diff --git a/packages/script/src/runtime/registry/google-tag-manager.ts b/packages/script/src/runtime/registry/google-tag-manager.ts
index 98a6a967..81a16c84 100644
--- a/packages/script/src/runtime/registry/google-tag-manager.ts
+++ b/packages/script/src/runtime/registry/google-tag-manager.ts
@@ -72,9 +72,17 @@ export interface GoogleTagManagerApi {
 
 /**
  * Enhanced window type with GTM
+ *
+ * `dataLayer` is deliberately NOT declared globally. Its name is configurable through the
+ * `l` / `dataLayer` options, so `window.dataLayer` is not guaranteed to exist, and declaring it
+ * here makes this package irreconcilable with any other package that declares `Window.dataLayer`
+ * (for example `@gtm-support/core`, used by `@gtm-support/vue-gtm`): the two declarations cannot
+ * merge, so consumers of both get an unsuppressable TS2430 at every one of their own `Window`
+ * augmentations. Read it through the typed proxy returned by `useScriptGoogleTagManager()`
+ * instead, which is also the only access path that respects a custom dataLayer name.
  */
 declare global {
-  interface Window extends GoogleTagManagerApi {}
+  interface Window extends Pick<GoogleTagManagerApi, 'google_tag_manager'> {}
 }
 
 export { GoogleTagManagerOptions }
diff --git a/test/types/global-window.test-d.ts b/test/types/global-window.test-d.ts
new file mode 100644
index 00000000..c940e36f
--- /dev/null
+++ b/test/types/global-window.test-d.ts
@@ -0,0 +1,31 @@
+import type { GoogleTagManagerApi } from '../../packages/script/src/runtime/registry/google-tag-manager'
+import { describe, expectTypeOf, it } from 'vitest'
+
+/**
+ * Regression guard for #852.
+ *
+ * `@gtm-support/core` (the engine behind `@gtm-support/vue-gtm`) declares
+ * `Window.dataLayer?: DataLayerObject[]`. Interface declarations merge, so if this package
+ * also declares `dataLayer` on the global `Window`, the merged `Window` fails this package's
+ * own `extends` clause. TypeScript then reports TS2430 at every one of the consumer's own
+ * `Window` augmentations, which the consumer cannot suppress.
+ *
+ * `dataLayer` must stay off the global `Window`. Its name is configurable through the
+ * `l` / `dataLayer` options anyway, so `window.dataLayer` was never guaranteed to exist.
+ * The proxy returned by `useScriptGoogleTagManager()` is the supported access path.
+ */
+describe('global `Window` augmentation', () => {
+  it('does not declare `dataLayer`', () => {
+    expectTypeOf<'dataLayer' extends keyof Window ? true : false>().toEqualTypeOf<false>()
+  })
+
+  it('keeps `google_tag_manager` declared', () => {
+    // `useScriptGoogleTagManager`'s `use()` reads `window.google_tag_manager` directly.
+    expectTypeOf<Window['google_tag_manager']>().toEqualTypeOf<GoogleTagManagerApi['google_tag_manager']>()
+  })
+
+  it('still types `dataLayer` on the GTM proxy API', () => {
+    expectTypeOf<GoogleTagManagerApi['dataLayer']>().not.toBeAny()
+    expectTypeOf<GoogleTagManagerApi['dataLayer']['push']>().toBeCallableWith({ event: 'test' })
+  })
+})