diff --git a/frontend/src/api/client.ts b/frontend/src/api/client.ts
index 540d69dc..7e5432f0 100644
--- a/frontend/src/api/client.ts
+++ b/frontend/src/api/client.ts
@@ -25,7 +25,12 @@ function handleGlobalError(error: AxiosError) {
   }
 
   const status = error.response.status
-  const data = error.response.data as { detail?: string }
+  const data = error.response.data as { detail?: string | unknown[] }
+
+  // Extract a displayable error message from the response.
+  // FastAPI returns `detail` as a string for most errors, but 422 validation
+  // errors return an array of objects — we must not pass those to toast.
+  const detail = typeof data?.detail === 'string' ? data.detail : undefined
 
   // Don't show toast for 401 (handled by refresh interceptor)
   if (status === 401) {
@@ -34,13 +39,13 @@ function handleGlobalError(error: AxiosError) {
 
   // Rate limit
   if (status === 429) {
-    toast.error(data?.detail || 'Too many requests — please try again shortly')
+    toast.error(detail || 'Too many requests — please try again shortly')
     return
   }
 
   // Client errors (4xx) — show backend detail if present
   if (status >= 400 && status < 500) {
-    toast.error(data?.detail || 'Invalid request')
+    toast.error(detail || 'Invalid request')
     return
   }
 
diff --git a/frontend/src/store/authStore.ts b/frontend/src/store/authStore.ts
index 5053fcbe..e06fbc32 100644
--- a/frontend/src/store/authStore.ts
+++ b/frontend/src/store/authStore.ts
@@ -48,8 +48,9 @@ export const useAuthStore = create<AuthState>()(
           // Fetch user info
           await get().fetchUser()
         } catch (error: unknown) {
-          const axiosErr = error as { response?: { data?: { detail?: string } } }
-          const message = axiosErr.response?.data?.detail || (error instanceof Error ? error.message : 'Login failed')
+          const axiosErr = error as { response?: { data?: { detail?: unknown } } }
+          const rawDetail = axiosErr.response?.data?.detail
+          const message = (typeof rawDetail === 'string' ? rawDetail : null) || (error instanceof Error ? error.message : 'Login failed')
           set({ error: message, isLoading: false })
           throw error
         }
@@ -62,8 +63,9 @@ export const useAuthStore = create<AuthState>()(
           // After registration, log the user in
           await get().login({ email: data.email, password: data.password })
         } catch (error: unknown) {
-          const axiosErr = error as { response?: { data?: { detail?: string } } }
-          const message = axiosErr.response?.data?.detail || (error instanceof Error ? error.message : 'Registration failed')
+          const axiosErr = error as { response?: { data?: { detail?: unknown } } }
+          const rawDetail = axiosErr.response?.data?.detail
+          const message = (typeof rawDetail === 'string' ? rawDetail : null) || (error instanceof Error ? error.message : 'Registration failed')
           set({ error: message, isLoading: false })
           throw error
         }