diff --git a/Client/tauri-client/tests/setup.ts b/Client/tauri-client/tests/setup.ts
new file mode 100644
index 00000000..f3ca64db
--- /dev/null
+++ b/Client/tauri-client/tests/setup.ts
@@ -0,0 +1,54 @@
+// Vitest global setup for the jsdom suite.
+//
+// Why this exists: Node 26 defines `localStorage` as a native global accessor
+// that returns `undefined` unless the process was started with
+// `--localstorage-file`. Vitest's jsdom environment makes `window === globalThis`,
+// so that native accessor shadows the one jsdom would otherwise install, and
+// every test touching `localStorage` throws. `sessionStorage` is unaffected —
+// jsdom's implementation still wins there.
+//
+// We deliberately do NOT pass `--localstorage-file`: it is file-backed and would
+// be shared by vitest's parallel workers, leaking state between test files.
+
+/**
+ * Minimal in-memory Web Storage implementation used to replace the shadowed
+ * `localStorage` global.
+ *
+ * Matches the spec on the three points app code depends on: `getItem` returns
+ * `null` (not `undefined`) for absent keys, `setItem` coerces values to strings,
+ * and `key(i)` enumerates in insertion order — src/lib/themes.ts:30 and
+ * src/components/settings/AdvancedTab.ts:351 both sweep storage via `key(i)`
+ * over `length`, so ordering is load-bearing.
+ */
+function createMemoryStorage(): Storage {
+  const store = new Map<string, string>();
+
+  return {
+    get length(): number {
+      return store.size;
+    },
+    key(index: number): string | null {
+      return [...store.keys()][index] ?? null;
+    },
+    getItem(key: string): string | null {
+      return store.get(String(key)) ?? null;
+    },
+    setItem(key: string, value: string): void {
+      store.set(String(key), String(value));
+    },
+    removeItem(key: string): void {
+      store.delete(String(key));
+    },
+    clear(): void {
+      store.clear();
+    },
+  } satisfies Storage;
+}
+
+if (typeof globalThis.localStorage === "undefined") {
+  Object.defineProperty(globalThis, "localStorage", {
+    value: createMemoryStorage(),
+    configurable: true,
+    writable: true,
+  });
+}
diff --git a/Client/tauri-client/vitest.config.ts b/Client/tauri-client/vitest.config.ts
index 02016d66..33cc4a22 100644
--- a/Client/tauri-client/vitest.config.ts
+++ b/Client/tauri-client/vitest.config.ts
@@ -13,6 +13,8 @@ export default defineConfig({
   },
   test: {
     environment: "jsdom",
+    // Restores `localStorage`, which Node 26 shadows out of the jsdom global.
+    setupFiles: ["./tests/setup.ts"],
     // Both the `tests/**/*.test.ts` suite and component-local
     // `src/**/*.test.ts` files are picked up.
     include: ["tests/**/*.test.ts", "src/**/*.test.ts"],