diff --git a/documentation/docs/98-reference/.generated/compile-warnings.md b/documentation/docs/98-reference/.generated/compile-warnings.md
index b049a40fb9..cc948e2547 100644
--- a/documentation/docs/98-reference/.generated/compile-warnings.md
+++ b/documentation/docs/98-reference/.generated/compile-warnings.md
@@ -654,6 +654,12 @@ HTML restricts where certain elements can appear. In case of a violation the bro
 
 This code will work when the component is rendered on the client (which is why this is a warning rather than an error), but if you use server rendering it will cause hydration to fail.
 
+### non_reactive_update
+
+```
+`%name%` is updated, but is not declared with `$state(...)`. Changing its value will not correctly trigger updates
+```
+
 ### options_deprecated_accessors
 
 ```
diff --git a/packages/svelte/messages/compile-warnings/script.md b/packages/svelte/messages/compile-warnings/script.md
index 20c535bd23..2c891b4fc7 100644
--- a/packages/svelte/messages/compile-warnings/script.md
+++ b/packages/svelte/messages/compile-warnings/script.md
@@ -6,6 +6,10 @@
 
 > Svelte 5 components are no longer classes. Instantiate them using `mount` or `hydrate` (imported from 'svelte') instead.
 
+## non_reactive_update
+
+> `%name%` is updated, but is not declared with `$state(...)`. Changing its value will not correctly trigger updates
+
 ## perf_avoid_inline_class
 
 > Avoid 'new class' — instead, declare the class at the top level scope
diff --git a/packages/svelte/src/compiler/warnings.js b/packages/svelte/src/compiler/warnings.js
index 78ab2aeb3e..f6ed7de7d5 100644
--- a/packages/svelte/src/compiler/warnings.js
+++ b/packages/svelte/src/compiler/warnings.js
@@ -97,6 +97,7 @@ export const codes = [
 	"options_renamed_ssr_dom",
 	"export_let_unused",
 	"legacy_component_creation",
+	"non_reactive_update",
 	"perf_avoid_inline_class",
 	"perf_avoid_nested_class",
 	"reactive_declaration_invalid_placement",
@@ -598,6 +599,15 @@ export function legacy_component_creation(node) {
 	w(node, "legacy_component_creation", "Svelte 5 components are no longer classes. Instantiate them using `mount` or `hydrate` (imported from 'svelte') instead.");
 }
 
+/**
+ * `%name%` is updated, but is not declared with `$state(...)`. Changing its value will not correctly trigger updates
+ * @param {null | NodeLike} node
+ * @param {string} name
+ */
+export function non_reactive_update(node, name) {
+	w(node, "non_reactive_update", `\`${name}\` is updated, but is not declared with \`$state(...)\`. Changing its value will not correctly trigger updates`);
+}
+
 /**
  * Avoid 'new class' — instead, declare the class at the top level scope
  * @param {null | NodeLike} node