diff --git a/src/app/utils/fetchUtil.ts b/src/app/utils/fetchUtil.ts
index aa445854..22327ebc 100644
--- a/src/app/utils/fetchUtil.ts
+++ b/src/app/utils/fetchUtil.ts
@@ -24,6 +24,27 @@ export interface searchParamsProps {
   searchParams: { [key: string]: string | string[] | undefined };
 }
 
+/** Prefer API `error` / `message` so SIGNAL 400s show the Chinese reason, not a traceId. */
+function userMessageFromErrorBody(body: string, fallback: string): string {
+  const trimmed = body.trim();
+  if (!trimmed) return fallback;
+  try {
+    const errorJson = JSON.parse(trimmed) as {
+      error?: unknown;
+      message?: unknown;
+    };
+    if (typeof errorJson.error === "string" && errorJson.error.trim()) {
+      return errorJson.error.trim();
+    }
+    if (typeof errorJson.message === "string" && errorJson.message.trim()) {
+      return errorJson.message.trim();
+    }
+  } catch {
+    return trimmed;
+  }
+  return trimmed;
+}
+
 export async function serverFetchWithNoContent(...args: FetchParams) {
   const response = await serverFetch(...args);
 
@@ -57,7 +78,7 @@ export async function serverFetchWithNoContent(...args: FetchParams) {
         }
         console.error(`Server error (${response.status}):`, errorMessage);
         throw new ServerFetchError(
-          `Server error: ${response.status} ${response.statusText}. ${errorMessage}`,
+          userMessageFromErrorBody(errorMessage, errorMessage),
           response
         );
     }
@@ -107,7 +128,10 @@ export async function serverFetchJson<T>(...args: FetchParams) {
         signOutUser();
       default:
         throw new ServerFetchError(
-          `Server error: ${response.status} ${response.statusText}. ${errorText}`,
+          userMessageFromErrorBody(
+            errorText,
+            `Server error: ${response.status} ${response.statusText}`,
+          ),
           response,
         );
     }