diff --git a/apps/api/src/routes/submission.ts b/apps/api/src/routes/submission.ts
index 0afd86a..8c78d14 100644
--- a/apps/api/src/routes/submission.ts
+++ b/apps/api/src/routes/submission.ts
@@ -28,7 +28,7 @@ import {
   requireContestAccess,
   type ContestEnv,
 } from "../services/contest"
-import { CodeFormatError, formatCode } from "../services/format-code"
+import { CodeFormatError, formatCode, mergesLines } from "../services/format-code"
 import { getBooleanOption } from "../services/options"
 import { problemSetLockCutoffs } from "../services/problemset"
 import { consumeToken } from "../services/throttling"
@@ -246,28 +246,30 @@ submissionRoutes.post("/submissions/:id/rejudge", requireTeacher, async (c) => {
 submissionRoutes.post("/code/format", requireAuth, async (c) => {
   const parsed = await parseBody(c, formatCodeRequestSchema, "Invalid format payload")
   if (!parsed.success) return parsed.response
-  // C / C++ 编不过就原样返回，不格式化（理由见 judge/run.ts 的 compileFails）。
-  // 代码照常提交、照常判成编译错误，只是不改动学生的代码
   const { code: source, language, problemId } = parsed.data
-  if (language === "c" || language === "cpp") {
-    const judgeLanguage = language === "c" ? "C" : "C++"
-    const [problem] =
-      problemId === undefined
-        ? []
-        : await db
-            .select({ template: schema.problem.template })
-            .from(schema.problem)
-            .where(eq(schema.problem.id, problemId))
-            .limit(1)
-    const rawTemplate = problem ? templateForLanguage(problem.template, judgeLanguage) : null
-    const template = rawTemplate ? parseProblemTemplate(rawTemplate) : null
-    const full = template ? `${template.prepend}\n${source}\n${template.append}` : source
-    if (await compileFails(judgeLanguage, full)) {
-      return success(c, { code: source } satisfies FormatCodeResponse)
-    }
-  }
   try {
     const code = await formatCode(source, language)
+    // C / C++ 编不过就原样返回，不格式化（理由见 judge/run.ts 的 compileFails）。
+    // 代码照常提交、照常判成编译错误，只是不改动学生的代码。
+    // 只有 clang-format 真把两行并成一行时才去判题机编：判题机和判题抢同一台机器的 CPU，
+    // 全班一起交 C++ 时每人都要排满 8 秒超时；而编得过的代码九成以上根本不并行
+    if ((language === "c" || language === "cpp") && mergesLines(source, code)) {
+      const judgeLanguage = language === "c" ? "C" : "C++"
+      const [problem] =
+        problemId === undefined
+          ? []
+          : await db
+              .select({ template: schema.problem.template })
+              .from(schema.problem)
+              .where(eq(schema.problem.id, problemId))
+              .limit(1)
+      const rawTemplate = problem ? templateForLanguage(problem.template, judgeLanguage) : null
+      const template = rawTemplate ? parseProblemTemplate(rawTemplate) : null
+      const full = template ? `${template.prepend}\n${source}\n${template.append}` : source
+      if (await compileFails(judgeLanguage, full)) {
+        return success(c, { code: source } satisfies FormatCodeResponse)
+      }
+    }
     return success(c, { code } satisfies FormatCodeResponse)
   } catch (error) {
     if (error instanceof CodeFormatError) {
diff --git a/apps/api/src/services/format-code.ts b/apps/api/src/services/format-code.ts
index 5bf71c2..0d126f4 100644
--- a/apps/api/src/services/format-code.ts
+++ b/apps/api/src/services/format-code.ts
@@ -82,6 +82,41 @@ async function checkPythonSyntax(code: string): Promise<string | null | undefine
   return result.exitCode === 1 && result.stdout ? result.stdout : undefined
 }
 
+/** 去掉全部空白之后的字符流，以及原来每个换行落在流里的哪个位置（首尾的不算） */
+function lineBreaks(code: string) {
+  let stream = ""
+  const breaks = new Set<number>()
+  for (const char of code) {
+    if (char === "\n") breaks.add(stream.length)
+    else if (!/\s/.test(char)) stream += char
+  }
+  breaks.delete(0)
+  breaks.delete(stream.length)
+  return { stream, breaks }
+}
+
+/**
+ * clang-format 有没有把学生原来分开的两行并成一行。并了才值得怀疑代码编不过
+ * （`int n；` 和下一行、少了分号的两行，见 judge/run.ts 的 compileFails）。
+ *
+ * 风格本身就会并的两种不算：`{` 挪到上一行末尾（BreakBeforeBraces: Attach），
+ * `}` 和后面的 `else` / `while` 接上。别的差异（包括 clang-format 动了非空白字符，
+ * 比如排了 #include 的顺序）一律算可疑 —— 宁可多编一次，也不放过一次并行。
+ */
+export function mergesLines(original: string, formatted: string) {
+  const before = lineBreaks(original)
+  const after = lineBreaks(formatted)
+  if (before.stream !== after.stream) return true
+  for (const at of before.breaks) {
+    if (after.breaks.has(at)) continue
+    const next = before.stream.slice(at)
+    if (next.startsWith("{")) continue
+    if (before.stream[at - 1] === "}" && /^(else|while)/.test(next)) continue
+    return true
+  }
+  return false
+}
+
 export async function formatCode(code: string, language: "python" | "c" | "cpp" | "sql") {
   if (language === "sql") return formatSql(code)
 
@@ -101,11 +136,13 @@ export async function formatCode(code: string, language: "python" | "c" | "cpp"
   }
 
   const filename = language === "c" ? "code.c" : "code.cpp"
+  // 不排 #include、不拆长字符串：学生的 printf("……") 被拆成 "……" "……" 两截看不懂；
+  // 这两样也是 mergesLines 唯一常见的误报来源（它们改了非空白字符）
   const result = await runFormatter(
     [
       config.clangFormatPath,
       `-assume-filename=${filename}`,
-      "-style={BasedOnStyle: LLVM, IndentWidth: 4, BreakBeforeBraces: Attach}",
+      "-style={BasedOnStyle: LLVM, IndentWidth: 4, BreakBeforeBraces: Attach, SortIncludes: Never, BreakStringLiterals: false}",
     ],
     code,
   )