diff --git a/CLAUDE.md b/CLAUDE.md
index b3b7e6a..914b72c 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -17,7 +17,7 @@ The backend was rewritten from Django in 2026-10 to cut memory use (~25 MB vs. 1
   - `src/auth.ts` — Django-compatible password hashing; JWT login, no cookies: `POST /account/login` returns a 15-min HS256 `access_token` (sent as `Authorization: Bearer`, signed with `JWT_SECRET`, random per process if unset) and a 14-day opaque `refresh_token` (only its sha256 is stored, in `auth_refresh_token`; `POST /account/refresh` trades it for a new access token, logout / password change / deactivation delete it). An invalid or expired Bearer token is a 401 even on public endpoints so the client knows to refresh. `requireUser/requireAdmin/requireSuper` (401 / 403)
   - `src/routes/` — `account`, `tasks` (tutorial / challenge / creation), `files` (task assets + image upload), `submission`, `showcase` (mounted under `/submission/showcase`), `prompt`
   - `src/services/` — `score.ts` (rating weights super×0.5, admin×0.3, normal×0.2, then Bayesian smoothing with C=3 and zones), `gradebook.ts`, `llm.ts` (OpenAI-compatible streaming via fetch; DeepSeek default, ARK for doubao models), `classifier.ts` (Bloom-level L1–L6 prompt tagging, runs in background), `cascade.ts` (Django's CASCADE deletes, done by hand), `pyround.ts` (`pyRound`/`pySum` reproduce Python's `round()` and `sum()` so scores match the old system bit-for-bit)
-  - `src/ws.ts` — `/ws/prompt/<task_id>/?token=<access_token>` (code generation chat) and `/ws/guidance/<task_id>/?token=...` (prompt coaching); messages per connection are processed serially; on disconnect the pending user message is deleted and the LLM request aborted
+  - `src/ws.ts` — `/ws/prompt/<task_id>/?token=<access_token>` (code generation chat) and `/ws/guidance/<task_id>/?token=...` (prompt coaching); one message per connection at a time (a message sent while the AI is still replying gets an error, payloads capped at 64 KB); on disconnect the pending user message is deleted and the LLM request aborted
   - `scripts/` — one-off CLIs: `create-superuser.ts`, `classify-prompts.ts`, `recalculate-scores.ts`
   - Roles: `super`, `admin`, `normal`. Task model is multi-table: `task_task` + one of `task_tutorial` / `task_challenge` / `task_creation` (`task_ptr_id`)
 
diff --git a/client/src/components/Login.vue b/client/src/components/Login.vue
index 79e57ec..aa970c2 100644
--- a/client/src/components/Login.vue
+++ b/client/src/components/Login.vue
@@ -48,9 +48,9 @@
           />
           <n-alert
             type="error"
-            v-if="showStudentError"
+            v-if="studentError"
             class="message"
-            title="登录失败，请检查密码"
+            :title="studentError"
           />
           <n-button
             block
@@ -79,9 +79,9 @@
           </n-form-item>
           <n-alert
             type="error"
-            v-if="showAdminError"
+            v-if="adminError"
             class="message"
-            title="登录失败，请检查用户名和密码"
+            :title="adminError"
           />
           <n-button
             block
@@ -113,7 +113,7 @@ const selectedClass = ref<string | null>(null)
 const selectedUsername = ref<string | null>(null)
 const studentPassword = ref("")
 const studentLoading = ref(false)
-const showStudentError = ref(false)
+const studentError = ref("")
 
 // Classes data
 const classes = ref<string[]>([])
@@ -135,7 +135,7 @@ const nameOptions = computed(() =>
 const adminName = ref("")
 const adminPassword = ref("")
 const adminLoading = ref(false)
-const showAdminError = ref(false)
+const adminError = ref("")
 
 // Load classes on mount (cached — not reloaded on tab switch)
 onMounted(async () => {
@@ -155,13 +155,13 @@ function onTabChange() {
   selectedClass.value = null
   selectedUsername.value = null
   studentPassword.value = ""
-  showStudentError.value = false
+  studentError.value = ""
   namesError.value = false
   names.value = []
   // Reset admin tab state
   adminName.value = ""
   adminPassword.value = ""
-  showAdminError.value = false
+  adminError.value = ""
 }
 
 async function onClassChange(classname: string) {
@@ -178,10 +178,15 @@ async function onClassChange(classname: string) {
   }
 }
 
+// 失败次数太多被暂时禁止登录时，显示服务端给的提示
+function loginError(err: any, fallback: string) {
+  return err.response?.status === 429 ? err.response.data.detail : fallback
+}
+
 async function submitStudent() {
   if (!selectedUsername.value || !studentPassword.value) return
   studentLoading.value = true
-  showStudentError.value = false
+  studentError.value = ""
   try {
     const data = await Account.login(
       selectedUsername.value,
@@ -194,8 +199,8 @@ async function submitStudent() {
     router.replace(
       window.location.pathname + window.location.search + window.location.hash,
     )
-  } catch {
-    showStudentError.value = true
+  } catch (err: any) {
+    studentError.value = loginError(err, "登录失败，请检查密码")
   } finally {
     studentLoading.value = false
   }
@@ -204,7 +209,7 @@ async function submitStudent() {
 async function submitAdmin() {
   if (!adminName.value || !adminPassword.value) return
   adminLoading.value = true
-  showAdminError.value = false
+  adminError.value = ""
   try {
     const data = await Account.login(adminName.value, adminPassword.value)
     user.username = data.username
@@ -214,8 +219,8 @@ async function submitAdmin() {
     router.replace(
       window.location.pathname + window.location.search + window.location.hash,
     )
-  } catch {
-    showAdminError.value = true
+  } catch (err: any) {
+    adminError.value = loginError(err, "登录失败，请检查用户名和密码")
   } finally {
     adminLoading.value = false
   }
diff --git a/client/src/components/editor/Editors.vue b/client/src/components/editor/Editors.vue
index 25dfeb5..9705412 100644
--- a/client/src/components/editor/Editors.vue
+++ b/client/src/components/editor/Editors.vue
@@ -79,11 +79,6 @@
 </template>
 <script lang="ts" setup>
 import { Icon } from "@iconify/vue"
-import prettier from "prettier/standalone"
-import * as htmlParser from "prettier/parser-html"
-import * as cssParser from "prettier/parser-postcss"
-import * as babelParser from "prettier/parser-babel"
-import * as estreeParser from "prettier/plugins/estree"
 import Editor from "./Editor.vue"
 import Toolbar from "./Toolbar.vue"
 import { html, css, js, tab, size, reset } from "../../store/editors"
@@ -105,6 +100,15 @@ function changeSize(num: number) {
 }
 
 async function formatCode() {
+  // prettier 加上各语言插件近 1MB，只在点“格式化”时才加载
+  const [prettier, htmlParser, cssParser, babelParser, estreeParser] =
+    await Promise.all([
+      import("prettier/standalone"),
+      import("prettier/parser-html"),
+      import("prettier/parser-postcss"),
+      import("prettier/parser-babel"),
+      import("prettier/plugins/estree"),
+    ])
   const [htmlFormatted, cssFormatted, jsFormatted] = await Promise.all([
     prettier.format(html.value, {
       parser: "html",
diff --git a/client/src/pages/Showcase.vue b/client/src/pages/Showcase.vue
index eac68ad..792047f 100644
--- a/client/src/pages/Showcase.vue
+++ b/client/src/pages/Showcase.vue
@@ -39,6 +39,7 @@
               <iframe
                 :srcdoc="buildSrcdoc(item)"
                 sandbox="allow-scripts"
+                loading="lazy"
                 scrolling="no"
                 class="preview-iframe"
               />
diff --git a/client/src/router.ts b/client/src/router.ts
index 9f8ca38..642918c 100644
--- a/client/src/router.ts
+++ b/client/src/router.ts
@@ -1,7 +1,8 @@
 import { createWebHistory, createRouter } from "vue-router"
 import { loginModal } from "./store/modal"
 
-import Workspace from "./pages/Workspace.vue"
+// 编辑器（CodeMirror 等）比较大，用到时才加载；几个路由共用同一个加载函数，切换时不会重新挂载
+const Workspace = () => import("./pages/Workspace.vue")
 import { hasLogin } from "./token"
 import { authed } from "./store/user"
 
diff --git a/server/src/routes/account.ts b/server/src/routes/account.ts
index 8485ee7..ee10111 100644
--- a/server/src/routes/account.ts
+++ b/server/src/routes/account.ts
@@ -13,7 +13,7 @@ import {
   requireUser,
   setPassword,
 } from "../auth"
-import { HttpError, body, bool, intParam, orNotFound, pageQuery, pageWindow, query, type Env } from "../http"
+import { HttpError, body, bool, intParam, orNotFound, pageQuery, pageWindow, query, type Ctx, type Env } from "../http"
 
 const router = new Hono<Env>()
 
@@ -54,10 +54,45 @@ async function insertUser(u: {
 const userExists = (username: string) =>
   !!get(`SELECT 1 FROM account_user WHERE username = $username`, { username })
 
+// 登录失败次数限制（进程内存里计数）：同一用户名 10 分钟内失败 10 次、同一 IP 失败 100 次后暂停登录。
+// 机房的电脑共用一个出口 IP，IP 的阈值要放宽，主要防的是对单个账号猜密码
+const LOGIN_WINDOW_MS = 10 * 60_000
+const MAX_FAILS_PER_USER = 10
+const MAX_FAILS_PER_IP = 100
+const loginFailures = new Map<string, { n: number; reset: number }>()
+
+const failCount = (key: string) => {
+  const f = loginFailures.get(key)
+  return f && f.reset > Date.now() ? f.n : 0
+}
+function recordFailure(key: string) {
+  const now = Date.now()
+  const f = loginFailures.get(key)
+  if (f && f.reset > now) f.n++
+  else loginFailures.set(key, { n: 1, reset: now + LOGIN_WINDOW_MS })
+}
+setInterval(() => {
+  const now = Date.now()
+  for (const [k, f] of loginFailures) if (f.reset <= now) loginFailures.delete(k)
+}, LOGIN_WINDOW_MS).unref()
+
+// 生产环境经过 Caddy，它会把 X-Forwarded-For 设成真实的客户端地址
+const clientIp = (c: Ctx) => c.req.header("x-forwarded-for")?.split(",")[0]?.trim() || "local"
+
 router.post("/login", async (c) => {
   const payload = await body(c, z.object({ username: z.string(), password: z.string() }))
+  const userKey = `user:${payload.username}`
+  const ipKey = `ip:${clientIp(c)}`
+  if (failCount(userKey) >= MAX_FAILS_PER_USER || failCount(ipKey) >= MAX_FAILS_PER_IP) {
+    throw new HttpError(429, "登录失败次数太多，请 10 分钟后再试")
+  }
   const user = await authenticate(payload.username, payload.password)
-  if (!user) throw new HttpError(401, "账号密码错误")
+  if (!user) {
+    recordFailure(userKey)
+    recordFailure(ipKey)
+    throw new HttpError(401, "账号密码错误")
+  }
+  loginFailures.delete(userKey)
   return c.json({ username: user.username, role: user.role, ...(await issueTokens(user)) })
 })
 
diff --git a/server/src/routes/submission.ts b/server/src/routes/submission.ts
index ea76b04..38d9187 100644
--- a/server/src/routes/submission.ts
+++ b/server/src/routes/submission.ts
@@ -314,18 +314,30 @@ router.get("/random-for-rating", (c) => {
   }
   if (user.role === Role.NORMAL && todayRatingCount(user.id) >= 30) return c.json(null)
 
-  // 只在索引上随机挑出 rowid，再读这一行的代码；否则每个候选都要读一遍 html/css/js
-  const base = `SELECT s.rowid AS rid FROM submission_submission s
-    WHERE s.user_id != $me
+  const cond = `s.user_id != $me
       AND NOT EXISTS (SELECT 1 FROM submission_rating r WHERE r.submission_id = s.id AND r.user_id = $me)
       ${excludeId ? "AND s.id != $exclude" : ""}`
   const params = { me: user.id, ...(excludeId ? { exclude: excludeId } : {}) }
-  const picked =
-    get<{ rid: number }>(
-      `${base} AND (SELECT COUNT(*) FROM submission_rating r WHERE r.submission_id = s.id) < 5
+  const fewRatings = `(SELECT COUNT(*) FROM submission_rating r WHERE r.submission_id = s.id) < 5`
+  // 每个学生生成代码时都会调这个接口，ORDER BY RANDOM() 要把所有提交过一遍（6 万条约 50ms，期间整个服务都卡住）。
+  // 先随机抽几十个 rowid 试试，抽到评分少于 5 个的就用；抽不中再退回全表随机
+  const maxRowid = get<{ m: number | null }>(`SELECT MAX(rowid) AS m FROM submission_submission`)!.m ?? 0
+  let picked: { rid: number } | null = null
+  for (let round = 0; round < 3 && maxRowid && !picked; round++) {
+    const sample = Array.from({ length: 64 }, () => 1 + Math.floor(Math.random() * maxRowid))
+    // 抽样作为参数传入（SQL 文本固定，db.query 按文本缓存语句，拼进 SQL 会让缓存越来越大）
+    picked = get<{ rid: number }>(
+      `SELECT s.rowid AS rid FROM submission_submission s
+       WHERE s.rowid IN (SELECT value FROM json_each($sample)) AND ${cond} AND ${fewRatings}
        ORDER BY RANDOM() LIMIT 1`,
-      params,
-    ) ?? get<{ rid: number }>(`${base} ORDER BY RANDOM() LIMIT 1`, params)
+      { ...params, sample: JSON.stringify(sample) },
+    )
+  }
+  // 只在索引上随机挑出 rowid，再读这一行的代码；否则每个候选都要读一遍 html/css/js
+  const base = `SELECT s.rowid AS rid FROM submission_submission s WHERE ${cond}`
+  picked ??=
+    get<{ rid: number }>(`${base} AND ${fewRatings} ORDER BY RANDOM() LIMIT 1`, params) ??
+    get<{ rid: number }>(`${base} ORDER BY RANDOM() LIMIT 1`, params)
   if (!picked) return c.json(null)
   const pick = get(
     `SELECT s.id, u.username, t.title AS task_title, t.display AS task_display,
diff --git a/server/src/ws.ts b/server/src/ws.ts
index 4b92330..35a1a12 100644
--- a/server/src/ws.ts
+++ b/server/src/ws.ts
@@ -21,8 +21,8 @@ export interface WsData {
   pendingUserMessageId: number | null
   abort: AbortController
   closed: boolean
-  // 串行处理消息（与 Django Channels 的行为一致）
-  queue: Promise<void>
+  // 正在处理一条消息（等 AI 回复）；期间再发来的直接拒绝，不排队，免得一次塞几百条刷模型费用
+  busy: boolean
   // 引导对话只在本次连接内保留上下文
   guidanceHistory: ChatMessage[]
 }
@@ -57,7 +57,7 @@ export async function upgrade(
       pendingUserMessageId: null,
       abort: new AbortController(),
       closed: false,
-      queue: Promise.resolve(),
+      busy: false,
       guidanceHistory: [],
     },
   })
@@ -185,6 +185,8 @@ export const websocket: WebSocketHandler<WsData> = {
   // AI 生成可能要一两分钟，期间没有客户端消息
   idleTimeout: 960,
   sendPings: true,
+  // 提示词不会这么长；Bun 默认 16MB
+  maxPayloadLength: 64 * 1024,
 
   open(ws) {
     const d = ws.data
@@ -222,13 +224,20 @@ export const websocket: WebSocketHandler<WsData> = {
     } catch {
       return
     }
+    if (ws.data.busy) {
+      send(ws, { type: "error", content: "上一条消息还在处理，请等 AI 回复完再发" })
+      return
+    }
     const handler = ws.data.kind === "prompt" ? handlePrompt : handleGuidance
-    ws.data.queue = ws.data.queue
-      .then(() => (ws.data.closed ? undefined : handler(ws, data)))
+    ws.data.busy = true
+    handler(ws, data)
       .catch((e) => {
         console.error("websocket handler failed:", e)
         send(ws, { type: "error", content: "服务器出错，请刷新页面重试" })
       })
+      .finally(() => {
+        ws.data.busy = false
+      })
   },
 
   close(ws) {
diff --git a/server/tests/api.test.ts b/server/tests/api.test.ts
index 169e5c3..2f42480 100644
--- a/server/tests/api.test.ts
+++ b/server/tests/api.test.ts
@@ -5,7 +5,7 @@ import { config } from "../src/config"
 import { mkdtemp, readFile, readdir, rm } from "node:fs/promises"
 import { tmpdir } from "node:os"
 import { join } from "node:path"
-import { client, freshDb, makeSubmission, makeTask, makeUser, uuid } from "./helpers"
+import { PASSWORD, client, freshDb, makeSubmission, makeTask, makeUser, uuid } from "./helpers"
 
 beforeEach(freshDb)
 
@@ -50,6 +50,17 @@ describe("权限", () => {
     expect(res.status).toBe(404)
   })
 
+  test("同一用户名连续登录失败 10 次后暂停登录", async () => {
+    makeUser("brute")
+    const anon = await client()
+    for (let i = 0; i < 10; i++) {
+      expect((await anon("POST", "/api/account/login", { username: "brute", password: "wrong" })).status).toBe(401)
+    }
+    // 密码对了也要等窗口过去
+    const res = await anon("POST", "/api/account/login", { username: "brute", password: PASSWORD })
+    expect(res.status).toBe(429)
+  })
+
   test("封号后 access token 和 refresh token 都失效", async () => {
     const id = makeUser("stu")
     makeUser("boss", "super")
@@ -176,6 +187,26 @@ describe("评分", () => {
   })
 })
 
+describe("随机评分", () => {
+  test("不给自己的、已评过的；没有可评的返回 null", async () => {
+    const me = makeUser("me")
+    const other = makeUser("other")
+    const task = makeTask("T")
+    makeSubmission(me, task)
+    const subs = Array.from({ length: 5 }, () => makeSubmission(other, task))
+    const c = await client("me")
+    const seen = new Set<string>()
+    for (let i = 0; i < 5; i++) {
+      const res = await c("GET", "/api/submission/random-for-rating")
+      expect(subs.map(uuid)).toContain(res.json.submission_id)
+      expect(seen.has(res.json.submission_id)).toBe(false)
+      seen.add(res.json.submission_id)
+      await c("PUT", `/api/submission/${res.json.submission_id}/score`, { score: 4 })
+    }
+    expect((await c("GET", "/api/submission/random-for-rating")).json).toBeNull()
+  })
+})
+
 describe("提交与删除", () => {
   test("手写提示词提交会生成对话，删除提交连带删除提示词", async () => {
     makeUser("stu")
diff --git a/server/tests/ws.test.ts b/server/tests/ws.test.ts
new file mode 100644
index 0000000..02b0e52
--- /dev/null
+++ b/server/tests/ws.test.ts
@@ -0,0 +1,134 @@
+import { afterAll, beforeAll, beforeEach, describe, expect, test } from "bun:test"
+import type { Server } from "bun"
+import { app } from "../src/app"
+import { config } from "../src/config"
+import { all } from "../src/db"
+import { upgrade, websocket, type WsData } from "../src/ws"
+import { client, freshDb, makeTask, makeUser } from "./helpers"
+
+// 假的模型接口：每段之间停一下，方便测试中途断开
+const PIECES = ["好的", "```html\n<p>hi</p>\n```"]
+const llm = Bun.serve({
+  port: 0,
+  fetch() {
+    const enc = new TextEncoder()
+    const stream = new ReadableStream({
+      async start(ctrl) {
+        for (const piece of PIECES) {
+          ctrl.enqueue(enc.encode(`data: ${JSON.stringify({ choices: [{ delta: { content: piece } }] })}\n\n`))
+          await Bun.sleep(150)
+        }
+        ctrl.enqueue(enc.encode("data: [DONE]\n\n"))
+        ctrl.close()
+      },
+    })
+    return new Response(stream, { headers: { "content-type": "text/event-stream" } })
+  },
+})
+
+let server: Server<WsData>
+const saved = { ...config }
+
+beforeAll(() => {
+  Object.assign(config, { llmApiKey: "k", llmBaseUrl: `http://localhost:${llm.port}` })
+  server = Bun.serve<WsData>({
+    port: 0,
+    async fetch(req, srv) {
+      const res = await upgrade(req, srv)
+      return res !== null ? res : app.fetch(req)
+    },
+    websocket,
+  })
+})
+
+afterAll(() => {
+  Object.assign(config, saved)
+  server.stop(true)
+  llm.stop(true)
+})
+
+beforeEach(freshDb)
+
+interface Conn {
+  ws: WebSocket
+  msgs: any[]
+  next: (type: string) => Promise<any>
+}
+
+// 握手失败时返回 null
+function connect(path: string): Promise<Conn | null> {
+  return new Promise((resolve) => {
+    const ws = new WebSocket(`ws://localhost:${server.port}${path}`)
+    const msgs: any[] = []
+    const waiters: { type: string; resolve: (m: any) => void }[] = []
+    ws.onmessage = (e) => {
+      const m = JSON.parse(String(e.data))
+      msgs.push(m)
+      const i = waiters.findIndex((w) => w.type === m.type)
+      if (i >= 0) waiters.splice(i, 1)[0]!.resolve(m)
+    }
+    const next = (type: string) =>
+      new Promise<any>((res) => {
+        const seen = msgs.find((m) => m.type === type && !m._taken)
+        if (seen) {
+          seen._taken = true
+          res(seen)
+        } else {
+          waiters.push({ type, resolve: (m) => ((m._taken = true), res(m)) })
+        }
+      })
+    ws.onopen = () => resolve({ ws, msgs, next })
+    ws.onerror = () => resolve(null)
+  })
+}
+
+async function setup() {
+  makeUser("stu")
+  const task = makeTask("挑战")
+  const stu = await client("stu")
+  return { task, token: stu.tokens.access_token }
+}
+
+describe("WebSocket 代码生成", () => {
+  test("正常生成：流式返回并保存一问一答", async () => {
+    const { task, token } = await setup()
+    const c = (await connect(`/ws/prompt/${task}/?token=${token}`))!
+    expect((await c.next("init")).messages).toEqual([])
+    c.ws.send(JSON.stringify({ type: "message", content: "做个页面" }))
+    const done = await c.next("complete")
+    expect(done.code.html).toBe("<p>hi</p>")
+    const streamed = c.msgs.filter((m) => m.type === "stream").map((m) => m.content).join("")
+    expect(streamed).toBe(PIECES.join(""))
+    expect(all(`SELECT role FROM prompt_message ORDER BY id`).map((m) => m.role)).toEqual(["user", "assistant"])
+    c.ws.close()
+  })
+
+  test("AI 回复中再发消息会被拒绝", async () => {
+    const { task, token } = await setup()
+    const c = (await connect(`/ws/prompt/${task}/?token=${token}`))!
+    await c.next("init")
+    c.ws.send(JSON.stringify({ type: "message", content: "第一条" }))
+    c.ws.send(JSON.stringify({ type: "message", content: "第二条" }))
+    expect((await c.next("error")).content).toContain("上一条消息还在处理")
+    await c.next("complete")
+    expect(all(`SELECT content FROM prompt_message WHERE role = 'user'`)).toEqual([{ content: "第一条" }])
+    c.ws.close()
+  })
+
+  test("中途断开：删掉没有回复的提问", async () => {
+    const { task, token } = await setup()
+    const c = (await connect(`/ws/prompt/${task}/?token=${token}`))!
+    await c.next("init")
+    c.ws.send(JSON.stringify({ type: "message", content: "做个页面" }))
+    await c.next("stream")
+    c.ws.close()
+    await Bun.sleep(400)
+    expect(all(`SELECT * FROM prompt_message`)).toEqual([])
+  })
+
+  test("token 无效或任务不存在时握手失败", async () => {
+    const { task, token } = await setup()
+    expect(await connect(`/ws/prompt/${task}/?token=bad`)).toBeNull()
+    expect(await connect(`/ws/prompt/9999/?token=${token}`)).toBeNull()
+  })
+})