diff --git a/workbench-ui/src/design/components/SplitPane/SplitPane.tsx b/workbench-ui/src/design/components/SplitPane/SplitPane.tsx
index e4ca6991..c93544d9 100644
--- a/workbench-ui/src/design/components/SplitPane/SplitPane.tsx
+++ b/workbench-ui/src/design/components/SplitPane/SplitPane.tsx
@@ -23,6 +23,11 @@ function clamp(value: number, min: number, max: number) {
   return Math.max(min, Math.min(max, value));
 }
 
+function getLocalStorage(): Storage | null {
+  if (typeof globalThis.localStorage === "undefined") return null;
+  return globalThis.localStorage;
+}
+
 export function SplitPane({
   direction,
   defaultSplit = 50,
@@ -32,7 +37,7 @@ export function SplitPane({
 }: SplitPaneProps) {
   const [split, setSplit] = useState(() => {
     if (id) {
-      const stored = localStorage.getItem(storageKey(id));
+      const stored = getLocalStorage()?.getItem(storageKey(id));
       if (stored !== null) {
         const parsed = Number(stored);
         if (!Number.isNaN(parsed)) return parsed;
@@ -44,7 +49,7 @@ export function SplitPane({
   const dragging = useRef(false);
 
   useEffect(() => {
-    if (id) localStorage.setItem(storageKey(id), String(split));
+    if (id) getLocalStorage()?.setItem(storageKey(id), String(split));
   }, [split, id]);
 
   const onPointerDown = useCallback(
diff --git a/workbench-ui/src/test/setup.ts b/workbench-ui/src/test/setup.ts
index 08fce702..d483af4b 100644
--- a/workbench-ui/src/test/setup.ts
+++ b/workbench-ui/src/test/setup.ts
@@ -6,3 +6,25 @@ Object.defineProperty(navigator, "clipboard", {
     writeText: vi.fn().mockResolvedValue(undefined),
   },
 });
+
+if (typeof globalThis.localStorage === "undefined") {
+  const store = new Map<string, string>();
+
+  Object.defineProperty(globalThis, "localStorage", {
+    configurable: true,
+    value: {
+      clear: vi.fn(() => store.clear()),
+      getItem: vi.fn((key: string) => store.get(key) ?? null),
+      key: vi.fn((index: number) => Array.from(store.keys())[index] ?? null),
+      removeItem: vi.fn((key: string) => {
+        store.delete(key);
+      }),
+      setItem: vi.fn((key: string, value: string) => {
+        store.set(key, String(value));
+      }),
+      get length() {
+        return store.size;
+      },
+    },
+  });
+}