diff --git a/frontend/src/api.ts b/frontend/src/api.ts
index 6404c97..9d1bd01 100644
--- a/frontend/src/api.ts
+++ b/frontend/src/api.ts
@@ -78,7 +78,13 @@ export class ForbiddenError extends ApiError {
 async function handleResponse<T>(response: Response): Promise<T> {
   if (!response.ok) {
     const error = await response.json().catch(() => ({ detail: 'Unknown error' }));
-    const message = error.detail || `HTTP ${response.status}`;
+    // Handle detail as string or object (backend may return structured errors)
+    let message: string;
+    if (typeof error.detail === 'object') {
+      message = JSON.stringify(error.detail);
+    } else {
+      message = error.detail || `HTTP ${response.status}`;
+    }
 
     if (response.status === 401) {
       throw new UnauthorizedError(message);