diff --git a/apps/svelte.dev/content/docs/svelte/02-runes/02-$state.md b/apps/svelte.dev/content/docs/svelte/02-runes/02-$state.md
index 9c849659d6..b3a2d0a3fe 100644
--- a/apps/svelte.dev/content/docs/svelte/02-runes/02-$state.md
+++ b/apps/svelte.dev/content/docs/svelte/02-runes/02-$state.md
@@ -68,16 +68,15 @@ 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() {
@@ -111,10 +110,9 @@ 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/apps/svelte.dev/content/docs/svelte/03-template-syntax/09-@attach.md b/apps/svelte.dev/content/docs/svelte/03-template-syntax/09-@attach.md
index 4db1d043c2..f342945a8a 100644
--- a/apps/svelte.dev/content/docs/svelte/03-template-syntax/09-@attach.md
+++ b/apps/svelte.dev/content/docs/svelte/03-template-syntax/09-@attach.md
@@ -3,7 +3,9 @@ NOTE: do not edit this file, it is generated in apps/svelte.dev/scripts/sync-doc
 title: {@attach ...}
 ---
 
-Attachments are functions that run when an element is mounted to the DOM. Optionally, they can return a function that is called when the element is later removed from the DOM.
+Attachments are functions that run in an [effect]($effect) when an element is mounted to the DOM or when [state]($state) read inside the function updates.
+
+Optionally, they can return a function that is called before the attachment re-runs, or after the element is later removed from the DOM.
 
 > [!NOTE]
 > Attachments are available in Svelte 5.29 and newer.
@@ -56,7 +58,7 @@ A useful pattern is for a function, such as `tooltip` in this example, to _retur
 </button>
 ```
 
-Since the `tooltip(content)` expression runs inside an [effect]($effect), the attachment will be destroyed and recreated whenever `content` changes.
+Since the `tooltip(content)` expression runs inside an [effect]($effect), the attachment will be destroyed and recreated whenever `content` changes. The same thing would happen for any state read _inside_ the attachment function when it first runs. (If this isn't what you want, see [Controlling when attachments re-run](#Controlling-when-attachments-re-run).)
 
 ## Inline attachments
 
@@ -127,6 +129,35 @@ This allows you to create _wrapper components_ that augment elements ([demo](/pl
 </Button>
 ```
 
+## Controlling when attachments re-run
+
+Attachments, unlike [actions](use), are fully reactive: `{@attach foo(bar)}` will re-run on changes to `foo` _or_ `bar` (or any state read inside `foo`):
+
+```js
+// @errors: 7006 2304 2552
+function foo(bar) {
+	return (node) => {
+		veryExpensiveSetupWork(node);
+		update(node, bar);
+	};
+}
+```
+
+In the rare case that this is a problem (for example, if `foo` does expensive and unavoidable setup work) consider passing the data inside a function and reading it in a child effect:
+
+```js
+// @errors: 7006 2304 2552
+function foo(+++getBar+++) {
+	return (node) => {
+		veryExpensiveSetupWork(node);
+
++++		$effect(() => {
+			update(node, getBar());
+		});+++
+	}
+}
+```
+
 ## Creating attachments programmatically
 
 To add attachments to an object that will be spread onto a component or element, use [`createAttachmentKey`](svelte-attachments#createAttachmentKey).
diff --git a/apps/svelte.dev/content/docs/svelte/98-reference/.generated/compile-errors.md b/apps/svelte.dev/content/docs/svelte/98-reference/.generated/compile-errors.md
index e8669ead53..db848a0299 100644
--- a/apps/svelte.dev/content/docs/svelte/98-reference/.generated/compile-errors.md
+++ b/apps/svelte.dev/content/docs/svelte/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/apps/svelte.dev/content/docs/svelte/98-reference/30-compiler-errors.md b/apps/svelte.dev/content/docs/svelte/98-reference/30-compiler-errors.md
index a2ebf53216..143137c77e 100644
--- a/apps/svelte.dev/content/docs/svelte/98-reference/30-compiler-errors.md
+++ b/apps/svelte.dev/content/docs/svelte/98-reference/30-compiler-errors.md
@@ -851,6 +851,38 @@ 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
 
 ```
@@ -860,7 +892,7 @@ 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