diff --git a/.changeset/sweet-islands-sell.md b/.changeset/sweet-islands-sell.md
new file mode 100644
index 0000000000..b4fbbf182c
--- /dev/null
+++ b/.changeset/sweet-islands-sell.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: avoid auto-parenthesis for special-keywords-only `MediaQuery`
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/01-basic-markup.md b/documentation/docs/03-template-syntax/01-basic-markup.md
index fe5f8b02aa..feecfe033e 100644
--- a/documentation/docs/03-template-syntax/01-basic-markup.md
+++ b/documentation/docs/03-template-syntax/01-basic-markup.md
@@ -82,12 +82,14 @@ As with elements, `name={name}` can be replaced with the `{name}` shorthand.
 <Widget foo={bar} answer={42} text="hello" />
 ```
 
+## Spread attributes
+
 _Spread attributes_ allow many attributes or properties to be passed to an element or component at once.
 
-An element or component can have multiple spread attributes, interspersed with regular ones.
+An element or component can have multiple spread attributes, interspersed with regular ones. Order matters — if `things.a` exists it will take precedence over `a="b"`, while `c="d"` would take precedence over `things.c`:
 
 ```svelte
-<Widget {...things} />
+<Widget a="b" {...things} c="d" />
 ```
 
 ## Events
diff --git a/documentation/docs/03-template-syntax/03-each.md b/documentation/docs/03-template-syntax/03-each.md
index 70666f6a57..006cadd152 100644
--- a/documentation/docs/03-template-syntax/03-each.md
+++ b/documentation/docs/03-template-syntax/03-each.md
@@ -43,7 +43,9 @@ An each block can also specify an _index_, equivalent to the second argument in
 {#each expression as name, index (key)}...{/each}
 ```
 
-If a _key_ expression is provided — which must uniquely identify each list item — Svelte will use it to diff the list when data changes, rather than adding or removing items at the end. The key can be any object, but strings and numbers are recommended since they allow identity to persist when the objects themselves change.
+If a _key_ expression is provided — which must uniquely identify each list item — Svelte will use it to intelligently update the list when data changes by inserting, moving and deleting items, rather than adding or removing items at the end and updating the state in the middle.
+
+The key can be any object, but strings and numbers are recommended since they allow identity to persist when the objects themselves change.
 
 ```svelte
 {#each items as item (item.id)}
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..d40ff09317 100644
--- a/packages/svelte/CHANGELOG.md
+++ b/packages/svelte/CHANGELOG.md
@@ -1,5 +1,23 @@
 # svelte
 
+## 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..e0f6afe6fc 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": "5.31.0",
   "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/blocks/each.js b/packages/svelte/src/internal/client/dom/blocks/each.js
index 92c953b541..2997664fa2 100644
--- a/packages/svelte/src/internal/client/dom/blocks/each.js
+++ b/packages/svelte/src/internal/client/dom/blocks/each.js
@@ -12,6 +12,7 @@ import {
 	hydrate_next,
 	hydrate_node,
 	hydrating,
+	read_hydration_instruction,
 	remove_nodes,
 	set_hydrate_node,
 	set_hydrating
@@ -160,7 +161,7 @@ export function each(node, flags, get_collection, get_key, render_fn, fallback_f
 		let mismatch = false;
 
 		if (hydrating) {
-			var is_else = /** @type {Comment} */ (anchor).data === HYDRATION_START_ELSE;
+			var is_else = read_hydration_instruction(anchor) === HYDRATION_START_ELSE;
 
 			if (is_else !== (length === 0)) {
 				// hydration mismatch — remove the server-rendered DOM and start over
diff --git a/packages/svelte/src/internal/client/dom/blocks/if.js b/packages/svelte/src/internal/client/dom/blocks/if.js
index 925abb9d9d..bf1098c3f4 100644
--- a/packages/svelte/src/internal/client/dom/blocks/if.js
+++ b/packages/svelte/src/internal/client/dom/blocks/if.js
@@ -4,6 +4,7 @@ import {
 	hydrate_next,
 	hydrate_node,
 	hydrating,
+	read_hydration_instruction,
 	remove_nodes,
 	set_hydrate_node,
 	set_hydrating
@@ -56,7 +57,8 @@ export function if_block(node, fn, [root_index, hydrate_index] = [0, 0]) {
 
 		if (hydrating && hydrate_index !== -1) {
 			if (root_index === 0) {
-				const data = /** @type {Comment} */ (anchor).data;
+				const data = read_hydration_instruction(anchor);
+
 				if (data === HYDRATION_START) {
 					hydrate_index = 0;
 				} else if (data === HYDRATION_START_ELSE) {
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/internal/client/dom/hydration.js b/packages/svelte/src/internal/client/dom/hydration.js
index 8523ff97d5..ab3256da82 100644
--- a/packages/svelte/src/internal/client/dom/hydration.js
+++ b/packages/svelte/src/internal/client/dom/hydration.js
@@ -103,3 +103,16 @@ export function remove_nodes() {
 		node = next;
 	}
 }
+
+/**
+ *
+ * @param {TemplateNode} node
+ */
+export function read_hydration_instruction(node) {
+	if (!node || node.nodeType !== 8) {
+		w.hydration_mismatch();
+		throw HYDRATION_ERROR;
+	}
+
+	return /** @type {Comment} */ (node).data;
+}
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..ed99ef6795 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 = '5.31.0';
 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/hydration/samples/cloudflare-mirage-borking-2/_config.js b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/_config.js
new file mode 100644
index 0000000000..56ba73b064
--- /dev/null
+++ b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/_config.js
@@ -0,0 +1,6 @@
+import { test } from '../../test';
+
+// https://github.com/sveltejs/svelte/issues/15819
+export default test({
+	expect_hydration_error: true
+});
diff --git a/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/_expected.html b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/_expected.html
new file mode 100644
index 0000000000..5179fb04a5
--- /dev/null
+++ b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/_expected.html
@@ -0,0 +1 @@
+<p>start</p><p>cond</p><!---->
diff --git a/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/_override.html b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/_override.html
new file mode 100644
index 0000000000..2a1c323288
--- /dev/null
+++ b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/_override.html
@@ -0,0 +1 @@
+<!--[--> <p>start</p><!--[--><p>cond</p><!--]--><!--]-->
diff --git a/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/main.svelte b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/main.svelte
new file mode 100644
index 0000000000..bfb4f2cdb8
--- /dev/null
+++ b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking-2/main.svelte
@@ -0,0 +1,5 @@
+<script>
+	const cond = true;
+</script>
+
+<p>start</p>{#if cond}<p>cond</p>{/if}
diff --git a/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/_config.js b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/_config.js
new file mode 100644
index 0000000000..56ba73b064
--- /dev/null
+++ b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/_config.js
@@ -0,0 +1,6 @@
+import { test } from '../../test';
+
+// https://github.com/sveltejs/svelte/issues/15819
+export default test({
+	expect_hydration_error: true
+});
diff --git a/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/_expected.html b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/_expected.html
new file mode 100644
index 0000000000..f6c03b87c1
--- /dev/null
+++ b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/_expected.html
@@ -0,0 +1 @@
+<p>start</p> pre123 mid<!---->
diff --git a/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/_override.html b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/_override.html
new file mode 100644
index 0000000000..c84efbb00b
--- /dev/null
+++ b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/_override.html
@@ -0,0 +1 @@
+<!--[--><p>start</p>pre<a>123</a> <!--[-->mid<!--]--><!--]-->
diff --git a/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/main.svelte b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/main.svelte
new file mode 100644
index 0000000000..2c9a94686e
--- /dev/null
+++ b/packages/svelte/tests/hydration/samples/cloudflare-mirage-borking/main.svelte
@@ -0,0 +1,9 @@
+<script>
+	let cond = true;
+</script>
+
+<p>start</p>
+pre123
+{#if cond}
+mid
+{/if}
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 bb958c5108..1fda9a36b8 100644
--- a/packages/svelte/types/index.d.ts
+++ b/packages/svelte/types/index.d.ts
@@ -1362,7 +1362,13 @@ 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