diff --git a/apps/api/src/judge/run.ts b/apps/api/src/judge/run.ts
index 5dd1cae..6c7481b 100644
--- a/apps/api/src/judge/run.ts
+++ b/apps/api/src/judge/run.ts
@@ -112,6 +112,27 @@ export async function postJudge(
   return response
 }
 
+/**
+ * 这份代码在判题机上编不编得过。给提交前的自动格式化用：clang-format 碰上编译不过的
+ * C 代码会把行并起来（`int n；` 和下一行、少了分号的两行），学生回头看到的就不是自己
+ * 写的代码了，报错的行号也对不上他记得的位置。api 容器里没有 gcc，而判题机的编译器、
+ * 编译参数本来就是判的标准，所以借它编一次。
+ *
+ * 判题机只有 /judge 一个入口，得带一组测试点：给一组空输入，跑的那一下限 1 秒。
+ * 判题机连不上、超时，一律当编得过 —— 那就和原来一样照常格式化。
+ */
+export async function compileFails(language: string, source: string) {
+  try {
+    const response = await postJudge(language, source, 1000, 256, [{ input: "", output: "" }], {
+      signal: AbortSignal.timeout(8000),
+    })
+    const body = (await response.json()) as JudgeResponse
+    return body.err === "CompileError"
+  } catch {
+    return false
+  }
+}
+
 async function requestJudge(
   language: string,
   code: string,
diff --git a/apps/api/src/routes/submission.ts b/apps/api/src/routes/submission.ts
index 4c838f9..35819ac 100644
--- a/apps/api/src/routes/submission.ts
+++ b/apps/api/src/routes/submission.ts
@@ -17,7 +17,9 @@ import { optionalAuth, requireAuth, requireTeacher } from "../auth/middleware"
 import type { AuthUser } from "../auth/session"
 import { db, schema } from "../db"
 import { failure, parseBody, success } from "../http"
+import { compileFails, templateForLanguage } from "../judge/run"
 import { JudgeStatus } from "../judge/status"
+import { parseProblemTemplate } from "../judge/template"
 import { judgeQueue } from "../queue"
 import {
   canAccessContest,
@@ -244,8 +246,28 @@ 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(parsed.data.code, parsed.data.language)
+    const code = await formatCode(source, language)
     return success(c, { code } satisfies FormatCodeResponse)
   } catch (error) {
     if (error instanceof CodeFormatError) {
diff --git a/apps/web/src/oj/api.ts b/apps/web/src/oj/api.ts
index c169948..1befaf8 100644
--- a/apps/web/src/oj/api.ts
+++ b/apps/web/src/oj/api.ts
@@ -134,7 +134,7 @@ export function submitCode(data: SubmitCodePayload) {
   return api.post<CreateSubmissionResponse>("submissions", data)
 }
 
-export function formatCode(data: { code: string; language: string }) {
+export function formatCode(data: { code: string; language: string; problemId: number }) {
   const languages: Record<string, string> = {
     Python: "python",
     C: "c",
@@ -144,6 +144,7 @@ export function formatCode(data: { code: string; language: string }) {
   return api.post<FormatCodeResponse>("code/format", {
     code: data.code,
     language: languages[data.language] ?? data.language.toLowerCase(),
+    problemId: data.problemId,
   })
 }
 
diff --git a/apps/web/src/oj/store/submission.ts b/apps/web/src/oj/store/submission.ts
index 6537bcc..72d1163 100644
--- a/apps/web/src/oj/store/submission.ts
+++ b/apps/web/src/oj/store/submission.ts
@@ -103,7 +103,8 @@ export const useSubmissionStore = defineStore("submission", () => {
     formattedBeforeSubmit.value = false
 
     // 0. 提交前自动格式化（Python 用 ruff，C/C++ 用 clang-format，SQL 用 sqlparse）。
-    //    Python 在格式化之前先由服务端的 CPython 查一遍语法，有错就不提交
+    //    Python 在格式化之前先由服务端的 CPython 查一遍语法，有错就不提交；
+    //    C / C++ 先在判题机上编一遍，编不过就原样交（格式化会把错的几行并成一行）
     const formatLang = LANGUAGE_FORMAT_VALUE[codeStore.code.language]
     if (["python", "c", "cpp", "sql"].includes(formatLang)) {
       isFormatting.value = true
@@ -111,6 +112,7 @@ export const useSubmissionStore = defineStore("submission", () => {
         const res = await formatCode({
           code: codeStore.code.value,
           language: formatLang,
+          problemId: problem.id,
         })
         if (stale()) return
         if (res.code !== codeStore.code.value) {
diff --git a/packages/contract/src/submission.ts b/packages/contract/src/submission.ts
index 470aeb1..a0b2297 100644
--- a/packages/contract/src/submission.ts
+++ b/packages/contract/src/submission.ts
@@ -534,6 +534,8 @@ export const todaySubmissionStatisticsSchema = z.object({
 export const formatCodeRequestSchema = z.object({
   code: z.string().max(1024 * 1024),
   language: z.enum(["python", "c", "cpp", "sql"]),
+  /** C / C++ 格式化前要先编一遍，得套上这道题的模板才编得过 */
+  problemId: z.number().int().optional(),
 })
 
 export const formatCodeResponseSchema = z.object({ code: z.string() })