diff --git a/tests/unit/setup.js b/tests/unit/setup.js
index 237eccb5..98f6e0c4 100644
--- a/tests/unit/setup.js
+++ b/tests/unit/setup.js
@@ -1,7 +1,9 @@
 import { afterEach, beforeEach, vi } from "vitest";
-import { enableAutoUnmount } from "@vue/test-utils";
 
-enableAutoUnmount(afterEach);
+// NOTE: do NOT call enableAutoUnmount here. Spec files that mount Vue
+// components already invoke it themselves (and some also import it directly,
+// which makes a second registration throw). We only provide the polyfills
+// for storage, observers, and URL.createObjectURL below.
 
 class TestResizeObserver {
   observe() {}
@@ -33,6 +35,59 @@ function createMatchMedia(query) {
   };
 }
 
+class InMemoryStorage {
+  constructor() {
+    this._map = new Map();
+  }
+  get length() {
+    return this._map.size;
+  }
+  key(index) {
+    return Array.from(this._map.keys())[index] ?? null;
+  }
+  getItem(key) {
+    return this._map.has(key) ? this._map.get(key) : null;
+  }
+  setItem(key, value) {
+    this._map.set(String(key), String(value));
+  }
+  removeItem(key) {
+    this._map.delete(key);
+  }
+  clear() {
+    this._map.clear();
+  }
+}
+
+const rootStorage = new InMemoryStorage();
+
+// Polyfill localStorage/sessionStorage at module-load time so component
+// modules that read `localStorage` during their own evaluation
+// (e.g. SessionUser.vue) work in jsdom 29 + vitest 4 environments that do not
+// provide one. Only attach when there is no real implementation already.
+if (!globalThis.localStorage || typeof globalThis.localStorage.setItem !== "function") {
+  try {
+    Object.defineProperty(globalThis, "localStorage", {
+      configurable: true,
+      writable: true,
+      value: rootStorage,
+    });
+  } catch {
+    // Some envs lock the global — best effort.
+  }
+}
+if (!globalThis.sessionStorage || typeof globalThis.sessionStorage.setItem !== "function") {
+  try {
+    Object.defineProperty(globalThis, "sessionStorage", {
+      configurable: true,
+      writable: true,
+      value: rootStorage,
+    });
+  } catch {
+    // best effort
+  }
+}
+
 function ensureBrowserMocks() {
   if (!globalThis.ResizeObserver) {
     globalThis.ResizeObserver = TestResizeObserver;
@@ -58,6 +113,30 @@ function ensureBrowserMocks() {
         value() {},
       });
     }
+
+    // jsdom 29 + vitest 4 in this environment ships sessionStorage but not
+    // localStorage. Polyfill a tiny in-memory implementation so tests that
+    // touch `window.localStorage` (auth tokens, cached selectors, etc.) keep
+    // working under jsdom.
+    try {
+      if (!window.localStorage || typeof window.localStorage.setItem !== "function") {
+        Object.defineProperty(window, "localStorage", {
+          configurable: true,
+          writable: true,
+          value: new InMemoryStorage(),
+        });
+      }
+    } catch {
+      // Some envs lock localStorage — leave it untouched.
+    }
+
+    if (!window.sessionStorage || typeof window.sessionStorage.setItem !== "function") {
+      Object.defineProperty(window, "sessionStorage", {
+        configurable: true,
+        writable: true,
+        value: new InMemoryStorage(),
+      });
+    }
   }
 
   if (globalThis.URL) {
diff --git a/vitest.config.js b/vitest.config.js
index 5f3fed74..f74359a7 100644
--- a/vitest.config.js
+++ b/vitest.config.js
@@ -20,8 +20,16 @@ export default defineConfig({
     },
   },
   test: {
+    // Default to node so the many source-reading specs keep working. Specs
+    // that mount Vue components opt into jsdom via a file-level
+    // `// @vitest-environment jsdom` pragma, which still gets the polyfills
+    // below because the env config keys match.
     environment: "node",
+    environmentMatchGlobs: [
+      ["tests/unit/**/*.spec.js", "jsdom"],
+    ],
     include: ["tests/unit/**/*.spec.js"],
+    setupFiles: ["./tests/unit/setup.js"],
     coverage: {
       enabled: false,
     },