diff --git a/packages/hydrojudge/src/judge/default.ts b/packages/hydrojudge/src/judge/default.ts
index be4d54bb0..ce33e0e6f 100644
--- a/packages/hydrojudge/src/judge/default.ts
+++ b/packages/hydrojudge/src/judge/default.ts
@@ -143,7 +143,8 @@ export const judge = async (ctx) => {
     ctx.total_score = 0;
     ctx.total_memory_usage_kb = 0;
     ctx.total_time_usage_ms = 0;
-    ctx.queue = new Queue({ concurrency: ctx.config.concurrency || 2 });
+    // sandbox seems cannot handle concurrent tasks with output file correctly
+    ctx.queue = new Queue({ concurrency: ctx.config.concurrency || (ctx.config.filename ? 1 : 2) });
     for (const sid in ctx.config.subtasks) tasks.push(judgeSubtask(ctx.config.subtasks[sid])(ctx));
     await Promise.all(tasks);
     ctx.stat.done = new Date();
diff --git a/packages/hydrojudge/src/sandbox.ts b/packages/hydrojudge/src/sandbox.ts
index e1a6b0c8e..964a8ea87 100644
--- a/packages/hydrojudge/src/sandbox.ts
+++ b/packages/hydrojudge/src/sandbox.ts
@@ -104,9 +104,7 @@ export async function run(execute, params?) {
     else ret.stdout = result.files.stdout || '';
     if (params.stderr) await fs.writeFile(params.stderr, result.files.stderr || '');
     else ret.stderr = result.files.stderr || '';
-    if (result.error) {
-        ret.error = result.error;
-    }
+    if (result.error) ret.error = result.error;
     ret.files = result.files;
     ret.fileIds = result.fileIds || {};
     return ret;