diff --git a/.changeset/funny-carrots-teach.md b/.changeset/funny-carrots-teach.md
new file mode 100644
index 000000000000..53ff135e84da
--- /dev/null
+++ b/.changeset/funny-carrots-teach.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: Add missing `AttachTag` in `Tag` union type inside the `AST` namespace from `"svelte/compiler"`
diff --git a/packages/svelte/src/compiler/types/template.d.ts b/packages/svelte/src/compiler/types/template.d.ts
index 6dec1f2dbe15..b51c9e9a8dc6 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/types/index.d.ts b/packages/svelte/types/index.d.ts
index bb958c510807..1fda9a36b8bb 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