
-
 	if (has_state) {
-		const id = state.scope.generate(`${node_id.name}_value`);
-		build_update_assignment(
-			state,
-			id,
-			// `<option>` is a special case: The value property reflects to the DOM. If the value is set to undefined,
-			// that means the value should be set to the empty string. To be able to do that when the value is
-			// initially undefined, we need to set a value that is guaranteed to be different.
-			element === 'option' ? b.object([]) : undefined,
-			value,
-			update
-		);
+		const id = b.id(state.scope.generate(`${node_id.name}_value`));
+
+		// `<option>` is a special case: The value property reflects to the DOM. If the value is set to undefined,
+		// that means the value should be set to the empty string. To be able to do that when the value is
+		// initially undefined, we need to set a value that is guaranteed to be different.
+		const init = element === 'option' ? b.object([]) : undefined;
+
+		state.init.push(b.var(id, init));
+		state.update.push(b.if(b.binary('!==', id, b.assignment('=', id, value)), b.block([update])));
 	} else {
 		state.init.push(update);
 	}
+
+	if (is_select_with_value) {
+		state.init.push(b.stmt(b.call('$.init_select', node_id)));
+	}
 }
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/utils.js b/packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/utils.js
index c861a51b7d..0bd9bfb128 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/utils.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/utils.js
@@ -165,20 +165,6 @@ export function parse_directive_name(name) {
 	return expression;
 }
 
-/**
- * @param {ComponentClientTransformState} state
- * @param {string} id
- * @param {Expression | undefined} init
- * @param {Expression} value
- * @param {ExpressionStatement} update
- */
-export function build_update_assignment(state, id, init, value, update) {
-	state.init.push(b.var(id, init));
-	state.update.push(
-		b.if(b.binary('!==', b.id(id), b.assignment('=', b.id(id), value)), b.block([update]))
-	);
-}
-
 /**
  * Serializes `bind:this` for components and elements.
  * @param {Identifier | MemberExpression | SequenceExpression} expression
diff --git a/packages/svelte/src/internal/client/dom/elements/attributes.js b/packages/svelte/src/internal/client/dom/elements/attributes.js
index 2d3d6a921d..5db685cf3e 100644
--- a/packages/svelte/src/internal/client/dom/elements/attributes.js
+++ b/packages/svelte/src/internal/client/dom/elements/attributes.js
@@ -20,7 +20,7 @@ import { clsx } from '../../../shared/attributes.js';
 import { set_class } from './class.js';
 import { set_style } from './style.js';
 import { ATTACHMENT_KEY, NAMESPACE_HTML } from '../../../../constants.js';
-import { block, branch, destroy_effect } from '../../reactivity/effects.js';
+import { block, branch, destroy_effect, effect } from '../../reactivity/effects.js';
 import { derived } from '../../reactivity/deriveds.js';
 import { init_select, select_option } from './bindings/select.js';
 
@@ -513,10 +513,12 @@ export function attribute_effect(
 	});
 
 	if (is_select) {
-		init_select(
-			/** @type {HTMLSelectElement} */ (element),
-			() => /** @type {Record<string | symbol, any>} */ (prev).value
-		);
+		var select = /** @type {HTMLSelectElement} */ (element);
+
+		effect(() => {
+			select_option(select, /** @type {Record<string | symbol, any>} */ (prev).value);
+			init_select(select);
+		});
 	}
 
 	inited = true;
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 e3263c65af..5e89686d86 100644
--- a/packages/svelte/src/internal/client/dom/elements/bindings/select.js
+++ b/packages/svelte/src/internal/client/dom/elements/bindings/select.js
@@ -1,6 +1,5 @@
-import { effect } from '../../../reactivity/effects.js';
+import { effect, teardown } from '../../../reactivity/effects.js';
 import { listen_to_event_and_reset_event } from './shared.js';
-import { untrack } from '../../../runtime.js';
 import { is } from '../../../proxy.js';
 import { is_array } from '../../../../shared/utils.js';
 import * as w from '../../../warnings.js';
@@ -51,40 +50,29 @@ export function select_option(select, value, mounting) {
  * current selection to the dom when it changes. Such
  * changes could for example occur when options are
  * inside an `#each` block.
- * @template V
  * @param {HTMLSelectElement} select
- * @param {() => V} [get_value]
  */
-export function init_select(select, get_value) {
-	let mounting = true;
-	effect(() => {
-		if (get_value) {
-			select_option(select, untrack(get_value), mounting);
-		}
-		mounting = false;
+export function init_select(select) {
+	var observer = new MutationObserver(() => {
+		// @ts-ignore
+		select_option(select, select.__value);
+		// Deliberately don't update the potential binding value,
+		// the model should be preserved unless explicitly changed
+	});
+
+	observer.observe(select, {
+		// Listen to option element changes
+		childList: true,
+		subtree: true, // because of <optgroup>
+		// Listen to option element value attribute changes
+		// (doesn't get notified of select value changes,
+		// because that property is not reflected as an attribute)
+		attributes: true,
+		attributeFilter: ['value']
+	});
 
-		var observer = new MutationObserver(() => {
-			// @ts-ignore
-			var value = select.__value;
-			select_option(select, value);
-			// Deliberately don't update the potential binding value,
-			// the model should be preserved unless explicitly changed
-		});
-
-		observer.observe(select, {
-			// Listen to option element changes
-			childList: true,
-			subtree: true, // because of <optgroup>
-			// Listen to option element value attribute changes
-			// (doesn't get notified of select value changes,
-			// because that property is not reflected as an attribute)
-			attributes: true,
-			attributeFilter: ['value']
-		});
-
-		return () => {
-			observer.disconnect();
-		};
+	teardown(() => {
+		observer.disconnect();
 	});
 }
 
@@ -136,7 +124,6 @@ export function bind_select_value(select, get, set = get) {
 		mounting = false;
 	});
 
-	// don't pass get_value, we already initialize it in the effect above
 	init_select(select);
 }
 
diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 56bc157f33..d057bfdf0d 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -294,7 +294,12 @@ export function update_reaction(reaction) {
 				reaction.deps = deps = new_deps;
 			}
 
-			if (!skip_reaction) {
+			if (
+				!skip_reaction ||
+				// Deriveds that already have reactions can cleanup, so we still add them as reactions
+				((flags & DERIVED) !== 0 &&
+					/** @type {import('#client').Derived} */ (reaction).reactions !== null)
+			) {
 				for (i = skipped_deps; i < deps.length; i++) {
 					(deps[i].reactions ??= []).push(reaction);
 				}
diff --git a/packages/svelte/src/version.js b/packages/svelte/src/version.js
index ec0126b17d..1c0aed6e87 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 = '5.34.8';
+export const VERSION = '5.34.9';
 export const PUBLIC_VERSION = '5';
diff --git a/packages/svelte/tests/runtime-runes/samples/inspect-state-unsafe-mutation/_config.js b/packages/svelte/tests/runtime-runes/samples/inspect-state-unsafe-mutation/_config.js
index 7e8fcd2d48..dcf3a8bc3d 100644
--- a/packages/svelte/tests/runtime-runes/samples/inspect-state-unsafe-mutation/_config.js
+++ b/packages/svelte/tests/runtime-runes/samples/inspect-state-unsafe-mutation/_config.js
@@ -5,5 +5,9 @@ export default test({
 	compileOptions: {
 		dev: true
 	},
-	error: 'state_unsafe_mutation'
+
+	error: 'state_unsafe_mutation',
+
+	// silence the logs
+	test({ logs }) {}
 });
diff --git a/packages/svelte/tests/runtime-runes/samples/legacy-runes-ambiguous-export-labeled/_config.js b/packages/svelte/tests/runtime-runes/samples/legacy-runes-ambiguous-export-labeled/_config.js
index 904fea3f65..c4cff665d2 100644
--- a/packages/svelte/tests/runtime-runes/samples/legacy-runes-ambiguous-export-labeled/_config.js
+++ b/packages/svelte/tests/runtime-runes/samples/legacy-runes-ambiguous-export-labeled/_config.js
@@ -6,7 +6,7 @@ export default test({
 	compileOptions: {
 		runes: undefined
 	},
-	async test({ assert, target }) {
+	async test({ assert, target, logs }) {
 		const p = target.querySelector('p');
 		const btn = target.querySelector('button');
 		flushSync(() => {
diff --git a/packages/svelte/tests/signals/test.ts b/packages/svelte/tests/signals/test.ts
index 78d7919e0f..719c936df0 100644
--- a/packages/svelte/tests/signals/test.ts
+++ b/packages/svelte/tests/signals/test.ts
@@ -112,6 +112,45 @@ describe('signals', () => {
 		};
 	});
 
+	test('unowned deriveds are not added as reactions but trigger effects', () => {
+		var obj = state<any>(undefined);
+
+		class C1 {
+			#v = state(0);
+			get v() {
+				return $.get(this.#v);
+			}
+			set v(v: number) {
+				set(this.#v, v);
+			}
+		}
+
+		return () => {
+			let d = derived(() => $.get(obj)?.v || '-');
+
+			const log: number[] = [];
+			assert.equal($.get(d), '-');
+
+			let destroy = effect_root(() => {
+				render_effect(() => {
+					log.push($.get(d));
+				});
+			});
+
+			set(obj, new C1());
+			flushSync();
+			assert.equal($.get(d), '-');
+			$.get(obj).v = 1;
+			flushSync();
+			assert.equal($.get(d), 1);
+			assert.deepEqual(log, ['-', 1]);
+			destroy();
+			// ensure we're not leaking reactions
+			assert.equal(obj.reactions, null);
+			assert.equal(d.reactions, null);
+		};
+	});
+
 	test('derived from state', () => {
 		const log: number[] = [];