diff --git a/src/test/setup.ts b/src/test/setup.ts
index e9083a3..b366e2a 100644
--- a/src/test/setup.ts
+++ b/src/test/setup.ts
@@ -1,3 +1,43 @@
+// jsdom does not reliably expose `localStorage` across environments (e.g. on
+// Node 26 its experimental built-in `localStorage` shadows the jsdom one, so
+// `localStorage` is undefined in tests). Provide a minimal in-memory shim so
+// tests that rely on `localStorage` run consistently regardless of the runtime.
+if (typeof globalThis.localStorage === 'undefined') {
+  class MemoryStorage {
+    private store = new Map<string, string>();
+
+    get length(): number {
+      return this.store.size;
+    }
+
+    key(index: number): string | null {
+      return Array.from(this.store.keys())[index] ?? null;
+    }
+
+    getItem(key: string): string | null {
+      return this.store.get(key) ?? null;
+    }
+
+    setItem(key: string, value: string): void {
+      this.store.set(key, String(value));
+    }
+
+    removeItem(key: string): void {
+      this.store.delete(key);
+    }
+
+    clear(): void {
+      this.store.clear();
+    }
+  }
+
+  const storage = new MemoryStorage();
+  globalThis.localStorage = storage as unknown as Storage;
+  if (typeof window !== 'undefined') {
+    window.localStorage = storage as unknown as Storage;
+  }
+}
+
 beforeEach(() => {
   if (typeof localStorage !== 'undefined') {
     localStorage.clear();