diff --git a/.changeset/silly-apples-remain.md b/.changeset/silly-apples-remain.md
deleted file mode 100644
index 10d43db550..0000000000
--- a/.changeset/silly-apples-remain.md
+++ /dev/null
@@ -1,5 +0,0 @@
----
-'svelte': patch
----
-
-fix: handle more hydration mismatches
diff --git a/documentation/docs/02-runes/02-$state.md b/documentation/docs/02-runes/02-$state.md
index aac006d2f5..f7314e2950 100644
--- a/documentation/docs/02-runes/02-$state.md
+++ b/documentation/docs/02-runes/02-$state.md
@@ -67,16 +67,15 @@ todos[0].done = !todos[0].done;
 
 ### Classes
 
-You can also use `$state` in class fields (whether public or private):
+You can also use `$state` in class fields (whether public or private), or as the first assignment to a property immediately inside the `constructor`:
 
 ```js
 // @errors: 7006 2554
 class Todo {
 	done = $state(false);
-	text = $state();
 
 	constructor(text) {
-		this.text = text;
+		this.text = $state(text);
 	}
 
 	reset() {
@@ -110,10 +109,9 @@ You can either use an inline function...
 // @errors: 7006 2554
 class Todo {
 	done = $state(false);
-	text = $state();
 
 	constructor(text) {
-		this.text = text;
+		this.text = $state(text);
 	}
 
 	+++reset = () => {+++
diff --git a/documentation/docs/03-template-syntax/09-@attach.md b/documentation/docs/03-template-syntax/09-@attach.md
index 1736a82e77..0d43ab6cfa 100644
--- a/documentation/docs/03-template-syntax/09-@attach.md
+++ b/documentation/docs/03-template-syntax/09-@attach.md
@@ -2,7 +2,9 @@
 title: {@attach ...}
 ---
 
-Attachments are functions that run when an element is mounted to the DOM. Optionally, they can return a function that is called when the element is later removed from the DOM.
+Attachments are functions that run in an [effect]($effect) when an element is mounted to the DOM or when [state]($state) read inside the function updates.
+
+Optionally, they can return a function that is called before the attachment re-runs, or after the element is later removed from the DOM.
 
 > [!NOTE]
 > Attachments are available in Svelte 5.29 and newer.
@@ -55,7 +57,7 @@ A useful pattern is for a function, such as `tooltip` in this example, to _retur
 </button>
 ```
 
-Since the `tooltip(content)` expression runs inside an [effect]($effect), the attachment will be destroyed and recreated whenever `content` changes.
+Since the `tooltip(content)` expression runs inside an [effect]($effect), the attachment will be destroyed and recreated whenever `content` changes. The same thing would happen for any state read _inside_ the attachment function when it first runs. (If this isn't what you want, see [Controlling when attachments re-run](#Controlling-when-attachments-re-run).)
 
 ## Inline attachments
 
@@ -126,9 +128,42 @@ This allows you to create _wrapper components_ that augment elements ([demo](/pl
 </Button>
 ```
 
-### Converting actions to attachments
+## Controlling when attachments re-run
+
+Attachments, unlike [actions](use), are fully reactive: `{@attach foo(bar)}` will re-run on changes to `foo` _or_ `bar` (or any state read inside `foo`):
+
+```js
+// @errors: 7006 2304 2552
+function foo(bar) {
+	return (node) => {
+		veryExpensiveSetupWork(node);
+		update(node, bar);
+	};
+}
+```
+
+In the rare case that this is a problem (for example, if `foo` does expensive and unavoidable setup work) consider passing the data inside a function and reading it in a child effect:
+
+```js
+// @errors: 7006 2304 2552
+function foo(+++getBar+++) {
+	return (node) => {
+		veryExpensiveSetupWork(node);
+
++++		$effect(() => {
+			update(node, getBar());
+		});+++
+	}
+}
+```
+
+## Creating attachments programmatically
+
+To add attachments to an object that will be spread onto a component or element, use [`createAttachmentKey`](svelte-attachments#createAttachmentKey).
 
-If you want to use this functionality on Components but you are using a library that only provides actions you can use the `fromAction` utility exported from `svelte/attachments` to convert between the two.
+## Converting actions to attachments
+
+If you want to use this functionality on components but you are using a library that only provides actions you can use the [`fromAction`](svelte/attachments#fromAction) utility.
 
 This function accept an action as the first argument and a function returning the arguments of the action as the second argument and returns an attachment.
 
@@ -146,9 +181,4 @@ This function accept an action as the first argument and a function returning th
 	{@attach fromAction(log, () => count)}
 >
 	{count}
-</Button>
-```
-
-## Creating attachments programmatically
-
-To add attachments to an object that will be spread onto a component or element, use [`createAttachmentKey`](svelte-attachments#createAttachmentKey).
+</Button>
\ No newline at end of file
diff --git a/documentation/docs/06-runtime/02-context.md b/documentation/docs/06-runtime/02-context.md
index 4204bcfe6d..f395de421c 100644
--- a/documentation/docs/06-runtime/02-context.md
+++ b/documentation/docs/06-runtime/02-context.md
@@ -125,7 +125,7 @@ In many cases this is perfectly fine, but there is a risk: if you mutate the sta
 ```svelte
 <!--- file: App.svelte ---->
 <script>
-	import { myGlobalState } from 'svelte';
+	import { myGlobalState } from './state.svelte.js';
 
 	let { data } = $props();
 
diff --git a/documentation/docs/98-reference/.generated/compile-errors.md b/documentation/docs/98-reference/.generated/compile-errors.md
index e8669ead53..db848a0299 100644
--- a/documentation/docs/98-reference/.generated/compile-errors.md
+++ b/documentation/docs/98-reference/.generated/compile-errors.md
@@ -846,6 +846,38 @@ Cannot reassign or bind to snippet parameter
 This snippet is shadowing the prop `%prop%` with the same name
 ```
 
+### state_field_duplicate
+
+```
+`%name%` has already been declared on this class
+```
+
+An assignment to a class field that uses a `$state` or `$derived` rune is considered a _state field declaration_. The declaration can happen in the class body...
+
+```js
+class Counter {
+	count = $state(0);
+}
+```
+
+...or inside the constructor...
+
+```js
+class Counter {
+	constructor() {
+		this.count = $state(0);
+	}
+}
+```
+
+...but it can only happen once.
+
+### state_field_invalid_assignment
+
+```
+Cannot assign to a state field before its declaration
+```
+
 ### state_invalid_export
 
 ```
@@ -855,7 +887,7 @@ Cannot export state from a module if it is reassigned. Either export a function
 ### state_invalid_placement
 
 ```
-`%rune%(...)` can only be used as a variable declaration initializer or a class field
+`%rune%(...)` can only be used as a variable declaration initializer, a class field declaration, or the first assignment to a class field at the top level of the constructor.
 ```
 
 ### store_invalid_scoped_subscription
diff --git a/packages/svelte/CHANGELOG.md b/packages/svelte/CHANGELOG.md
index 2b65a1889f..4508a9b054 100644
--- a/packages/svelte/CHANGELOG.md
+++ b/packages/svelte/CHANGELOG.md
@@ -1,5 +1,29 @@
 # svelte
 
+## 5.31.1
+
+### Patch Changes
+
+- fix: avoid auto-parenthesis for special-keywords-only `MediaQuery` ([#15937](https://github.com/sveltejs/svelte/pull/15937))
+
+## 5.31.0
+
+### Minor Changes
+
+- feat: allow state fields to be declared inside class constructors ([#15820](https://github.com/sveltejs/svelte/pull/15820))
+
+### Patch Changes
+
+- fix: Add missing `AttachTag` in `Tag` union type inside the `AST` namespace from `"svelte/compiler"` ([#15946](https://github.com/sveltejs/svelte/pull/15946))
+
+## 5.30.2
+
+### Patch Changes
+
+- fix: falsy attachments types ([#15939](https://github.com/sveltejs/svelte/pull/15939))
+
+- fix: handle more hydration mismatches ([#15851](https://github.com/sveltejs/svelte/pull/15851))
+
 ## 5.30.1
 
 ### Patch Changes
diff --git a/packages/svelte/elements.d.ts b/packages/svelte/elements.d.ts
index c637137365..237e96c699 100644
--- a/packages/svelte/elements.d.ts
+++ b/packages/svelte/elements.d.ts
@@ -863,8 +863,8 @@ export interface HTMLAttributes<T extends EventTarget> extends AriaAttributes, D
 	// allow any data- attribute
 	[key: `data-${string}`]: any;
 
-	// allow any attachment
-	[key: symbol]: Attachment<T>;
+	// allow any attachment and falsy values (by using false we prevent the usage of booleans values by themselves)
+	[key: symbol]: Attachment<T> | false | undefined | null;
 }
 
 export type HTMLAttributeAnchorTarget = '_self' | '_blank' | '_parent' | '_top' | (string & {});
diff --git a/packages/svelte/messages/compile-errors/script.md b/packages/svelte/messages/compile-errors/script.md
index aabcbeae48..e11975aef2 100644
--- a/packages/svelte/messages/compile-errors/script.md
+++ b/packages/svelte/messages/compile-errors/script.md
@@ -212,13 +212,41 @@ It's possible to export a snippet from a `<script module>` block, but only if it
 
 > Cannot reassign or bind to snippet parameter
 
+## state_field_duplicate
+
+> `%name%` has already been declared on this class
+
+An assignment to a class field that uses a `$state` or `$derived` rune is considered a _state field declaration_. The declaration can happen in the class body...
+
+```js
+class Counter {
+	count = $state(0);
+}
+```
+
+...or inside the constructor...
+
+```js
+class Counter {
+	constructor() {
+		this.count = $state(0);
+	}
+}
+```
+
+...but it can only happen once.
+
+## state_field_invalid_assignment
+
+> Cannot assign to a state field before its declaration
+
 ## state_invalid_export
 
 > Cannot export state from a module if it is reassigned. Either export a function returning the state value or only mutate the state value's properties
 
 ## state_invalid_placement
 
-> `%rune%(...)` can only be used as a variable declaration initializer or a class field
+> `%rune%(...)` can only be used as a variable declaration initializer, a class field declaration, or the first assignment to a class field at the top level of the constructor.
 
 ## store_invalid_scoped_subscription
 
diff --git a/packages/svelte/package.json b/packages/svelte/package.json
index b6e8d06c63..45a875d19b 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": "5.30.1",
+  "version": "5.31.1",
   "type": "module",
   "types": "./types/index.d.ts",
   "engines": {
@@ -136,7 +136,7 @@
   ],
   "scripts": {
     "build": "node scripts/process-messages && rollup -c && pnpm generate:types && node scripts/check-treeshakeability.js",
-    "dev": "node scripts/process-messages && rollup -cw",
+    "dev": "node scripts/process-messages -w & rollup -cw",
     "check": "tsc --project tsconfig.runtime.json && tsc && cd ./tests/types && tsc",
     "check:watch": "tsc --watch",
     "generate:version": "node ./scripts/generate-version.js",
diff --git a/packages/svelte/scripts/process-messages/index.js b/packages/svelte/scripts/process-messages/index.js
index 80619acfa7..81c59271de 100644
--- a/packages/svelte/scripts/process-messages/index.js
+++ b/packages/svelte/scripts/process-messages/index.js
@@ -1,409 +1,441 @@
 // @ts-check
+import process from 'node:process';
 import fs from 'node:fs';
 import * as acorn from 'acorn';
 import { walk } from 'zimmerframe';
 import * as esrap from 'esrap';
 
-/** @type {Record<string, Record<string, { messages: string[], details: string | null }>>} */
-const messages = {};
-const seen = new Set();
-
 const DIR = '../../documentation/docs/98-reference/.generated';
-fs.rmSync(DIR, { force: true, recursive: true });
-fs.mkdirSync(DIR);
 
-for (const category of fs.readdirSync('messages')) {
-	if (category.startsWith('.')) continue;
+const watch = process.argv.includes('-w');
 
-	messages[category] = {};
+function run() {
+	/** @type {Record<string, Record<string, { messages: string[], details: string | null }>>} */
+	const messages = {};
+	const seen = new Set();
 
-	for (const file of fs.readdirSync(`messages/${category}`)) {
-		if (!file.endsWith('.md')) continue;
+	fs.rmSync(DIR, { force: true, recursive: true });
+	fs.mkdirSync(DIR);
 
-		const markdown = fs
-			.readFileSync(`messages/${category}/${file}`, 'utf-8')
-			.replace(/\r\n/g, '\n');
+	for (const category of fs.readdirSync('messages')) {
+		if (category.startsWith('.')) continue;
 
-		const sorted = [];
+		messages[category] = {};
 
-		for (const match of markdown.matchAll(/## ([\w]+)\n\n([^]+?)(?=$|\n\n## )/g)) {
-			const [_, code, text] = match;
+		for (const file of fs.readdirSync(`messages/${category}`)) {
+			if (!file.endsWith('.md')) continue;
 
-			if (seen.has(code)) {
-				throw new Error(`Duplicate message code ${category}/${code}`);
-			}
+			const markdown = fs
+				.readFileSync(`messages/${category}/${file}`, 'utf-8')
+				.replace(/\r\n/g, '\n');
 
-			sorted.push({ code, _ });
+			const sorted = [];
 
-			const sections = text.trim().split('\n\n');
-			const details = [];
+			for (const match of markdown.matchAll(/## ([\w]+)\n\n([^]+?)(?=$|\n\n## )/g)) {
+				const [_, code, text] = match;
 
-			while (!sections[sections.length - 1].startsWith('> ')) {
-				details.unshift(/** @type {string} */ (sections.pop()));
-			}
+				if (seen.has(code)) {
+					throw new Error(`Duplicate message code ${category}/${code}`);
+				}
+
+				sorted.push({ code, _ });
 
-			if (sections.length === 0) {
-				throw new Error('No message text');
+				const sections = text.trim().split('\n\n');
+				const details = [];
+
+				while (!sections[sections.length - 1].startsWith('> ')) {
+					details.unshift(/** @type {string} */ (sections.pop()));
+				}
+
+				if (sections.length === 0) {
+					throw new Error('No message text');
+				}
+
+				seen.add(code);
+				messages[category][code] = {
+					messages: sections.map((section) => section.replace(/^> /gm, '').replace(/^>\n/gm, '\n')),
+					details: details.join('\n\n')
+				};
 			}
 
-			seen.add(code);
-			messages[category][code] = {
-				messages: sections.map((section) => section.replace(/^> /gm, '').replace(/^>\n/gm, '\n')),
-				details: details.join('\n\n')
-			};
+			sorted.sort((a, b) => (a.code < b.code ? -1 : 1));
+
+			fs.writeFileSync(
+				`messages/${category}/${file}`,
+				sorted.map((x) => x._.trim()).join('\n\n') + '\n'
+			);
 		}
 
-		sorted.sort((a, b) => (a.code < b.code ? -1 : 1));
 		fs.writeFileSync(
-			`messages/${category}/${file}`,
-			sorted.map((x) => x._.trim()).join('\n\n') + '\n'
+			`${DIR}/${category}.md`,
+			'<!-- This file is generated by scripts/process-messages/index.js. Do not edit! -->\n\n' +
+				Object.entries(messages[category])
+					.map(([code, { messages, details }]) => {
+						const chunks = [
+							`### ${code}`,
+							...messages.map((message) => '```\n' + message + '\n```')
+						];
+
+						if (details) {
+							chunks.push(details);
+						}
+
+						return chunks.join('\n\n');
+					})
+					.sort()
+					.join('\n\n') +
+				'\n'
 		);
 	}
 
-	fs.writeFileSync(
-		`${DIR}/${category}.md`,
-		'<!-- This file is generated by scripts/process-messages/index.js. Do not edit! -->\n\n' +
-			Object.entries(messages[category])
-				.map(([code, { messages, details }]) => {
-					const chunks = [`### ${code}`, ...messages.map((message) => '```\n' + message + '\n```')];
-
-					if (details) {
-						chunks.push(details);
-					}
-
-					return chunks.join('\n\n');
-				})
-				.sort()
-				.join('\n\n') +
-			'\n'
-	);
-}
-
-/**
- * @param {string} name
- * @param {string} dest
- */
-function transform(name, dest) {
-	const source = fs
-		.readFileSync(new URL(`./templates/${name}.js`, import.meta.url), 'utf-8')
-		.replace(/\r\n/g, '\n');
-
 	/**
-	 * @type {Array<{
-	 * 	type: string;
-	 * 	value: string;
-	 * 	start: number;
-	 * 	end: number
-	 * }>}
+	 * @param {string} name
+	 * @param {string} dest
 	 */
-	const comments = [];
-
-	let ast = acorn.parse(source, {
-		ecmaVersion: 'latest',
-		sourceType: 'module',
-		onComment: (block, value, start, end) => {
-			if (block && /\n/.test(value)) {
-				let a = start;
-				while (a > 0 && source[a - 1] !== '\n') a -= 1;
-
-				let b = a;
-				while (/[ \t]/.test(source[b])) b += 1;
-
-				const indentation = source.slice(a, b);
-				value = value.replace(new RegExp(`^${indentation}`, 'gm'), '');
-			}
-
-			comments.push({ type: block ? 'Block' : 'Line', value, start, end });
-		}
-	});
+	function transform(name, dest) {
+		const source = fs
+			.readFileSync(new URL(`./templates/${name}.js`, import.meta.url), 'utf-8')
+			.replace(/\r\n/g, '\n');
 
-	ast = walk(ast, null, {
-		_(node, { next }) {
-			let comment;
+		/**
+		 * @type {Array<{
+		 * 	type: string;
+		 * 	value: string;
+		 * 	start: number;
+		 * 	end: number
+		 * }>}
+		 */
+		const comments = [];
+
+		let ast = acorn.parse(source, {
+			ecmaVersion: 'latest',
+			sourceType: 'module',
+			onComment: (block, value, start, end) => {
+				if (block && /\n/.test(value)) {
+					let a = start;
+					while (a > 0 && source[a - 1] !== '\n') a -= 1;
+
+					let b = a;
+					while (/[ \t]/.test(source[b])) b += 1;
+
+					const indentation = source.slice(a, b);
+					value = value.replace(new RegExp(`^${indentation}`, 'gm'), '');
+				}
 
-			while (comments[0] && comments[0].start < node.start) {
-				comment = comments.shift();
-				// @ts-expect-error
-				(node.leadingComments ||= []).push(comment);
+				comments.push({ type: block ? 'Block' : 'Line', value, start, end });
 			}
+		});
 
-			next();
-
-			if (comments[0]) {
-				const slice = source.slice(node.end, comments[0].start);
+		ast = walk(ast, null, {
+			_(node, { next }) {
+				let comment;
 
-				if (/^[,) \t]*$/.test(slice)) {
+				while (comments[0] && comments[0].start < node.start) {
+					comment = comments.shift();
 					// @ts-expect-error
-					node.trailingComments = [comments.shift()];
+					(node.leadingComments ||= []).push(comment);
 				}
-			}
-		},
-		// @ts-expect-error
-		Identifier(node, context) {
-			if (node.name === 'CODES') {
-				return {
-					type: 'ArrayExpression',
-					elements: Object.keys(messages[name]).map((code) => ({
-						type: 'Literal',
-						value: code
-					}))
-				};
-			}
-		}
-	});
-
-	if (comments.length > 0) {
-		// @ts-expect-error
-		(ast.trailingComments ||= []).push(...comments);
-	}
-
-	const category = messages[name];
-
-	// find the `export function CODE` node
-	const index = ast.body.findIndex((node) => {
-		if (
-			node.type === 'ExportNamedDeclaration' &&
-			node.declaration &&
-			node.declaration.type === 'FunctionDeclaration'
-		) {
-			return node.declaration.id.name === 'CODE';
-		}
-	});
-
-	if (index === -1) throw new Error(`missing export function CODE in ${name}.js`);
 
-	const template_node = ast.body[index];
-	ast.body.splice(index, 1);
+				next();
 
-	for (const code in category) {
-		const { messages } = category[code];
-		/** @type {string[]} */
-		const vars = [];
+				if (comments[0]) {
+					const slice = source.slice(node.end, comments[0].start);
 
-		const group = messages.map((text, i) => {
-			for (const match of text.matchAll(/%(\w+)%/g)) {
-				const name = match[1];
-				if (!vars.includes(name)) {
-					vars.push(match[1]);
+					if (/^[,) \t]*$/.test(slice)) {
+						// @ts-expect-error
+						node.trailingComments = [comments.shift()];
+					}
+				}
+			},
+			// @ts-expect-error
+			Identifier(node, context) {
+				if (node.name === 'CODES') {
+					return {
+						type: 'ArrayExpression',
+						elements: Object.keys(messages[name]).map((code) => ({
+							type: 'Literal',
+							value: code
+						}))
+					};
 				}
 			}
-
-			return {
-				text,
-				vars: vars.slice()
-			};
 		});
 
-		/** @type {import('estree').Expression} */
-		let message = { type: 'Literal', value: '' };
-		let prev_vars;
+		if (comments.length > 0) {
+			// @ts-expect-error
+			(ast.trailingComments ||= []).push(...comments);
+		}
 
-		for (let i = 0; i < group.length; i += 1) {
-			const { text, vars } = group[i];
+		const category = messages[name];
 
-			if (vars.length === 0) {
-				message = {
-					type: 'Literal',
-					value: text
-				};
-				prev_vars = vars;
-				continue;
+		// find the `export function CODE` node
+		const index = ast.body.findIndex((node) => {
+			if (
+				node.type === 'ExportNamedDeclaration' &&
+				node.declaration &&
+				node.declaration.type === 'FunctionDeclaration'
+			) {
+				return node.declaration.id.name === 'CODE';
 			}
+		});
 
-			const parts = text.split(/(%\w+%)/);
+		if (index === -1) throw new Error(`missing export function CODE in ${name}.js`);
 
-			/** @type {import('estree').Expression[]} */
-			const expressions = [];
+		const template_node = ast.body[index];
+		ast.body.splice(index, 1);
 
-			/** @type {import('estree').TemplateElement[]} */
-			const quasis = [];
+		for (const code in category) {
+			const { messages } = category[code];
+			/** @type {string[]} */
+			const vars = [];
 
-			for (let i = 0; i < parts.length; i += 1) {
-				const part = parts[i];
-				if (i % 2 === 0) {
-					const str = part.replace(/(`|\${)/g, '\\$1');
-					quasis.push({
-						type: 'TemplateElement',
-						value: { raw: str, cooked: str },
-						tail: i === parts.length - 1
-					});
-				} else {
-					expressions.push({
-						type: 'Identifier',
-						name: part.slice(1, -1)
-					});
+			const group = messages.map((text, i) => {
+				for (const match of text.matchAll(/%(\w+)%/g)) {
+					const name = match[1];
+					if (!vars.includes(name)) {
+						vars.push(match[1]);
+					}
 				}
-			}
+
+				return {
+					text,
+					vars: vars.slice()
+				};
+			});
 
 			/** @type {import('estree').Expression} */
-			const expression = {
-				type: 'TemplateLiteral',
-				expressions,
-				quasis
-			};
-
-			if (prev_vars) {
-				if (vars.length === prev_vars.length) {
-					throw new Error('Message overloads must have new parameters');
+			let message = { type: 'Literal', value: '' };
+			let prev_vars;
+
+			for (let i = 0; i < group.length; i += 1) {
+				const { text, vars } = group[i];
+
+				if (vars.length === 0) {
+					message = {
+						type: 'Literal',
+						value: text
+					};
+					prev_vars = vars;
+					continue;
 				}
 
-				message = {
-					type: 'ConditionalExpression',
-					test: {
-						type: 'Identifier',
-						name: vars[prev_vars.length]
-					},
-					consequent: expression,
-					alternate: message
-				};
-			} else {
-				message = expression;
-			}
+				const parts = text.split(/(%\w+%)/);
 
-			prev_vars = vars;
-		}
+				/** @type {import('estree').Expression[]} */
+				const expressions = [];
 
-		const clone = walk(/** @type {import('estree').Node} */ (template_node), null, {
-			// @ts-expect-error Block is a block comment, which is not recognised
-			Block(node, context) {
-				if (!node.value.includes('PARAMETER')) return;
-
-				const value = /** @type {string} */ (node.value)
-					.split('\n')
-					.map((line) => {
-						if (line === ' * MESSAGE') {
-							return messages[messages.length - 1]
-								.split('\n')
-								.map((line) => ` * ${line}`)
-								.join('\n');
-						}
+				/** @type {import('estree').TemplateElement[]} */
+				const quasis = [];
 
-						if (line.includes('PARAMETER')) {
-							return vars
-								.map((name, i) => {
-									const optional = i >= group[0].vars.length;
+				for (let i = 0; i < parts.length; i += 1) {
+					const part = parts[i];
+					if (i % 2 === 0) {
+						const str = part.replace(/(`|\${)/g, '\\$1');
+						quasis.push({
+							type: 'TemplateElement',
+							value: { raw: str, cooked: str },
+							tail: i === parts.length - 1
+						});
+					} else {
+						expressions.push({
+							type: 'Identifier',
+							name: part.slice(1, -1)
+						});
+					}
+				}
 
-									return optional
-										? ` * @param {string | undefined | null} [${name}]`
-										: ` * @param {string} ${name}`;
-								})
-								.join('\n');
-						}
+				/** @type {import('estree').Expression} */
+				const expression = {
+					type: 'TemplateLiteral',
+					expressions,
+					quasis
+				};
 
-						return line;
-					})
-					.filter((x) => x !== '')
-					.join('\n');
+				if (prev_vars) {
+					if (vars.length === prev_vars.length) {
+						throw new Error('Message overloads must have new parameters');
+					}
 
-				if (value !== node.value) {
-					return { ...node, value };
+					message = {
+						type: 'ConditionalExpression',
+						test: {
+							type: 'Identifier',
+							name: vars[prev_vars.length]
+						},
+						consequent: expression,
+						alternate: message
+					};
+				} else {
+					message = expression;
 				}
-			},
-			FunctionDeclaration(node, context) {
-				if (node.id.name !== 'CODE') return;
 
-				const params = [];
+				prev_vars = vars;
+			}
 
-				for (const param of node.params) {
-					if (param.type === 'Identifier' && param.name === 'PARAMETER') {
-						params.push(...vars.map((name) => ({ type: 'Identifier', name })));
-					} else {
-						params.push(param);
+			const clone = walk(/** @type {import('estree').Node} */ (template_node), null, {
+				// @ts-expect-error Block is a block comment, which is not recognised
+				Block(node, context) {
+					if (!node.value.includes('PARAMETER')) return;
+
+					const value = /** @type {string} */ (node.value)
+						.split('\n')
+						.map((line) => {
+							if (line === ' * MESSAGE') {
+								return messages[messages.length - 1]
+									.split('\n')
+									.map((line) => ` * ${line}`)
+									.join('\n');
+							}
+
+							if (line.includes('PARAMETER')) {
+								return vars
+									.map((name, i) => {
+										const optional = i >= group[0].vars.length;
+
+										return optional
+											? ` * @param {string | undefined | null} [${name}]`
+											: ` * @param {string} ${name}`;
+									})
+									.join('\n');
+							}
+
+							return line;
+						})
+						.filter((x) => x !== '')
+						.join('\n');
+
+					if (value !== node.value) {
+						return { ...node, value };
 					}
-				}
+				},
+				FunctionDeclaration(node, context) {
+					if (node.id.name !== 'CODE') return;
+
+					const params = [];
 
-				return /** @type {import('estree').FunctionDeclaration} */ ({
-					.../** @type {import('estree').FunctionDeclaration} */ (context.next()),
-					params,
-					id: {
-						...node.id,
-						name: code
+					for (const param of node.params) {
+						if (param.type === 'Identifier' && param.name === 'PARAMETER') {
+							params.push(...vars.map((name) => ({ type: 'Identifier', name })));
+						} else {
+							params.push(param);
+						}
 					}
-				});
-			},
-			TemplateLiteral(node, context) {
-				/** @type {import('estree').TemplateElement} */
-				let quasi = {
-					type: 'TemplateElement',
-					value: {
-						...node.quasis[0].value
-					},
-					tail: node.quasis[0].tail
-				};
 
-				/** @type {import('estree').TemplateLiteral} */
-				let out = {
-					type: 'TemplateLiteral',
-					quasis: [quasi],
-					expressions: []
-				};
+					return /** @type {import('estree').FunctionDeclaration} */ ({
+						.../** @type {import('estree').FunctionDeclaration} */ (context.next()),
+						params,
+						id: {
+							...node.id,
+							name: code
+						}
+					});
+				},
+				TemplateLiteral(node, context) {
+					/** @type {import('estree').TemplateElement} */
+					let quasi = {
+						type: 'TemplateElement',
+						value: {
+							...node.quasis[0].value
+						},
+						tail: node.quasis[0].tail
+					};
 
-				for (let i = 0; i < node.expressions.length; i += 1) {
-					const q = structuredClone(node.quasis[i + 1]);
-					const e = node.expressions[i];
+					/** @type {import('estree').TemplateLiteral} */
+					let out = {
+						type: 'TemplateLiteral',
+						quasis: [quasi],
+						expressions: []
+					};
 
-					if (e.type === 'Literal' && e.value === 'CODE') {
-						quasi.value.raw += code + q.value.raw;
-						continue;
-					}
+					for (let i = 0; i < node.expressions.length; i += 1) {
+						const q = structuredClone(node.quasis[i + 1]);
+						const e = node.expressions[i];
 
-					if (e.type === 'Identifier' && e.name === 'MESSAGE') {
-						if (message.type === 'Literal') {
-							const str = /** @type {string} */ (message.value).replace(/(`|\${)/g, '\\$1');
-							quasi.value.raw += str + q.value.raw;
+						if (e.type === 'Literal' && e.value === 'CODE') {
+							quasi.value.raw += code + q.value.raw;
 							continue;
 						}
 
-						if (message.type === 'TemplateLiteral') {
-							const m = structuredClone(message);
-							quasi.value.raw += m.quasis[0].value.raw;
-							out.quasis.push(...m.quasis.slice(1));
-							out.expressions.push(...m.expressions);
-							quasi = m.quasis[m.quasis.length - 1];
-							quasi.value.raw += q.value.raw;
-							continue;
+						if (e.type === 'Identifier' && e.name === 'MESSAGE') {
+							if (message.type === 'Literal') {
+								const str = /** @type {string} */ (message.value).replace(/(`|\${)/g, '\\$1');
+								quasi.value.raw += str + q.value.raw;
+								continue;
+							}
+
+							if (message.type === 'TemplateLiteral') {
+								const m = structuredClone(message);
+								quasi.value.raw += m.quasis[0].value.raw;
+								out.quasis.push(...m.quasis.slice(1));
+								out.expressions.push(...m.expressions);
+								quasi = m.quasis[m.quasis.length - 1];
+								quasi.value.raw += q.value.raw;
+								continue;
+							}
 						}
+
+						out.quasis.push((quasi = q));
+						out.expressions.push(/** @type {import('estree').Expression} */ (context.visit(e)));
 					}
 
-					out.quasis.push((quasi = q));
-					out.expressions.push(/** @type {import('estree').Expression} */ (context.visit(e)));
+					return out;
+				},
+				Literal(node) {
+					if (node.value === 'CODE') {
+						return {
+							type: 'Literal',
+							value: code
+						};
+					}
+				},
+				Identifier(node) {
+					if (node.name !== 'MESSAGE') return;
+					return message;
 				}
+			});
 
-				return out;
-			},
-			Literal(node) {
-				if (node.value === 'CODE') {
-					return {
-						type: 'Literal',
-						value: code
-					};
-				}
-			},
-			Identifier(node) {
-				if (node.name !== 'MESSAGE') return;
-				return message;
-			}
-		});
+			// @ts-expect-error
+			ast.body.push(clone);
+		}
+
+		const module = esrap.print(ast);
 
-		// @ts-expect-error
-		ast.body.push(clone);
+		fs.writeFileSync(
+			dest,
+			`/* This file is generated by scripts/process-messages/index.js. Do not edit! */\n\n` +
+				module.code,
+			'utf-8'
+		);
 	}
 
-	const module = esrap.print(ast);
+	transform('compile-errors', 'src/compiler/errors.js');
+	transform('compile-warnings', 'src/compiler/warnings.js');
 
-	fs.writeFileSync(
-		dest,
-		`/* This file is generated by scripts/process-messages/index.js. Do not edit! */\n\n` +
-			module.code,
-		'utf-8'
-	);
+	transform('client-warnings', 'src/internal/client/warnings.js');
+	transform('client-errors', 'src/internal/client/errors.js');
+	transform('server-errors', 'src/internal/server/errors.js');
+	transform('shared-errors', 'src/internal/shared/errors.js');
+	transform('shared-warnings', 'src/internal/shared/warnings.js');
 }
 
-transform('compile-errors', 'src/compiler/errors.js');
-transform('compile-warnings', 'src/compiler/warnings.js');
+if (watch) {
+	let running = false;
+	let timeout;
+
+	fs.watch('messages', { recursive: true }, (type, file) => {
+		if (running) {
+			timeout ??= setTimeout(() => {
+				running = false;
+				timeout = null;
+			});
+		} else {
+			running = true;
+
+			// eslint-disable-next-line no-console
+			console.log('Regenerating messages...');
+			run();
+		}
+	});
+}
 
-transform('client-warnings', 'src/internal/client/warnings.js');
-transform('client-errors', 'src/internal/client/errors.js');
-transform('server-errors', 'src/internal/server/errors.js');
-transform('shared-errors', 'src/internal/shared/errors.js');
-transform('shared-warnings', 'src/internal/shared/warnings.js');
+run();
diff --git a/packages/svelte/src/compiler/errors.js b/packages/svelte/src/compiler/errors.js
index c99f597468..25e72340c6 100644
--- a/packages/svelte/src/compiler/errors.js
+++ b/packages/svelte/src/compiler/errors.js
@@ -461,6 +461,25 @@ export function snippet_parameter_assignment(node) {
 	e(node, 'snippet_parameter_assignment', `Cannot reassign or bind to snippet parameter\nhttps://svelte.dev/e/snippet_parameter_assignment`);
 }
 
+/**
+ * `%name%` has already been declared on this class
+ * @param {null | number | NodeLike} node
+ * @param {string} name
+ * @returns {never}
+ */
+export function state_field_duplicate(node, name) {
+	e(node, 'state_field_duplicate', `\`${name}\` has already been declared on this class\nhttps://svelte.dev/e/state_field_duplicate`);
+}
+
+/**
+ * Cannot assign to a state field before its declaration
+ * @param {null | number | NodeLike} node
+ * @returns {never}
+ */
+export function state_field_invalid_assignment(node) {
+	e(node, 'state_field_invalid_assignment', `Cannot assign to a state field before its declaration\nhttps://svelte.dev/e/state_field_invalid_assignment`);
+}
+
 /**
  * Cannot export state from a module if it is reassigned. Either export a function returning the state value or only mutate the state value's properties
  * @param {null | number | NodeLike} node
@@ -471,13 +490,13 @@ export function state_invalid_export(node) {
 }
 
 /**
- * `%rune%(...)` can only be used as a variable declaration initializer or a class field
+ * `%rune%(...)` can only be used as a variable declaration initializer, a class field declaration, or the first assignment to a class field at the top level of the constructor.
  * @param {null | number | NodeLike} node
  * @param {string} rune
  * @returns {never}
  */
 export function state_invalid_placement(node, rune) {
-	e(node, 'state_invalid_placement', `\`${rune}(...)\` can only be used as a variable declaration initializer or a class field\nhttps://svelte.dev/e/state_invalid_placement`);
+	e(node, 'state_invalid_placement', `\`${rune}(...)\` can only be used as a variable declaration initializer, a class field declaration, or the first assignment to a class field at the top level of the constructor.\nhttps://svelte.dev/e/state_invalid_placement`);
 }
 
 /**
diff --git a/packages/svelte/src/compiler/phases/2-analyze/index.js b/packages/svelte/src/compiler/phases/2-analyze/index.js
index 09cb41b23f..fded183b86 100644
--- a/packages/svelte/src/compiler/phases/2-analyze/index.js
+++ b/packages/svelte/src/compiler/phases/2-analyze/index.js
@@ -48,6 +48,7 @@ import { Literal } from './visitors/Literal.js';
 import { MemberExpression } from './visitors/MemberExpression.js';
 import { NewExpression } from './visitors/NewExpression.js';
 import { OnDirective } from './visitors/OnDirective.js';
+import { PropertyDefinition } from './visitors/PropertyDefinition.js';
 import { RegularElement } from './visitors/RegularElement.js';
 import { RenderTag } from './visitors/RenderTag.js';
 import { SlotElement } from './visitors/SlotElement.js';
@@ -164,6 +165,7 @@ const visitors = {
 	MemberExpression,
 	NewExpression,
 	OnDirective,
+	PropertyDefinition,
 	RegularElement,
 	RenderTag,
 	SlotElement,
@@ -256,7 +258,8 @@ export function analyze_module(ast, options) {
 		accessors: false,
 		runes: true,
 		immutable: true,
-		tracing: false
+		tracing: false,
+		classes: new Map()
 	};
 
 	walk(
@@ -265,7 +268,7 @@ export function analyze_module(ast, options) {
 			scope,
 			scopes,
 			analysis: /** @type {ComponentAnalysis} */ (analysis),
-			derived_state: [],
+			state_fields: new Map(),
 			// TODO the following are not needed for modules, but we have to pass them in order to avoid type error,
 			// and reducing the type would result in a lot of tedious type casts elsewhere - find a good solution one day
 			ast_type: /** @type {any} */ (null),
@@ -429,6 +432,7 @@ export function analyze_component(root, source, options) {
 		elements: [],
 		runes,
 		tracing: false,
+		classes: new Map(),
 		immutable: runes || options.immutable,
 		exports: [],
 		uses_props: false,
@@ -624,7 +628,7 @@ export function analyze_component(root, source, options) {
 				has_props_rune: false,
 				component_slots: new Set(),
 				expression: null,
-				derived_state: [],
+				state_fields: new Map(),
 				function_depth: scope.function_depth,
 				reactive_statement: null
 			};
@@ -691,7 +695,7 @@ export function analyze_component(root, source, options) {
 				reactive_statement: null,
 				component_slots: new Set(),
 				expression: null,
-				derived_state: [],
+				state_fields: new Map(),
 				function_depth: scope.function_depth
 			};
 
diff --git a/packages/svelte/src/compiler/phases/2-analyze/types.d.ts b/packages/svelte/src/compiler/phases/2-analyze/types.d.ts
index 17c8123de1..080239bac0 100644
--- a/packages/svelte/src/compiler/phases/2-analyze/types.d.ts
+++ b/packages/svelte/src/compiler/phases/2-analyze/types.d.ts
@@ -1,6 +1,6 @@
 import type { Scope } from '../scope.js';
 import type { ComponentAnalysis, ReactiveStatement } from '../types.js';
-import type { AST, ExpressionMetadata, ValidatedCompileOptions } from '#compiler';
+import type { AST, ExpressionMetadata, StateField, ValidatedCompileOptions } from '#compiler';
 
 export interface AnalysisState {
 	scope: Scope;
@@ -18,7 +18,10 @@ export interface AnalysisState {
 	component_slots: Set<string>;
 	/** Information about the current expression/directive/block value */
 	expression: ExpressionMetadata | null;
-	derived_state: { name: string; private: boolean }[];
+
+	/** Used to analyze class state */
+	state_fields: Map<string, StateField>;
+
 	function_depth: number;
 
 	// legacy stuff
diff --git a/packages/svelte/src/compiler/phases/2-analyze/visitors/AssignmentExpression.js b/packages/svelte/src/compiler/phases/2-analyze/visitors/AssignmentExpression.js
index a64c89cd88..673c79f2df 100644
--- a/packages/svelte/src/compiler/phases/2-analyze/visitors/AssignmentExpression.js
+++ b/packages/svelte/src/compiler/phases/2-analyze/visitors/AssignmentExpression.js
@@ -8,7 +8,7 @@ import { validate_assignment } from './shared/utils.js';
  * @param {Context} context
  */
 export function AssignmentExpression(node, context) {
-	validate_assignment(node, node.left, context.state);
+	validate_assignment(node, node.left, context);
 
 	if (context.state.reactive_statement) {
 		const id = node.left.type === 'MemberExpression' ? object(node.left) : node.left;
diff --git a/packages/svelte/src/compiler/phases/2-analyze/visitors/Attribute.js b/packages/svelte/src/compiler/phases/2-analyze/visitors/Attribute.js
index 3ba81767cc..773aa59744 100644
--- a/packages/svelte/src/compiler/phases/2-analyze/visitors/Attribute.js
+++ b/packages/svelte/src/compiler/phases/2-analyze/visitors/Attribute.js
@@ -211,7 +211,7 @@ function get_delegated_event(event_name, handler, context) {
 
 		if (
 			binding !== null &&
-			// Bail out if the the binding is a rest param
+			// Bail out if the binding is a rest param
 			(binding.declaration_kind === 'rest_param' ||
 				// Bail out if we reference anything from the EachBlock (for now) that mutates in non-runes mode,
 				(((!context.state.analysis.runes && binding.kind === 'each') ||
diff --git a/packages/svelte/src/compiler/phases/2-analyze/visitors/BindDirective.js b/packages/svelte/src/compiler/phases/2-analyze/visitors/BindDirective.js
index 18ea79262b..9f02e7fa5a 100644
--- a/packages/svelte/src/compiler/phases/2-analyze/visitors/BindDirective.js
+++ b/packages/svelte/src/compiler/phases/2-analyze/visitors/BindDirective.js
@@ -158,7 +158,7 @@ export function BindDirective(node, context) {
 		return;
 	}
 
-	validate_assignment(node, node.expression, context.state);
+	validate_assignment(node, node.expression, context);
 
 	const assignee = node.expression;
 	const left = object(assignee);
diff --git a/packages/svelte/src/compiler/phases/2-analyze/visitors/CallExpression.js b/packages/svelte/src/compiler/phases/2-analyze/visitors/CallExpression.js
index 904817b014..33abb52cac 100644
--- a/packages/svelte/src/compiler/phases/2-analyze/visitors/CallExpression.js
+++ b/packages/svelte/src/compiler/phases/2-analyze/visitors/CallExpression.js
@@ -114,12 +114,13 @@ export function CallExpression(node, context) {
 		case '$state':
 		case '$state.raw':
 		case '$derived':
-		case '$derived.by':
-			if (
-				(parent.type !== 'VariableDeclarator' ||
-					get_parent(context.path, -3).type === 'ConstTag') &&
-				!(parent.type === 'PropertyDefinition' && !parent.static && !parent.computed)
-			) {
+		case '$derived.by': {
+			const valid =
+				is_variable_declaration(parent, context) ||
+				is_class_property_definition(parent) ||
+				is_class_property_assignment_at_constructor_root(parent, context);
+
+			if (!valid) {
 				e.state_invalid_placement(node, rune);
 			}
 
@@ -130,6 +131,7 @@ export function CallExpression(node, context) {
 			}
 
 			break;
+		}
 
 		case '$effect':
 		case '$effect.pre':
@@ -270,3 +272,40 @@ function get_function_label(nodes) {
 		return parent.id.name;
 	}
 }
+
+/**
+ * @param {AST.SvelteNode} parent
+ * @param {Context} context
+ */
+function is_variable_declaration(parent, context) {
+	return parent.type === 'VariableDeclarator' && get_parent(context.path, -3).type !== 'ConstTag';
+}
+
+/**
+ * @param {AST.SvelteNode} parent
+ */
+function is_class_property_definition(parent) {
+	return parent.type === 'PropertyDefinition' && !parent.static && !parent.computed;
+}
+
+/**
+ * @param {AST.SvelteNode} node
+ * @param {Context} context
+ */
+function is_class_property_assignment_at_constructor_root(node, context) {
+	if (
+		node.type === 'AssignmentExpression' &&
+		node.operator === '=' &&
+		node.left.type === 'MemberExpression' &&
+		node.left.object.type === 'ThisExpression' &&
+		((node.left.property.type === 'Identifier' && !node.left.computed) ||
+			node.left.property.type === 'PrivateIdentifier' ||
+			node.left.property.type === 'Literal')
+	) {
+		// MethodDefinition (-5) -> FunctionExpression (-4) -> BlockStatement (-3) -> ExpressionStatement (-2) -> AssignmentExpression (-1)
+		const parent = get_parent(context.path, -5);
+		return parent?.type === 'MethodDefinition' && parent.kind === 'constructor';
+	}
+
+	return false;
+}
diff --git a/packages/svelte/src/compiler/phases/2-analyze/visitors/ClassBody.js b/packages/svelte/src/compiler/phases/2-analyze/visitors/ClassBody.js
index 0463e4da85..ffc39ac00d 100644
--- a/packages/svelte/src/compiler/phases/2-analyze/visitors/ClassBody.js
+++ b/packages/svelte/src/compiler/phases/2-analyze/visitors/ClassBody.js
@@ -1,30 +1,107 @@
-/** @import { ClassBody } from 'estree' */
+/** @import { AssignmentExpression, CallExpression, ClassBody, PropertyDefinition, Expression, PrivateIdentifier, MethodDefinition } from 'estree' */
+/** @import { StateField } from '#compiler' */
 /** @import { Context } from '../types' */
+import * as b from '#compiler/builders';
 import { get_rune } from '../../scope.js';
+import * as e from '../../../errors.js';
+import { is_state_creation_rune } from '../../../../utils.js';
+import { get_name } from '../../nodes.js';
+import { regex_invalid_identifier_chars } from '../../patterns.js';
 
 /**
  * @param {ClassBody} node
  * @param {Context} context
  */
 export function ClassBody(node, context) {
-	/** @type {{name: string, private: boolean}[]} */
-	const derived_state = [];
+	if (!context.state.analysis.runes) {
+		context.next();
+		return;
+	}
+
+	/** @type {string[]} */
+	const private_ids = [];
 
-	for (const definition of node.body) {
+	for (const prop of node.body) {
 		if (
-			definition.type === 'PropertyDefinition' &&
-			(definition.key.type === 'PrivateIdentifier' || definition.key.type === 'Identifier') &&
-			definition.value?.type === 'CallExpression'
+			(prop.type === 'MethodDefinition' || prop.type === 'PropertyDefinition') &&
+			prop.key.type === 'PrivateIdentifier'
 		) {
-			const rune = get_rune(definition.value, context.state.scope);
-			if (rune === '$derived' || rune === '$derived.by') {
-				derived_state.push({
-					name: definition.key.name,
-					private: definition.key.type === 'PrivateIdentifier'
-				});
+			private_ids.push(prop.key.name);
+		}
+	}
+
+	/** @type {Map<string, StateField>} */
+	const state_fields = new Map();
+
+	context.state.analysis.classes.set(node, state_fields);
+
+	/** @type {MethodDefinition | null} */
+	let constructor = null;
+
+	/**
+	 * @param {PropertyDefinition | AssignmentExpression} node
+	 * @param {Expression | PrivateIdentifier} key
+	 * @param {Expression | null | undefined} value
+	 */
+	function handle(node, key, value) {
+		const name = get_name(key);
+		if (name === null) return;
+
+		const rune = get_rune(value, context.state.scope);
+
+		if (rune && is_state_creation_rune(rune)) {
+			if (state_fields.has(name)) {
+				e.state_field_duplicate(node, name);
 			}
+
+			state_fields.set(name, {
+				node,
+				type: rune,
+				// @ts-expect-error for public state this is filled out in a moment
+				key: key.type === 'PrivateIdentifier' ? key : null,
+				value: /** @type {CallExpression} */ (value)
+			});
+		}
+	}
+
+	for (const child of node.body) {
+		if (child.type === 'PropertyDefinition' && !child.computed && !child.static) {
+			handle(child, child.key, child.value);
 		}
+
+		if (child.type === 'MethodDefinition' && child.kind === 'constructor') {
+			constructor = child;
+		}
+	}
+
+	if (constructor) {
+		for (const statement of constructor.value.body.body) {
+			if (statement.type !== 'ExpressionStatement') continue;
+			if (statement.expression.type !== 'AssignmentExpression') continue;
+
+			const { left, right } = statement.expression;
+
+			if (left.type !== 'MemberExpression') continue;
+			if (left.object.type !== 'ThisExpression') continue;
+			if (left.computed && left.property.type !== 'Literal') continue;
+
+			handle(statement.expression, left.property, right);
+		}
+	}
+
+	for (const [name, field] of state_fields) {
+		if (name[0] === '#') {
+			continue;
+		}
+
+		let deconflicted = name.replace(regex_invalid_identifier_chars, '_');
+		while (private_ids.includes(deconflicted)) {
+			deconflicted = '_' + deconflicted;
+		}
+
+		private_ids.push(deconflicted);
+		field.key = b.private_id(deconflicted);
 	}
 
-	context.next({ ...context.state, derived_state });
+	context.next({ ...context.state, state_fields });
 }
diff --git a/packages/svelte/src/compiler/phases/2-analyze/visitors/PropertyDefinition.js b/packages/svelte/src/compiler/phases/2-analyze/visitors/PropertyDefinition.js
new file mode 100644
index 0000000000..99d05cb47c
--- /dev/null
+++ b/packages/svelte/src/compiler/phases/2-analyze/visitors/PropertyDefinition.js
@@ -0,0 +1,21 @@
+/** @import { PropertyDefinition } from 'estree' */
+/** @import { Context } from '../types' */
+import * as e from '../../../errors.js';
+import { get_name } from '../../nodes.js';
+
+/**
+ * @param {PropertyDefinition} node
+ * @param {Context} context
+ */
+export function PropertyDefinition(node, context) {
+	const name = get_name(node.key);
+	const field = name && context.state.state_fields.get(name);
+
+	if (field && node !== field.node && node.value) {
+		if (/** @type {number} */ (node.start) < /** @type {number} */ (field.node.start)) {
+			e.state_field_invalid_assignment(node);
+		}
+	}
+
+	context.next();
+}
diff --git a/packages/svelte/src/compiler/phases/2-analyze/visitors/UpdateExpression.js b/packages/svelte/src/compiler/phases/2-analyze/visitors/UpdateExpression.js
index 741effc67a..13f4b9019e 100644
--- a/packages/svelte/src/compiler/phases/2-analyze/visitors/UpdateExpression.js
+++ b/packages/svelte/src/compiler/phases/2-analyze/visitors/UpdateExpression.js
@@ -8,7 +8,7 @@ import { validate_assignment } from './shared/utils.js';
  * @param {Context} context
  */
 export function UpdateExpression(node, context) {
-	validate_assignment(node, node.argument, context.state);
+	validate_assignment(node, node.argument, context);
 
 	if (context.state.reactive_statement) {
 		const id = node.argument.type === 'MemberExpression' ? object(node.argument) : node.argument;
diff --git a/packages/svelte/src/compiler/phases/2-analyze/visitors/shared/utils.js b/packages/svelte/src/compiler/phases/2-analyze/visitors/shared/utils.js
index 12e21c386c..d7b682da08 100644
--- a/packages/svelte/src/compiler/phases/2-analyze/visitors/shared/utils.js
+++ b/packages/svelte/src/compiler/phases/2-analyze/visitors/shared/utils.js
@@ -4,24 +4,25 @@
 /** @import { Scope } from '../../../scope' */
 /** @import { NodeLike } from '../../../../errors.js' */
 import * as e from '../../../../errors.js';
-import { extract_identifiers } from '../../../../utils/ast.js';
+import { extract_identifiers, get_parent } from '../../../../utils/ast.js';
 import * as w from '../../../../warnings.js';
 import * as b from '#compiler/builders';
 import { get_rune } from '../../../scope.js';
+import { get_name } from '../../../nodes.js';
 
 /**
  * @param {AssignmentExpression | UpdateExpression | AST.BindDirective} node
  * @param {Pattern | Expression} argument
- * @param {AnalysisState} state
+ * @param {Context} context
  */
-export function validate_assignment(node, argument, state) {
-	validate_no_const_assignment(node, argument, state.scope, node.type === 'BindDirective');
+export function validate_assignment(node, argument, context) {
+	validate_no_const_assignment(node, argument, context.state.scope, node.type === 'BindDirective');
 
 	if (argument.type === 'Identifier') {
-		const binding = state.scope.get(argument.name);
+		const binding = context.state.scope.get(argument.name);
 
-		if (state.analysis.runes) {
-			if (binding?.node === state.analysis.props_id) {
+		if (context.state.analysis.runes) {
+			if (binding?.node === context.state.analysis.props_id) {
 				e.constant_assignment(node, '$props.id()');
 			}
 
@@ -34,6 +35,41 @@ export function validate_assignment(node, argument, state) {
 			e.snippet_parameter_assignment(node);
 		}
 	}
+
+	if (argument.type === 'MemberExpression' && argument.object.type === 'ThisExpression') {
+		const name =
+			argument.computed && argument.property.type !== 'Literal'
+				? null
+				: get_name(argument.property);
+
+		const field = name !== null && context.state.state_fields?.get(name);
+
+		// check we're not assigning to a state field before its declaration in the constructor
+		if (field && field.node.type === 'AssignmentExpression' && node !== field.node) {
+			let i = context.path.length;
+			while (i--) {
+				const parent = context.path[i];
+
+				if (
+					parent.type === 'FunctionDeclaration' ||
+					parent.type === 'FunctionExpression' ||
+					parent.type === 'ArrowFunctionExpression'
+				) {
+					const grandparent = get_parent(context.path, i - 1);
+
+					if (
+						grandparent.type === 'MethodDefinition' &&
+						grandparent.kind === 'constructor' &&
+						/** @type {number} */ (node.start) < /** @type {number} */ (field.node.start)
+					) {
+						e.state_field_invalid_assignment(node);
+					}
+
+					break;
+				}
+			}
+		}
+	}
 }
 
 /**
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js b/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js
index 9a2f4dd34c..f2eda3a7d2 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js
@@ -163,8 +163,7 @@ export function client_component(analysis, options) {
 		},
 		events: new Set(),
 		preserve_whitespace: options.preserveWhitespace,
-		public_state: new Map(),
-		private_state: new Map(),
+		state_fields: new Map(),
 		transform: {},
 		in_constructor: false,
 		instance_level_snippets: [],
@@ -671,8 +670,7 @@ export function client_module(analysis, options) {
 		options,
 		scope: analysis.module.scope,
 		scopes: analysis.module.scopes,
-		public_state: new Map(),
-		private_state: new Map(),
+		state_fields: new Map(),
 		transform: {},
 		in_constructor: false
 	};
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/types.d.ts b/packages/svelte/src/compiler/phases/3-transform/client/types.d.ts
index 243e1c64a3..763b6773c7 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/types.d.ts
+++ b/packages/svelte/src/compiler/phases/3-transform/client/types.d.ts
@@ -9,15 +9,12 @@ import type {
 	UpdateExpression,
 	VariableDeclaration
 } from 'estree';
-import type { AST, Namespace, ValidatedCompileOptions } from '#compiler';
+import type { AST, Namespace, StateField, ValidatedCompileOptions } from '#compiler';
 import type { TransformState } from '../types.js';
 import type { ComponentAnalysis } from '../../types.js';
 import type { SourceLocation } from '#shared';
 
 export interface ClientTransformState extends TransformState {
-	readonly private_state: Map<string, StateField>;
-	readonly public_state: Map<string, StateField>;
-
 	/**
 	 * `true` if the current lexical scope belongs to a class constructor. this allows
 	 * us to rewrite `this.foo` as `this.#foo.value`
@@ -94,11 +91,6 @@ export interface ComponentClientTransformState extends ClientTransformState {
 	readonly module_level_snippets: VariableDeclaration[];
 }
 
-export interface StateField {
-	kind: 'state' | 'raw_state' | 'derived' | 'derived_by';
-	id: PrivateIdentifier;
-}
-
 export type Context = import('zimmerframe').Context<AST.SvelteNode, ClientTransformState>;
 export type Visitors = import('zimmerframe').Visitors<AST.SvelteNode, any>;
 
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 e3f9450050..a46b318601 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
@@ -11,6 +11,8 @@ import { dev, locate_node } from '../../../../state.js';
 import { should_proxy } from '../utils.js';
 import { visit_assignment_expression } from '../../shared/assignments.js';
 import { validate_mutation } from './shared/utils.js';
+import { get_rune } from '../../../scope.js';
+import { get_name } from '../../../nodes.js';
 
 /**
  * @param {AssignmentExpression} node
@@ -50,25 +52,42 @@ const callees = {
  * @returns {Expression | null}
  */
 function build_assignment(operator, left, right, context) {
-	// Handle class private/public state assignment cases
-	if (
-		context.state.analysis.runes &&
-		left.type === 'MemberExpression' &&
-		left.property.type === 'PrivateIdentifier'
-	) {
-		const private_state = context.state.private_state.get(left.property.name);
+	if (context.state.analysis.runes && left.type === 'MemberExpression') {
+		const name = get_name(left.property);
+		const field = name && context.state.state_fields.get(name);
+
+		if (field) {
+			// special case — state declaration in class constructor
+			if (field.node.type === 'AssignmentExpression' && left === field.node.left) {
+				const rune = get_rune(right, context.state.scope);
+
+				if (rune) {
+					const child_state = {
+						...context.state,
+						in_constructor: rune !== '$derived' && rune !== '$derived.by'
+					};
+
+					return b.assignment(
+						operator,
+						b.member(b.this, field.key),
+						/** @type {Expression} */ (context.visit(right, child_state))
+					);
+				}
+			}
 
-		if (private_state !== undefined) {
-			let value = /** @type {Expression} */ (
-				context.visit(build_assignment_value(operator, left, right))
-			);
+			// special case — assignment to private state field
+			if (left.property.type === 'PrivateIdentifier') {
+				let value = /** @type {Expression} */ (
+					context.visit(build_assignment_value(operator, left, right))
+				);
 
-			const needs_proxy =
-				private_state.kind === 'state' &&
-				is_non_coercive_operator(operator) &&
-				should_proxy(value, context.state.scope);
+				const needs_proxy =
+					field.type === '$state' &&
+					is_non_coercive_operator(operator) &&
+					should_proxy(value, context.state.scope);
 
-			return b.call('$.set', left, value, needs_proxy && b.true);
+				return b.call('$.set', left, value, needs_proxy && b.true);
+			}
 		}
 	}
 
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/visitors/CallExpression.js b/packages/svelte/src/compiler/phases/3-transform/client/visitors/CallExpression.js
index b110f8eae8..665be9e23b 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/visitors/CallExpression.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/visitors/CallExpression.js
@@ -4,19 +4,52 @@ import { dev, is_ignored } from '../../../../state.js';
 import * as b from '#compiler/builders';
 import { get_rune } from '../../../scope.js';
 import { transform_inspect_rune } from '../../utils.js';
+import { should_proxy } from '../utils.js';
 
 /**
  * @param {CallExpression} node
  * @param {Context} context
  */
 export function CallExpression(node, context) {
-	switch (get_rune(node, context.state.scope)) {
+	const rune = get_rune(node, context.state.scope);
+
+	switch (rune) {
 		case '$host':
 			return b.id('$$props.$$host');
 
 		case '$effect.tracking':
 			return b.call('$.effect_tracking');
 
+		// transform state field assignments in constructors
+		case '$state':
+		case '$state.raw': {
+			let arg = node.arguments[0];
+
+			/** @type {Expression | undefined} */
+			let value = undefined;
+
+			if (arg) {
+				value = /** @type {Expression} */ (context.visit(node.arguments[0]));
+
+				if (
+					rune === '$state' &&
+					should_proxy(/** @type {Expression} */ (arg), context.state.scope)
+				) {
+					value = b.call('$.proxy', value);
+				}
+			}
+
+			return b.call('$.state', value);
+		}
+
+		case '$derived':
+		case '$derived.by': {
+			let fn = /** @type {Expression} */ (context.visit(node.arguments[0]));
+			if (rune === '$derived') fn = b.thunk(fn);
+
+			return b.call('$.derived', fn);
+		}
+
 		case '$state.snapshot':
 			return b.call(
 				'$.snapshot',
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/visitors/ClassBody.js b/packages/svelte/src/compiler/phases/3-transform/client/visitors/ClassBody.js
index f3ebd94069..71a8fc3d05 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/visitors/ClassBody.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/visitors/ClassBody.js
@@ -1,184 +1,96 @@
-/** @import { ClassBody, Expression, Identifier, Literal, MethodDefinition, PrivateIdentifier, PropertyDefinition } from 'estree' */
-/** @import { Context, StateField } from '../types' */
+/** @import { CallExpression, ClassBody, MethodDefinition, PropertyDefinition, StaticBlock } from 'estree' */
+/** @import { StateField } from '#compiler' */
+/** @import { Context } from '../types' */
 import * as b from '#compiler/builders';
-import { regex_invalid_identifier_chars } from '../../../patterns.js';
-import { get_rune } from '../../../scope.js';
-import { should_proxy } from '../utils.js';
+import { get_name } from '../../../nodes.js';
 
 /**
  * @param {ClassBody} node
  * @param {Context} context
  */
 export function ClassBody(node, context) {
-	if (!context.state.analysis.runes) {
+	const state_fields = context.state.analysis.classes.get(node);
+
+	if (!state_fields) {
+		// in legacy mode, do nothing
 		context.next();
 		return;
 	}
 
-	/** @type {Map<string, StateField>} */
-	const public_state = new Map();
+	/** @type {Array<MethodDefinition | PropertyDefinition | StaticBlock>} */
+	const body = [];
 
-	/** @type {Map<string, StateField>} */
-	const private_state = new Map();
+	const child_state = { ...context.state, state_fields };
 
-	/** @type {Map<(MethodDefinition|PropertyDefinition)["key"], string>} */
-	const definition_names = new Map();
+	for (const [name, field] of state_fields) {
+		if (name[0] === '#') {
+			continue;
+		}
 
-	/** @type {string[]} */
-	const private_ids = [];
+		// insert backing fields for stuff declared in the constructor
+		if (field.node.type === 'AssignmentExpression') {
+			const member = b.member(b.this, field.key);
 
-	for (const definition of node.body) {
-		if (
-			(definition.type === 'PropertyDefinition' || definition.type === 'MethodDefinition') &&
-			(definition.key.type === 'Identifier' ||
-				definition.key.type === 'PrivateIdentifier' ||
-				definition.key.type === 'Literal')
-		) {
-			const type = definition.key.type;
-			const name = get_name(definition.key, public_state);
-			if (!name) continue;
-
-			// we store the deconflicted name in the map so that we can access it later
-			definition_names.set(definition.key, name);
-
-			const is_private = type === 'PrivateIdentifier';
-			if (is_private) private_ids.push(name);
-
-			if (definition.value?.type === 'CallExpression') {
-				const rune = get_rune(definition.value, context.state.scope);
-				if (
-					rune === '$state' ||
-					rune === '$state.raw' ||
-					rune === '$derived' ||
-					rune === '$derived.by'
-				) {
-					/** @type {StateField} */
-					const field = {
-						kind:
-							rune === '$state'
-								? 'state'
-								: rune === '$state.raw'
-									? 'raw_state'
-									: rune === '$derived.by'
-										? 'derived_by'
-										: 'derived',
-						// @ts-expect-error this is set in the next pass
-						id: is_private ? definition.key : null
-					};
-
-					if (is_private) {
-						private_state.set(name, field);
-					} else {
-						public_state.set(name, field);
-					}
-				}
-			}
-		}
-	}
+			const should_proxy = field.type === '$state' && true; // TODO
 
-	// each `foo = $state()` needs a backing `#foo` field
-	for (const [name, field] of public_state) {
-		let deconflicted = name;
-		while (private_ids.includes(deconflicted)) {
-			deconflicted = '_' + deconflicted;
-		}
+			const key = b.key(name);
 
-		private_ids.push(deconflicted);
-		field.id = b.private_id(deconflicted);
-	}
+			body.push(
+				b.prop_def(field.key, null),
 
-	/** @type {Array<MethodDefinition | PropertyDefinition>} */
-	const body = [];
+				b.method('get', key, [], [b.return(b.call('$.get', member))]),
 
-	const child_state = { ...context.state, public_state, private_state };
+				b.method(
+					'set',
+					key,
+					[b.id('value')],
+					[b.stmt(b.call('$.set', member, b.id('value'), should_proxy && b.true))]
+				)
+			);
+		}
+	}
 
 	// Replace parts of the class body
 	for (const definition of node.body) {
-		if (
-			definition.type === 'PropertyDefinition' &&
-			(definition.key.type === 'Identifier' ||
-				definition.key.type === 'PrivateIdentifier' ||
-				definition.key.type === 'Literal')
-		) {
-			const name = definition_names.get(definition.key);
-			if (!name) continue;
-
-			const is_private = definition.key.type === 'PrivateIdentifier';
-			const field = (is_private ? private_state : public_state).get(name);
-
-			if (definition.value?.type === 'CallExpression' && field !== undefined) {
-				let value = null;
-
-				if (definition.value.arguments.length > 0) {
-					const init = /** @type {Expression} **/ (
-						context.visit(definition.value.arguments[0], child_state)
-					);
-
-					value =
-						field.kind === 'state'
-							? b.call(
-									'$.state',
-									should_proxy(init, context.state.scope) ? b.call('$.proxy', init) : init
-								)
-							: field.kind === 'raw_state'
-								? b.call('$.state', init)
-								: field.kind === 'derived_by'
-									? b.call('$.derived', init)
-									: b.call('$.derived', b.thunk(init));
-				} else {
-					// if no arguments, we know it's state as `$derived()` is a compile error
-					value = b.call('$.state');
-				}
-
-				if (is_private) {
-					body.push(b.prop_def(field.id, value));
-				} else {
-					// #foo;
-					const member = b.member(b.this, field.id);
-					body.push(b.prop_def(field.id, value));
-
-					// get foo() { return this.#foo; }
-					body.push(b.method('get', definition.key, [], [b.return(b.call('$.get', member))]));
-
-					// set foo(value) { this.#foo = value; }
-					const val = b.id('value');
-
-					body.push(
-						b.method(
-							'set',
-							definition.key,
-							[val],
-							[b.stmt(b.call('$.set', member, val, field.kind === 'state' && b.true))]
-						)
-					);
-				}
-				continue;
-			}
+		if (definition.type !== 'PropertyDefinition') {
+			body.push(
+				/** @type {MethodDefinition | StaticBlock} */ (context.visit(definition, child_state))
+			);
+			continue;
 		}
 
-		body.push(/** @type {MethodDefinition} **/ (context.visit(definition, child_state)));
-	}
+		const name = get_name(definition.key);
+		const field = name && /** @type {StateField} */ (state_fields.get(name));
 
-	return { ...node, body };
-}
+		if (!field) {
+			body.push(/** @type {PropertyDefinition} */ (context.visit(definition, child_state)));
+			continue;
+		}
 
-/**
- * @param {Identifier | PrivateIdentifier | Literal} node
- * @param {Map<string, StateField>} public_state
- */
-function get_name(node, public_state) {
-	if (node.type === 'Literal') {
-		let name = node.value?.toString().replace(regex_invalid_identifier_chars, '_');
-
-		// the above could generate conflicts because it has to generate a valid identifier
-		// so stuff like `0` and `1` or `state%` and `state^` will result in the same string
-		// so we have to de-conflict. We can only check `public_state` because private state
-		// can't have literal keys
-		while (name && public_state.has(name)) {
-			name = '_' + name;
+		if (name[0] === '#') {
+			body.push(/** @type {PropertyDefinition} */ (context.visit(definition, child_state)));
+		} else if (field.node === definition) {
+			const member = b.member(b.this, field.key);
+
+			const should_proxy = field.type === '$state' && true; // TODO
+
+			body.push(
+				b.prop_def(
+					field.key,
+					/** @type {CallExpression} */ (context.visit(field.value, child_state))
+				),
+
+				b.method('get', definition.key, [], [b.return(b.call('$.get', member))]),
+
+				b.method(
+					'set',
+					definition.key,
+					[b.id('value')],
+					[b.stmt(b.call('$.set', member, b.id('value'), should_proxy && b.true))]
+				)
+			);
 		}
-		return name;
-	} else {
-		return node.name;
 	}
+
+	return { ...node, body };
 }
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/visitors/MemberExpression.js b/packages/svelte/src/compiler/phases/3-transform/client/visitors/MemberExpression.js
index ab88345ddd..7e89b0e705 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/visitors/MemberExpression.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/visitors/MemberExpression.js
@@ -9,9 +9,11 @@ import * as b from '#compiler/builders';
 export function MemberExpression(node, context) {
 	// rewrite `this.#foo` as `this.#foo.v` inside a constructor
 	if (node.property.type === 'PrivateIdentifier') {
-		const field = context.state.private_state.get(node.property.name);
+		const field = context.state.state_fields.get('#' + node.property.name);
+
 		if (field) {
-			return context.state.in_constructor && (field.kind === 'raw_state' || field.kind === 'state')
+			return context.state.in_constructor &&
+				(field.type === '$state.raw' || field.type === '$state')
 				? b.member(node, 'v')
 				: b.call('$.get', node);
 		}
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 ab981878ad..1d39289f3d 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
@@ -564,7 +564,7 @@ export function build_style_directives_object(style_directives, context) {
 
 /**
  * Serializes an assignment to an element property by adding relevant statements to either only
- * the init or the the init and update arrays, depending on whether or not the value is dynamic.
+ * the init or the init and update arrays, depending on whether or not the value is dynamic.
  * Resulting code for static looks something like this:
  * ```js
  * element.property = value;
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/visitors/UpdateExpression.js b/packages/svelte/src/compiler/phases/3-transform/client/visitors/UpdateExpression.js
index 96be119b84..66952eb298 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/visitors/UpdateExpression.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/visitors/UpdateExpression.js
@@ -15,7 +15,7 @@ export function UpdateExpression(node, context) {
 		argument.type === 'MemberExpression' &&
 		argument.object.type === 'ThisExpression' &&
 		argument.property.type === 'PrivateIdentifier' &&
-		context.state.private_state.has(argument.property.name)
+		context.state.state_fields.has('#' + argument.property.name)
 	) {
 		let fn = '$.update';
 		if (node.prefix) fn += '_pre';
diff --git a/packages/svelte/src/compiler/phases/3-transform/server/transform-server.js b/packages/svelte/src/compiler/phases/3-transform/server/transform-server.js
index e7896991d9..3498b2636b 100644
--- a/packages/svelte/src/compiler/phases/3-transform/server/transform-server.js
+++ b/packages/svelte/src/compiler/phases/3-transform/server/transform-server.js
@@ -23,7 +23,6 @@ import { Identifier } from './visitors/Identifier.js';
 import { IfBlock } from './visitors/IfBlock.js';
 import { KeyBlock } from './visitors/KeyBlock.js';
 import { LabeledStatement } from './visitors/LabeledStatement.js';
-import { MemberExpression } from './visitors/MemberExpression.js';
 import { PropertyDefinition } from './visitors/PropertyDefinition.js';
 import { RegularElement } from './visitors/RegularElement.js';
 import { RenderTag } from './visitors/RenderTag.js';
@@ -49,7 +48,6 @@ const global_visitors = {
 	ExpressionStatement,
 	Identifier,
 	LabeledStatement,
-	MemberExpression,
 	PropertyDefinition,
 	UpdateExpression,
 	VariableDeclaration
@@ -99,7 +97,7 @@ export function server_component(analysis, options) {
 		template: /** @type {any} */ (null),
 		namespace: options.namespace,
 		preserve_whitespace: options.preserveWhitespace,
-		private_derived: new Map(),
+		state_fields: new Map(),
 		skip_hydration_boundaries: false
 	};
 
@@ -395,7 +393,7 @@ export function server_module(analysis, options) {
 		// to be present for `javascript_visitors_legacy` and so is included in module
 		// transform state as well as component transform state
 		legacy_reactive_statements: new Map(),
-		private_derived: new Map()
+		state_fields: new Map()
 	};
 
 	const module = /** @type {Program} */ (
diff --git a/packages/svelte/src/compiler/phases/3-transform/server/types.d.ts b/packages/svelte/src/compiler/phases/3-transform/server/types.d.ts
index 971271642c..adde7480cb 100644
--- a/packages/svelte/src/compiler/phases/3-transform/server/types.d.ts
+++ b/packages/svelte/src/compiler/phases/3-transform/server/types.d.ts
@@ -2,12 +2,10 @@ import type { Expression, Statement, ModuleDeclaration, LabeledStatement } from
 import type { AST, Namespace, ValidatedCompileOptions } from '#compiler';
 import type { TransformState } from '../types.js';
 import type { ComponentAnalysis } from '../../types.js';
-import type { StateField } from '../client/types.js';
 
 export interface ServerTransformState extends TransformState {
 	/** The $: calls, which will be ordered in the end */
 	readonly legacy_reactive_statements: Map<LabeledStatement, Statement>;
-	readonly private_derived: Map<string, StateField>;
 }
 
 export interface ComponentServerTransformState extends ServerTransformState {
diff --git a/packages/svelte/src/compiler/phases/3-transform/server/visitors/AssignmentExpression.js b/packages/svelte/src/compiler/phases/3-transform/server/visitors/AssignmentExpression.js
index 071a12f9bc..280c16dbd2 100644
--- a/packages/svelte/src/compiler/phases/3-transform/server/visitors/AssignmentExpression.js
+++ b/packages/svelte/src/compiler/phases/3-transform/server/visitors/AssignmentExpression.js
@@ -3,6 +3,8 @@
 /** @import { Context, ServerTransformState } from '../types.js' */
 import * as b from '#compiler/builders';
 import { build_assignment_value } from '../../../../utils/ast.js';
+import { get_name } from '../../../nodes.js';
+import { get_rune } from '../../../scope.js';
 import { visit_assignment_expression } from '../../shared/assignments.js';
 
 /**
@@ -22,6 +24,29 @@ export function AssignmentExpression(node, context) {
  * @returns {Expression | null}
  */
 function build_assignment(operator, left, right, context) {
+	if (context.state.analysis.runes && left.type === 'MemberExpression') {
+		const name = get_name(left.property);
+		const field = name && context.state.state_fields.get(name);
+
+		// special case — state declaration in class constructor
+		if (field && field.node.type === 'AssignmentExpression' && left === field.node.left) {
+			const rune = get_rune(right, context.state.scope);
+
+			if (rune) {
+				const key =
+					left.property.type === 'PrivateIdentifier' || rune === '$state' || rune === '$state.raw'
+						? left.property
+						: field.key;
+
+				return b.assignment(
+					operator,
+					b.member(b.this, key, key.type === 'Literal'),
+					/** @type {Expression} */ (context.visit(right))
+				);
+			}
+		}
+	}
+
 	let object = left;
 
 	while (object.type === 'MemberExpression') {
diff --git a/packages/svelte/src/compiler/phases/3-transform/server/visitors/CallExpression.js b/packages/svelte/src/compiler/phases/3-transform/server/visitors/CallExpression.js
index 5bcbdee9fb..e36dc820b3 100644
--- a/packages/svelte/src/compiler/phases/3-transform/server/visitors/CallExpression.js
+++ b/packages/svelte/src/compiler/phases/3-transform/server/visitors/CallExpression.js
@@ -25,6 +25,15 @@ export function CallExpression(node, context) {
 		return b.arrow([], b.block([]));
 	}
 
+	if (rune === '$state' || rune === '$state.raw') {
+		return node.arguments[0] ? context.visit(node.arguments[0]) : b.void0;
+	}
+
+	if (rune === '$derived' || rune === '$derived.by') {
+		const fn = /** @type {Expression} */ (context.visit(node.arguments[0]));
+		return b.call('$.once', rune === '$derived' ? b.thunk(fn) : fn);
+	}
+
 	if (rune === '$state.snapshot') {
 		return b.call(
 			'$.snapshot',
diff --git a/packages/svelte/src/compiler/phases/3-transform/server/visitors/ClassBody.js b/packages/svelte/src/compiler/phases/3-transform/server/visitors/ClassBody.js
index c0ebdeae08..6797b0beff 100644
--- a/packages/svelte/src/compiler/phases/3-transform/server/visitors/ClassBody.js
+++ b/packages/svelte/src/compiler/phases/3-transform/server/visitors/ClassBody.js
@@ -1,120 +1,77 @@
-/** @import { ClassBody, Expression, MethodDefinition, PropertyDefinition } from 'estree' */
+/** @import { CallExpression, ClassBody, MethodDefinition, PropertyDefinition, StaticBlock } from 'estree' */
 /** @import { Context } from '../types.js' */
-/** @import { StateField } from '../../client/types.js' */
-import { dev } from '../../../../state.js';
 import * as b from '#compiler/builders';
-import { get_rune } from '../../../scope.js';
+import { get_name } from '../../../nodes.js';
 
 /**
  * @param {ClassBody} node
  * @param {Context} context
  */
 export function ClassBody(node, context) {
-	if (!context.state.analysis.runes) {
+	const state_fields = context.state.analysis.classes.get(node);
+
+	if (!state_fields) {
+		// in legacy mode, do nothing
 		context.next();
 		return;
 	}
 
-	/** @type {Map<string, StateField>} */
-	const public_derived = new Map();
+	/** @type {Array<MethodDefinition | PropertyDefinition | StaticBlock>} */
+	const body = [];
 
-	/** @type {Map<string, StateField>} */
-	const private_derived = new Map();
+	const child_state = { ...context.state, state_fields };
 
-	/** @type {string[]} */
-	const private_ids = [];
+	for (const [name, field] of state_fields) {
+		if (name[0] === '#') {
+			continue;
+		}
 
-	for (const definition of node.body) {
+		// insert backing fields for stuff declared in the constructor
 		if (
-			definition.type === 'PropertyDefinition' &&
-			(definition.key.type === 'Identifier' || definition.key.type === 'PrivateIdentifier')
+			field &&
+			field.node.type === 'AssignmentExpression' &&
+			(field.type === '$derived' || field.type === '$derived.by')
 		) {
-			const { type, name } = definition.key;
-
-			const is_private = type === 'PrivateIdentifier';
-			if (is_private) private_ids.push(name);
+			const member = b.member(b.this, field.key);
 
-			if (definition.value?.type === 'CallExpression') {
-				const rune = get_rune(definition.value, context.state.scope);
-				if (rune === '$derived' || rune === '$derived.by') {
-					/** @type {StateField} */
-					const field = {
-						kind: rune === '$derived.by' ? 'derived_by' : 'derived',
-						// @ts-expect-error this is set in the next pass
-						id: is_private ? definition.key : null
-					};
-
-					if (is_private) {
-						private_derived.set(name, field);
-					} else {
-						public_derived.set(name, field);
-					}
-				}
-			}
+			body.push(
+				b.prop_def(field.key, null),
+				b.method('get', b.key(name), [], [b.return(b.call(member))])
+			);
 		}
 	}
 
-	// each `foo = $derived()` needs a backing `#foo` field
-	for (const [name, field] of public_derived) {
-		let deconflicted = name;
-		while (private_ids.includes(deconflicted)) {
-			deconflicted = '_' + deconflicted;
-		}
-
-		private_ids.push(deconflicted);
-		field.id = b.private_id(deconflicted);
-	}
-
-	/** @type {Array<MethodDefinition | PropertyDefinition>} */
-	const body = [];
-
-	const child_state = { ...context.state, private_derived };
-
 	// Replace parts of the class body
 	for (const definition of node.body) {
-		if (
-			definition.type === 'PropertyDefinition' &&
-			(definition.key.type === 'Identifier' || definition.key.type === 'PrivateIdentifier')
-		) {
-			const name = definition.key.name;
-
-			const is_private = definition.key.type === 'PrivateIdentifier';
-			const field = (is_private ? private_derived : public_derived).get(name);
+		if (definition.type !== 'PropertyDefinition') {
+			body.push(
+				/** @type {MethodDefinition | StaticBlock} */ (context.visit(definition, child_state))
+			);
+			continue;
+		}
 
-			if (definition.value?.type === 'CallExpression' && field !== undefined) {
-				const init = /** @type {Expression} **/ (
-					context.visit(definition.value.arguments[0], child_state)
-				);
-				const value =
-					field.kind === 'derived_by' ? b.call('$.once', init) : b.call('$.once', b.thunk(init));
+		const name = get_name(definition.key);
+		const field = name && state_fields.get(name);
 
-				if (is_private) {
-					body.push(b.prop_def(field.id, value));
-				} else {
-					// #foo;
-					const member = b.member(b.this, field.id);
-					body.push(b.prop_def(field.id, value));
+		if (!field) {
+			body.push(/** @type {PropertyDefinition} */ (context.visit(definition, child_state)));
+			continue;
+		}
 
-					// get foo() { return this.#foo; }
-					body.push(b.method('get', definition.key, [], [b.return(b.call(member))]));
+		if (name[0] === '#' || field.type === '$state' || field.type === '$state.raw') {
+			body.push(/** @type {PropertyDefinition} */ (context.visit(definition, child_state)));
+		} else if (field.node === definition) {
+			const member = b.member(b.this, field.key);
 
-					if (dev && (field.kind === 'derived' || field.kind === 'derived_by')) {
-						body.push(
-							b.method(
-								'set',
-								definition.key,
-								[b.id('_')],
-								[b.throw_error(`Cannot update a derived property ('${name}')`)]
-							)
-						);
-					}
-				}
+			body.push(
+				b.prop_def(
+					field.key,
+					/** @type {CallExpression} */ (context.visit(field.value, child_state))
+				),
 
-				continue;
-			}
+				b.method('get', definition.key, [], [b.return(b.call(member))])
+			);
 		}
-
-		body.push(/** @type {MethodDefinition} **/ (context.visit(definition, child_state)));
 	}
 
 	return { ...node, body };
diff --git a/packages/svelte/src/compiler/phases/3-transform/server/visitors/MemberExpression.js b/packages/svelte/src/compiler/phases/3-transform/server/visitors/MemberExpression.js
deleted file mode 100644
index 73631395e6..0000000000
--- a/packages/svelte/src/compiler/phases/3-transform/server/visitors/MemberExpression.js
+++ /dev/null
@@ -1,23 +0,0 @@
-/** @import { MemberExpression } from 'estree' */
-/** @import { Context } from '../types.js' */
-import * as b from '#compiler/builders';
-
-/**
- * @param {MemberExpression} node
- * @param {Context} context
- */
-export function MemberExpression(node, context) {
-	if (
-		context.state.analysis.runes &&
-		node.object.type === 'ThisExpression' &&
-		node.property.type === 'PrivateIdentifier'
-	) {
-		const field = context.state.private_derived.get(node.property.name);
-
-		if (field) {
-			return b.call(node);
-		}
-	}
-
-	context.next();
-}
diff --git a/packages/svelte/src/compiler/phases/3-transform/types.d.ts b/packages/svelte/src/compiler/phases/3-transform/types.d.ts
index 5d860207dd..887ad447b9 100644
--- a/packages/svelte/src/compiler/phases/3-transform/types.d.ts
+++ b/packages/svelte/src/compiler/phases/3-transform/types.d.ts
@@ -1,5 +1,5 @@
 import type { Scope } from '../scope.js';
-import type { AST, ValidatedModuleCompileOptions } from '#compiler';
+import type { AST, StateField, ValidatedModuleCompileOptions } from '#compiler';
 import type { Analysis } from '../types.js';
 
 export interface TransformState {
@@ -7,4 +7,6 @@ export interface TransformState {
 	readonly options: ValidatedModuleCompileOptions;
 	readonly scope: Scope;
 	readonly scopes: Map<AST.SvelteNode, Scope>;
+
+	readonly state_fields: Map<string, StateField>;
 }
diff --git a/packages/svelte/src/compiler/phases/nodes.js b/packages/svelte/src/compiler/phases/nodes.js
index 003c3a2c49..2043747ed0 100644
--- a/packages/svelte/src/compiler/phases/nodes.js
+++ b/packages/svelte/src/compiler/phases/nodes.js
@@ -1,4 +1,6 @@
+/** @import { Expression, PrivateIdentifier } from 'estree' */
 /** @import { AST, ExpressionMetadata } from '#compiler' */
+
 /**
  * All nodes that can appear elsewhere than the top level, have attributes and can contain children
  */
@@ -64,3 +66,14 @@ export function create_expression_metadata() {
 		has_call: false
 	};
 }
+
+/**
+ * @param {Expression | PrivateIdentifier} node
+ */
+export function get_name(node) {
+	if (node.type === 'Literal') return String(node.value);
+	if (node.type === 'PrivateIdentifier') return '#' + node.name;
+	if (node.type === 'Identifier') return node.name;
+
+	return null;
+}
diff --git a/packages/svelte/src/compiler/phases/types.d.ts b/packages/svelte/src/compiler/phases/types.d.ts
index f98cbe1415..67cbd75ff8 100644
--- a/packages/svelte/src/compiler/phases/types.d.ts
+++ b/packages/svelte/src/compiler/phases/types.d.ts
@@ -1,6 +1,15 @@
-import type { AST, Binding } from '#compiler';
-import type { Identifier, LabeledStatement, Node, Program } from 'estree';
+import type { AST, Binding, StateField } from '#compiler';
+import type {
+	AssignmentExpression,
+	ClassBody,
+	Identifier,
+	LabeledStatement,
+	Node,
+	Program
+} from 'estree';
 import type { Scope, ScopeRoot } from './scope.js';
+import type { StateCreationRuneName } from '../../utils.js';
+import type { AnalysisState } from './2-analyze/types.js';
 
 export interface Js {
 	ast: Program;
@@ -29,6 +38,8 @@ export interface Analysis {
 	immutable: boolean;
 	tracing: boolean;
 
+	classes: Map<ClassBody, Map<string, StateField>>;
+
 	// TODO figure out if we can move this to ComponentAnalysis
 	accessors: boolean;
 }
diff --git a/packages/svelte/src/compiler/types/index.d.ts b/packages/svelte/src/compiler/types/index.d.ts
index 616c346ad3..ef52552a2e 100644
--- a/packages/svelte/src/compiler/types/index.d.ts
+++ b/packages/svelte/src/compiler/types/index.d.ts
@@ -2,6 +2,13 @@ import type { SourceMap } from 'magic-string';
 import type { Binding } from '../phases/scope.js';
 import type { AST, Namespace } from './template.js';
 import type { ICompileDiagnostic } from '../utils/compile_diagnostic.js';
+import type { StateCreationRuneName } from '../../utils.js';
+import type {
+	AssignmentExpression,
+	CallExpression,
+	PrivateIdentifier,
+	PropertyDefinition
+} from 'estree';
 
 /** The return value of `compile` from `svelte/compiler` */
 export interface CompileResult {
@@ -269,6 +276,13 @@ export interface ExpressionMetadata {
 	has_call: boolean;
 }
 
+export interface StateField {
+	type: StateCreationRuneName;
+	node: PropertyDefinition | AssignmentExpression;
+	key: PrivateIdentifier;
+	value: CallExpression;
+}
+
 export * from './template.js';
 
 export { Binding, Scope } from '../phases/scope.js';
diff --git a/packages/svelte/src/compiler/types/template.d.ts b/packages/svelte/src/compiler/types/template.d.ts
index 6dec1f2dbe..b51c9e9a8d 100644
--- a/packages/svelte/src/compiler/types/template.d.ts
+++ b/packages/svelte/src/compiler/types/template.d.ts
@@ -547,7 +547,13 @@ export namespace AST {
 		| AST.SvelteWindow
 		| AST.SvelteBoundary;
 
-	export type Tag = AST.ExpressionTag | AST.HtmlTag | AST.ConstTag | AST.DebugTag | AST.RenderTag;
+	export type Tag =
+		| AST.AttachTag
+		| AST.ConstTag
+		| AST.DebugTag
+		| AST.ExpressionTag
+		| AST.HtmlTag
+		| AST.RenderTag;
 
 	export type TemplateNode =
 		| AST.Root
diff --git a/packages/svelte/src/internal/client/dom/elements/class.js b/packages/svelte/src/internal/client/dom/elements/class.js
index fc081b8956..038ce33f3e 100644
--- a/packages/svelte/src/internal/client/dom/elements/class.js
+++ b/packages/svelte/src/internal/client/dom/elements/class.js
@@ -24,7 +24,7 @@ export function set_class(dom, is_html, value, hash, prev_classes, next_classes)
 		if (!hydrating || next_class_name !== dom.getAttribute('class')) {
 			// Removing the attribute when the value is only an empty string causes
 			// performance issues vs simply making the className an empty string. So
-			// we should only remove the class if the the value is nullish
+			// we should only remove the class if the value is nullish
 			// and there no hash/directives :
 			if (next_class_name == null) {
 				dom.removeAttribute('class');
diff --git a/packages/svelte/src/reactivity/media-query.js b/packages/svelte/src/reactivity/media-query.js
index 22310df18d..d286709719 100644
--- a/packages/svelte/src/reactivity/media-query.js
+++ b/packages/svelte/src/reactivity/media-query.js
@@ -3,6 +3,19 @@ import { ReactiveValue } from './reactive-value.js';
 
 const parenthesis_regex = /\(.+\)/;
 
+// these keywords are valid media queries but they need to be without parenthesis
+//
+// eg: new MediaQuery('screen')
+//
+// however because of the auto-parenthesis logic in the constructor since there's no parentehesis
+// in the media query they'll be surrounded by parenthesis
+//
+// however we can check if the media query is only composed of these keywords
+// and skip the auto-parenthesis
+//
+// https://github.com/sveltejs/svelte/issues/15930
+const non_parenthesized_keywords = new Set(['all', 'print', 'screen', 'and', 'or', 'not', 'only']);
+
 /**
  * Creates a media query and provides a `current` property that reflects whether or not it matches.
  *
@@ -27,7 +40,12 @@ export class MediaQuery extends ReactiveValue {
 	 * @param {boolean} [fallback] Fallback value for the server
 	 */
 	constructor(query, fallback) {
-		let final_query = parenthesis_regex.test(query) ? query : `(${query})`;
+		let final_query =
+			parenthesis_regex.test(query) ||
+			// we need to use `some` here because technically this `window.matchMedia('random,screen')` still returns true
+			query.split(/[\s,]+/).some((keyword) => non_parenthesized_keywords.has(keyword.trim()))
+				? query
+				: `(${query})`;
 		const q = window.matchMedia(final_query);
 		super(
 			() => q.matches,
diff --git a/packages/svelte/src/utils.js b/packages/svelte/src/utils.js
index ada318e85a..921eaec57c 100644
--- a/packages/svelte/src/utils.js
+++ b/packages/svelte/src/utils.js
@@ -428,15 +428,19 @@ export function is_mathml(name) {
 	return MATHML_ELEMENTS.includes(name);
 }
 
-const RUNES = /** @type {const} */ ([
+export const STATE_CREATION_RUNES = /** @type {const} */ ([
 	'$state',
 	'$state.raw',
+	'$derived',
+	'$derived.by'
+]);
+
+const RUNES = /** @type {const} */ ([
+	...STATE_CREATION_RUNES,
 	'$state.snapshot',
 	'$props',
 	'$props.id',
 	'$bindable',
-	'$derived',
-	'$derived.by',
 	'$effect',
 	'$effect.pre',
 	'$effect.tracking',
@@ -447,12 +451,24 @@ const RUNES = /** @type {const} */ ([
 	'$host'
 ]);
 
+/** @typedef {RUNES[number]} RuneName */
+
 /**
  * @param {string} name
- * @returns {name is RUNES[number]}
+ * @returns {name is RuneName}
  */
 export function is_rune(name) {
-	return RUNES.includes(/** @type {RUNES[number]} */ (name));
+	return RUNES.includes(/** @type {RuneName} */ (name));
+}
+
+/** @typedef {STATE_CREATION_RUNES[number]} StateCreationRuneName */
+
+/**
+ * @param {string} name
+ * @returns {name is StateCreationRuneName}
+ */
+export function is_state_creation_rune(name) {
+	return STATE_CREATION_RUNES.includes(/** @type {StateCreationRuneName} */ (name));
 }
 
 /** List of elements that require raw contents and should not have SSR comments put in them */
diff --git a/packages/svelte/src/version.js b/packages/svelte/src/version.js
index b849318036..e42e6741bd 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 = '5.30.1';
+export const VERSION = '5.31.1';
 export const PUBLIC_VERSION = '5';
diff --git a/packages/svelte/tests/compiler-errors/samples/class-state-field-static/_config.js b/packages/svelte/tests/compiler-errors/samples/class-state-field-static/_config.js
index e6551031cc..f5b116117e 100644
--- a/packages/svelte/tests/compiler-errors/samples/class-state-field-static/_config.js
+++ b/packages/svelte/tests/compiler-errors/samples/class-state-field-static/_config.js
@@ -4,7 +4,7 @@ export default test({
 	error: {
 		code: 'state_invalid_placement',
 		message:
-			'`$state(...)` can only be used as a variable declaration initializer or a class field',
+			'`$state(...)` can only be used as a variable declaration initializer, a class field declaration, or the first assignment to a class field at the top level of the constructor.',
 		position: [33, 41]
 	}
 });
diff --git a/packages/svelte/tests/compiler-errors/samples/runes-no-rune-each/_config.js b/packages/svelte/tests/compiler-errors/samples/runes-no-rune-each/_config.js
index 1185ca6b45..592b2634a3 100644
--- a/packages/svelte/tests/compiler-errors/samples/runes-no-rune-each/_config.js
+++ b/packages/svelte/tests/compiler-errors/samples/runes-no-rune-each/_config.js
@@ -3,6 +3,7 @@ import { test } from '../../test';
 export default test({
 	error: {
 		code: 'state_invalid_placement',
-		message: '`$state(...)` can only be used as a variable declaration initializer or a class field'
+		message:
+			'`$state(...)` can only be used as a variable declaration initializer, a class field declaration, or the first assignment to a class field at the top level of the constructor.'
 	}
 });
diff --git a/packages/svelte/tests/compiler-errors/samples/runes-wrong-derived-placement/_config.js b/packages/svelte/tests/compiler-errors/samples/runes-wrong-derived-placement/_config.js
index 476d7239a3..f31748d26a 100644
--- a/packages/svelte/tests/compiler-errors/samples/runes-wrong-derived-placement/_config.js
+++ b/packages/svelte/tests/compiler-errors/samples/runes-wrong-derived-placement/_config.js
@@ -4,6 +4,6 @@ export default test({
 	error: {
 		code: 'state_invalid_placement',
 		message:
-			'`$derived(...)` can only be used as a variable declaration initializer or a class field'
+			'`$derived(...)` can only be used as a variable declaration initializer, a class field declaration, or the first assignment to a class field at the top level of the constructor.'
 	}
 });
diff --git a/packages/svelte/tests/compiler-errors/samples/runes-wrong-state-placement/_config.js b/packages/svelte/tests/compiler-errors/samples/runes-wrong-state-placement/_config.js
index 1185ca6b45..592b2634a3 100644
--- a/packages/svelte/tests/compiler-errors/samples/runes-wrong-state-placement/_config.js
+++ b/packages/svelte/tests/compiler-errors/samples/runes-wrong-state-placement/_config.js
@@ -3,6 +3,7 @@ import { test } from '../../test';
 export default test({
 	error: {
 		code: 'state_invalid_placement',
-		message: '`$state(...)` can only be used as a variable declaration initializer or a class field'
+		message:
+			'`$state(...)` can only be used as a variable declaration initializer, a class field declaration, or the first assignment to a class field at the top level of the constructor.'
 	}
 });
diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-constructor-closure-private/_config.js b/packages/svelte/tests/runtime-runes/samples/class-state-constructor-closure-private-1/_config.js
similarity index 100%
rename from packages/svelte/tests/runtime-runes/samples/class-state-constructor-closure-private/_config.js
rename to packages/svelte/tests/runtime-runes/samples/class-state-constructor-closure-private-1/_config.js
diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-constructor-closure-private/main.svelte b/packages/svelte/tests/runtime-runes/samples/class-state-constructor-closure-private-1/main.svelte
similarity index 100%
rename from packages/svelte/tests/runtime-runes/samples/class-state-constructor-closure-private/main.svelte
rename to packages/svelte/tests/runtime-runes/samples/class-state-constructor-closure-private-1/main.svelte
diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-constructor-closure-private-2/_config.js b/packages/svelte/tests/runtime-runes/samples/class-state-constructor-closure-private-2/_config.js
new file mode 100644
index 0000000000..dd847ce2f2
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/class-state-constructor-closure-private-2/_config.js
@@ -0,0 +1,13 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	html: `<button>10</button>`,
+	ssrHtml: `<button>0</button>`,
+
+	async test({ assert, target }) {
+		flushSync();
+
+		assert.htmlEqual(target.innerHTML, `<button>10</button>`);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-constructor-closure-private-2/main.svelte b/packages/svelte/tests/runtime-runes/samples/class-state-constructor-closure-private-2/main.svelte
new file mode 100644
index 0000000000..3d8ea41418
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/class-state-constructor-closure-private-2/main.svelte
@@ -0,0 +1,13 @@
+<script>
+	class Counter {
+		constructor() {
+			this.count = $state(0);
+			$effect(() => {
+				this.count = 10;
+			});
+		}
+	}
+	const counter = new Counter();
+</script>
+
+<button on:click={() => counter.count++}>{counter.count}</button>
diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-constructor-closure-private-3/_config.js b/packages/svelte/tests/runtime-runes/samples/class-state-constructor-closure-private-3/_config.js
new file mode 100644
index 0000000000..dd847ce2f2
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/class-state-constructor-closure-private-3/_config.js
@@ -0,0 +1,13 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	html: `<button>10</button>`,
+	ssrHtml: `<button>0</button>`,
+
+	async test({ assert, target }) {
+		flushSync();
+
+		assert.htmlEqual(target.innerHTML, `<button>10</button>`);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-constructor-closure-private-3/main.svelte b/packages/svelte/tests/runtime-runes/samples/class-state-constructor-closure-private-3/main.svelte
new file mode 100644
index 0000000000..47b8c901eb
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/class-state-constructor-closure-private-3/main.svelte
@@ -0,0 +1,12 @@
+<script>
+	const counter = new class Counter {
+		constructor() {
+			this.count = $state(0);
+			$effect(() => {
+				this.count = 10;
+			});
+		}
+	}
+</script>
+
+<button on:click={() => counter.count++}>{counter.count}</button>
diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-constructor-conflicting-get-name/_config.js b/packages/svelte/tests/runtime-runes/samples/class-state-constructor-conflicting-get-name/_config.js
new file mode 100644
index 0000000000..f47bee71df
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/class-state-constructor-conflicting-get-name/_config.js
@@ -0,0 +1,3 @@
+import { test } from '../../test';
+
+export default test({});
diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-constructor-conflicting-get-name/main.svelte b/packages/svelte/tests/runtime-runes/samples/class-state-constructor-conflicting-get-name/main.svelte
new file mode 100644
index 0000000000..e2c4f302b3
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/class-state-constructor-conflicting-get-name/main.svelte
@@ -0,0 +1,9 @@
+<script>
+	class Test {
+		0 = $state();
+
+		constructor() {
+			this[1] = $state();
+		}
+	}
+</script>
\ No newline at end of file
diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-constructor-derived-unowned/_config.js b/packages/svelte/tests/runtime-runes/samples/class-state-constructor-derived-unowned/_config.js
new file mode 100644
index 0000000000..4cf1aea213
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/class-state-constructor-derived-unowned/_config.js
@@ -0,0 +1,45 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	// The component context class instance gets shared between tests, strangely, causing hydration to fail?
+	mode: ['client', 'server'],
+
+	async test({ assert, target, logs }) {
+		const btn = target.querySelector('button');
+
+		flushSync(() => {
+			btn?.click();
+		});
+
+		assert.deepEqual(logs, [0, 'class trigger false', 'local trigger false', 1]);
+
+		flushSync(() => {
+			btn?.click();
+		});
+
+		assert.deepEqual(logs, [0, 'class trigger false', 'local trigger false', 1, 2]);
+
+		flushSync(() => {
+			btn?.click();
+		});
+
+		assert.deepEqual(logs, [0, 'class trigger false', 'local trigger false', 1, 2, 3]);
+
+		flushSync(() => {
+			btn?.click();
+		});
+
+		assert.deepEqual(logs, [
+			0,
+			'class trigger false',
+			'local trigger false',
+			1,
+			2,
+			3,
+			4,
+			'class trigger true',
+			'local trigger true'
+		]);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-constructor-derived-unowned/main.svelte b/packages/svelte/tests/runtime-runes/samples/class-state-constructor-derived-unowned/main.svelte
new file mode 100644
index 0000000000..03687d01bb
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/class-state-constructor-derived-unowned/main.svelte
@@ -0,0 +1,37 @@
+<script module>
+	class SomeLogic {
+		trigger() {
+			this.someValue++;
+		}
+
+		constructor() {
+			this.someValue = $state(0);
+			this.isAboveThree = $derived(this.someValue > 3);
+		}
+	}
+
+	const someLogic = new SomeLogic();
+</script>
+
+<script>
+	function increment() {
+		someLogic.trigger();
+	}
+
+	let localDerived = $derived(someLogic.someValue > 3);
+
+	$effect(() => {
+		console.log(someLogic.someValue);
+	});
+	$effect(() => {
+		console.log('class trigger ' + someLogic.isAboveThree)
+	});
+	$effect(() => {
+		console.log('local trigger ' + localDerived)
+	});
+
+</script>
+
+<button on:click={increment}>
+	clicks: {someLogic.someValue}
+</button>
diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-constructor-predeclared-field/_config.js b/packages/svelte/tests/runtime-runes/samples/class-state-constructor-predeclared-field/_config.js
new file mode 100644
index 0000000000..02cf36d900
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/class-state-constructor-predeclared-field/_config.js
@@ -0,0 +1,20 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	html: `<button>0</button>`,
+
+	test({ assert, target }) {
+		const btn = target.querySelector('button');
+
+		flushSync(() => {
+			btn?.click();
+		});
+		assert.htmlEqual(target.innerHTML, `<button>1</button>`);
+
+		flushSync(() => {
+			btn?.click();
+		});
+		assert.htmlEqual(target.innerHTML, `<button>2</button>`);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-constructor-predeclared-field/main.svelte b/packages/svelte/tests/runtime-runes/samples/class-state-constructor-predeclared-field/main.svelte
new file mode 100644
index 0000000000..5dbbb10afd
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/class-state-constructor-predeclared-field/main.svelte
@@ -0,0 +1,12 @@
+<script>
+	class Counter {
+		count;
+
+		constructor(count) {
+			this.count = $state(count);
+		}
+	}
+	const counter = new Counter(0);
+</script>
+
+<button onclick={() => counter.count++}>{counter.count}</button>
diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-constructor-subclass/_config.js b/packages/svelte/tests/runtime-runes/samples/class-state-constructor-subclass/_config.js
new file mode 100644
index 0000000000..32cca6c693
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/class-state-constructor-subclass/_config.js
@@ -0,0 +1,20 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	html: `<button>10: 20</button>`,
+
+	test({ assert, target }) {
+		const btn = target.querySelector('button');
+
+		flushSync(() => {
+			btn?.click();
+		});
+		assert.htmlEqual(target.innerHTML, `<button>11: 22</button>`);
+
+		flushSync(() => {
+			btn?.click();
+		});
+		assert.htmlEqual(target.innerHTML, `<button>12: 24</button>`);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-constructor-subclass/main.svelte b/packages/svelte/tests/runtime-runes/samples/class-state-constructor-subclass/main.svelte
new file mode 100644
index 0000000000..d8feb554cd
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/class-state-constructor-subclass/main.svelte
@@ -0,0 +1,22 @@
+<script>
+	class Counter {
+		constructor(initial) {
+			this.count = $state(initial);
+		}
+
+		increment = () => {
+			this.count++;
+		}
+	}
+
+	class PluggableCounter extends Counter {
+		constructor(initial, plugin) {
+			super(initial)
+			this.custom = $derived(plugin(this.count));
+		}
+	}
+
+	const counter = new PluggableCounter(10, (count) => count * 2);
+</script>
+
+<button onclick={counter.increment}>{counter.count}: {counter.custom}</button>
diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-constructor/_config.js b/packages/svelte/tests/runtime-runes/samples/class-state-constructor/_config.js
new file mode 100644
index 0000000000..f35dc57228
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/class-state-constructor/_config.js
@@ -0,0 +1,20 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	html: `<button>20</button>`,
+
+	test({ assert, target }) {
+		const btn = target.querySelector('button');
+
+		flushSync(() => {
+			btn?.click();
+		});
+		assert.htmlEqual(target.innerHTML, `<button>22</button>`);
+
+		flushSync(() => {
+			btn?.click();
+		});
+		assert.htmlEqual(target.innerHTML, `<button>24</button>`);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-constructor/main.svelte b/packages/svelte/tests/runtime-runes/samples/class-state-constructor/main.svelte
new file mode 100644
index 0000000000..aa8ba1658b
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/class-state-constructor/main.svelte
@@ -0,0 +1,18 @@
+<script>
+	class Counter {
+		/** @type {number} */
+		#count;
+
+		constructor(initial) {
+			this.#count = $state(initial);
+			this.doubled = $derived(this.#count * 2);
+		}
+
+		increment = () => {
+			this.#count++;
+		}
+	}
+	const counter = new Counter(10);
+</script>
+
+<button onclick={counter.increment}>{counter.doubled}</button>
diff --git a/packages/svelte/tests/runtime-runes/samples/media-query/_config.js b/packages/svelte/tests/runtime-runes/samples/media-query/_config.js
index f7a4ca05f5..d8b202955a 100644
--- a/packages/svelte/tests/runtime-runes/samples/media-query/_config.js
+++ b/packages/svelte/tests/runtime-runes/samples/media-query/_config.js
@@ -5,5 +5,10 @@ export default test({
 	async test({ window }) {
 		expect(window.matchMedia).toHaveBeenCalledWith('(max-width: 599px), (min-width: 900px)');
 		expect(window.matchMedia).toHaveBeenCalledWith('(min-width: 900px)');
+		expect(window.matchMedia).toHaveBeenCalledWith('screen');
+		expect(window.matchMedia).toHaveBeenCalledWith('not print');
+		expect(window.matchMedia).toHaveBeenCalledWith('screen,print');
+		expect(window.matchMedia).toHaveBeenCalledWith('screen,      print');
+		expect(window.matchMedia).toHaveBeenCalledWith('screen,      random');
 	}
 });
diff --git a/packages/svelte/tests/runtime-runes/samples/media-query/main.svelte b/packages/svelte/tests/runtime-runes/samples/media-query/main.svelte
index 446a9213dd..fe07ed8ab0 100644
--- a/packages/svelte/tests/runtime-runes/samples/media-query/main.svelte
+++ b/packages/svelte/tests/runtime-runes/samples/media-query/main.svelte
@@ -3,4 +3,9 @@
 
 	const mq = new MediaQuery("(max-width: 599px), (min-width: 900px)");
 	const mq2 = new MediaQuery("min-width: 900px");
+	const mq3 = new MediaQuery("screen");
+	const mq4 = new MediaQuery("not print");
+	const mq5 = new MediaQuery("screen,print");
+	const mq6 = new MediaQuery("screen,      print");
+	const mq7 = new MediaQuery("screen,      random");
 </script>
diff --git a/packages/svelte/tests/validator/samples/class-state-constructor-1/errors.json b/packages/svelte/tests/validator/samples/class-state-constructor-1/errors.json
new file mode 100644
index 0000000000..82765c51c1
--- /dev/null
+++ b/packages/svelte/tests/validator/samples/class-state-constructor-1/errors.json
@@ -0,0 +1,14 @@
+[
+	{
+		"code": "state_field_duplicate",
+		"message": "`count` has already been declared on this class",
+		"start": {
+			"line": 5,
+			"column": 2
+		},
+		"end": {
+			"line": 5,
+			"column": 24
+		}
+	}
+]
diff --git a/packages/svelte/tests/validator/samples/class-state-constructor-1/input.svelte.js b/packages/svelte/tests/validator/samples/class-state-constructor-1/input.svelte.js
new file mode 100644
index 0000000000..05cd4d9d9d
--- /dev/null
+++ b/packages/svelte/tests/validator/samples/class-state-constructor-1/input.svelte.js
@@ -0,0 +1,7 @@
+export class Counter {
+	count = $state(0);
+
+	constructor() {
+		this.count = $state(0);
+	}
+}
diff --git a/packages/svelte/tests/validator/samples/class-state-constructor-10/errors.json b/packages/svelte/tests/validator/samples/class-state-constructor-10/errors.json
new file mode 100644
index 0000000000..c4cb0991d0
--- /dev/null
+++ b/packages/svelte/tests/validator/samples/class-state-constructor-10/errors.json
@@ -0,0 +1,14 @@
+[
+	{
+		"code": "state_field_invalid_assignment",
+		"message": "Cannot assign to a state field before its declaration",
+		"start": {
+			"line": 4,
+			"column": 3
+		},
+		"end": {
+			"line": 4,
+			"column": 18
+		}
+	}
+]
diff --git a/packages/svelte/tests/validator/samples/class-state-constructor-10/input.svelte.js b/packages/svelte/tests/validator/samples/class-state-constructor-10/input.svelte.js
new file mode 100644
index 0000000000..e5ad562727
--- /dev/null
+++ b/packages/svelte/tests/validator/samples/class-state-constructor-10/input.svelte.js
@@ -0,0 +1,9 @@
+export class Counter {
+	constructor() {
+		if (true) {
+			this.count = -1;
+		}
+
+		this.count = $state(0);
+	}
+}
diff --git a/packages/svelte/tests/validator/samples/class-state-constructor-2/errors.json b/packages/svelte/tests/validator/samples/class-state-constructor-2/errors.json
new file mode 100644
index 0000000000..82765c51c1
--- /dev/null
+++ b/packages/svelte/tests/validator/samples/class-state-constructor-2/errors.json
@@ -0,0 +1,14 @@
+[
+	{
+		"code": "state_field_duplicate",
+		"message": "`count` has already been declared on this class",
+		"start": {
+			"line": 5,
+			"column": 2
+		},
+		"end": {
+			"line": 5,
+			"column": 24
+		}
+	}
+]
diff --git a/packages/svelte/tests/validator/samples/class-state-constructor-2/input.svelte.js b/packages/svelte/tests/validator/samples/class-state-constructor-2/input.svelte.js
new file mode 100644
index 0000000000..e37be4b3e6
--- /dev/null
+++ b/packages/svelte/tests/validator/samples/class-state-constructor-2/input.svelte.js
@@ -0,0 +1,7 @@
+export class Counter {
+	constructor() {
+		this.count = $state(0);
+		this.count = 1;
+		this.count = $state(0);
+	}
+}
diff --git a/packages/svelte/tests/validator/samples/class-state-constructor-3/errors.json b/packages/svelte/tests/validator/samples/class-state-constructor-3/errors.json
new file mode 100644
index 0000000000..175c41f98c
--- /dev/null
+++ b/packages/svelte/tests/validator/samples/class-state-constructor-3/errors.json
@@ -0,0 +1,14 @@
+[
+	{
+		"code": "state_field_duplicate",
+		"message": "`count` has already been declared on this class",
+		"start": {
+			"line": 5,
+			"column": 2
+		},
+		"end": {
+			"line": 5,
+			"column": 28
+		}
+	}
+]
diff --git a/packages/svelte/tests/validator/samples/class-state-constructor-3/input.svelte.js b/packages/svelte/tests/validator/samples/class-state-constructor-3/input.svelte.js
new file mode 100644
index 0000000000..f9196ff3cd
--- /dev/null
+++ b/packages/svelte/tests/validator/samples/class-state-constructor-3/input.svelte.js
@@ -0,0 +1,7 @@
+export class Counter {
+	constructor() {
+		this.count = $state(0);
+		this.count = 1;
+		this.count = $state.raw(0);
+	}
+}
diff --git a/packages/svelte/tests/validator/samples/class-state-constructor-4/errors.json b/packages/svelte/tests/validator/samples/class-state-constructor-4/errors.json
new file mode 100644
index 0000000000..9f959874c8
--- /dev/null
+++ b/packages/svelte/tests/validator/samples/class-state-constructor-4/errors.json
@@ -0,0 +1,14 @@
+[
+	{
+		"code": "state_invalid_placement",
+		"message": "`$state(...)` can only be used as a variable declaration initializer, a class field declaration, or the first assignment to a class field at the top level of the constructor.",
+		"start": {
+			"line": 4,
+			"column": 16
+		},
+		"end": {
+			"line": 4,
+			"column": 25
+		}
+	}
+]
diff --git a/packages/svelte/tests/validator/samples/class-state-constructor-4/input.svelte.js b/packages/svelte/tests/validator/samples/class-state-constructor-4/input.svelte.js
new file mode 100644
index 0000000000..bf1aada1b5
--- /dev/null
+++ b/packages/svelte/tests/validator/samples/class-state-constructor-4/input.svelte.js
@@ -0,0 +1,7 @@
+export class Counter {
+	constructor() {
+		if (true) {
+			this.count = $state(0);
+		}
+	}
+}
diff --git a/packages/svelte/tests/validator/samples/class-state-constructor-5/errors.json b/packages/svelte/tests/validator/samples/class-state-constructor-5/errors.json
new file mode 100644
index 0000000000..af2f30dade
--- /dev/null
+++ b/packages/svelte/tests/validator/samples/class-state-constructor-5/errors.json
@@ -0,0 +1,14 @@
+[
+	{
+		"code": "state_field_duplicate",
+		"message": "`count` has already been declared on this class",
+		"start": {
+			"line": 5,
+			"column": 2
+		},
+		"end": {
+			"line": 5,
+			"column": 27
+		}
+	}
+]
diff --git a/packages/svelte/tests/validator/samples/class-state-constructor-5/input.svelte.js b/packages/svelte/tests/validator/samples/class-state-constructor-5/input.svelte.js
new file mode 100644
index 0000000000..bc3d19a14f
--- /dev/null
+++ b/packages/svelte/tests/validator/samples/class-state-constructor-5/input.svelte.js
@@ -0,0 +1,7 @@
+export class Counter {
+	// prettier-ignore
+	'count' = $state(0);
+	constructor() {
+		this['count'] = $state(0);
+	}
+}
diff --git a/packages/svelte/tests/validator/samples/class-state-constructor-6/errors.json b/packages/svelte/tests/validator/samples/class-state-constructor-6/errors.json
new file mode 100644
index 0000000000..ae7a47f31b
--- /dev/null
+++ b/packages/svelte/tests/validator/samples/class-state-constructor-6/errors.json
@@ -0,0 +1,14 @@
+[
+	{
+		"code": "state_field_duplicate",
+		"message": "`count` has already been declared on this class",
+		"start": {
+			"line": 4,
+			"column": 2
+		},
+		"end": {
+			"line": 4,
+			"column": 27
+		}
+	}
+]
diff --git a/packages/svelte/tests/validator/samples/class-state-constructor-6/input.svelte.js b/packages/svelte/tests/validator/samples/class-state-constructor-6/input.svelte.js
new file mode 100644
index 0000000000..2ebe52e685
--- /dev/null
+++ b/packages/svelte/tests/validator/samples/class-state-constructor-6/input.svelte.js
@@ -0,0 +1,6 @@
+export class Counter {
+	count = $state(0);
+	constructor() {
+		this['count'] = $state(0);
+	}
+}
diff --git a/packages/svelte/tests/validator/samples/class-state-constructor-7/errors.json b/packages/svelte/tests/validator/samples/class-state-constructor-7/errors.json
new file mode 100644
index 0000000000..64e56f8d5c
--- /dev/null
+++ b/packages/svelte/tests/validator/samples/class-state-constructor-7/errors.json
@@ -0,0 +1,14 @@
+[
+	{
+		"code": "state_invalid_placement",
+		"message": "`$state(...)` can only be used as a variable declaration initializer, a class field declaration, or the first assignment to a class field at the top level of the constructor.",
+		"start": {
+			"line": 5,
+			"column": 16
+		},
+		"end": {
+			"line": 5,
+			"column": 25
+		}
+	}
+]
diff --git a/packages/svelte/tests/validator/samples/class-state-constructor-7/input.svelte.js b/packages/svelte/tests/validator/samples/class-state-constructor-7/input.svelte.js
new file mode 100644
index 0000000000..50c8559837
--- /dev/null
+++ b/packages/svelte/tests/validator/samples/class-state-constructor-7/input.svelte.js
@@ -0,0 +1,7 @@
+const count = 'count';
+
+export class Counter {
+	constructor() {
+		this[count] = $state(0);
+	}
+}
diff --git a/packages/svelte/tests/validator/samples/class-state-constructor-8/errors.json b/packages/svelte/tests/validator/samples/class-state-constructor-8/errors.json
new file mode 100644
index 0000000000..2e0bd10ff8
--- /dev/null
+++ b/packages/svelte/tests/validator/samples/class-state-constructor-8/errors.json
@@ -0,0 +1,14 @@
+[
+	{
+		"code": "state_field_invalid_assignment",
+		"message": "Cannot assign to a state field before its declaration",
+		"start": {
+			"line": 3,
+			"column": 2
+		},
+		"end": {
+			"line": 3,
+			"column": 17
+		}
+	}
+]
diff --git a/packages/svelte/tests/validator/samples/class-state-constructor-8/input.svelte.js b/packages/svelte/tests/validator/samples/class-state-constructor-8/input.svelte.js
new file mode 100644
index 0000000000..0a76c6fec9
--- /dev/null
+++ b/packages/svelte/tests/validator/samples/class-state-constructor-8/input.svelte.js
@@ -0,0 +1,6 @@
+export class Counter {
+	constructor() {
+		this.count = -1;
+		this.count = $state(0);
+	}
+}
diff --git a/packages/svelte/tests/validator/samples/class-state-constructor-9/errors.json b/packages/svelte/tests/validator/samples/class-state-constructor-9/errors.json
new file mode 100644
index 0000000000..b7dd4c8ed4
--- /dev/null
+++ b/packages/svelte/tests/validator/samples/class-state-constructor-9/errors.json
@@ -0,0 +1,14 @@
+[
+	{
+		"code": "state_field_invalid_assignment",
+		"message": "Cannot assign to a state field before its declaration",
+		"start": {
+			"line": 2,
+			"column": 1
+		},
+		"end": {
+			"line": 2,
+			"column": 12
+		}
+	}
+]
diff --git a/packages/svelte/tests/validator/samples/class-state-constructor-9/input.svelte.js b/packages/svelte/tests/validator/samples/class-state-constructor-9/input.svelte.js
new file mode 100644
index 0000000000..a8469e13af
--- /dev/null
+++ b/packages/svelte/tests/validator/samples/class-state-constructor-9/input.svelte.js
@@ -0,0 +1,7 @@
+export class Counter {
+	count = -1;
+
+	constructor() {
+		this.count = $state(0);
+	}
+}
diff --git a/packages/svelte/tests/validator/samples/const-tag-invalid-rune-usage/errors.json b/packages/svelte/tests/validator/samples/const-tag-invalid-rune-usage/errors.json
index 32594e4268..e1906b181a 100644
--- a/packages/svelte/tests/validator/samples/const-tag-invalid-rune-usage/errors.json
+++ b/packages/svelte/tests/validator/samples/const-tag-invalid-rune-usage/errors.json
@@ -1,7 +1,7 @@
 [
 	{
 		"code": "state_invalid_placement",
-		"message": "`$derived(...)` can only be used as a variable declaration initializer or a class field",
+		"message": "`$derived(...)` can only be used as a variable declaration initializer, a class field declaration, or the first assignment to a class field at the top level of the constructor.",
 		"start": {
 			"line": 2,
 			"column": 15
diff --git a/packages/svelte/types/index.d.ts b/packages/svelte/types/index.d.ts
index 5520ccabe9..fa0d701422 100644
--- a/packages/svelte/types/index.d.ts
+++ b/packages/svelte/types/index.d.ts
@@ -1420,7 +1420,13 @@ declare module 'svelte/compiler' {
 			| AST.SvelteWindow
 			| AST.SvelteBoundary;
 
-		export type Tag = AST.ExpressionTag | AST.HtmlTag | AST.ConstTag | AST.DebugTag | AST.RenderTag;
+		export type Tag =
+			| AST.AttachTag
+			| AST.ConstTag
+			| AST.DebugTag
+			| AST.ExpressionTag
+			| AST.HtmlTag
+			| AST.RenderTag;
 
 		export type TemplateNode =
 			| AST.Root