diff --git a/apps/web/package.json b/apps/web/package.json
index 4702fde..20053ee 100644
--- a/apps/web/package.json
+++ b/apps/web/package.json
@@ -53,6 +53,7 @@
     "vue-codemirror": "^6.1.1",
     "vue-router": "^5.2.0",
     "y-codemirror.next": "^0.3.6",
+    "y-protocols": "1.0.7",
     "y-webrtc": "^10.3.0",
     "yjs": "^13.6.32"
   },
diff --git a/apps/web/src/shared/components/CollabModal.vue b/apps/web/src/shared/components/CollabModal.vue
new file mode 100644
index 0000000..179a8ab
--- /dev/null
+++ b/apps/web/src/shared/components/CollabModal.vue
@@ -0,0 +1,93 @@
+<script setup lang="ts">
+import { cpp } from "@codemirror/lang-cpp"
+import { bracketMatching } from "@codemirror/language"
+import { closeBrackets } from "@codemirror/autocomplete"
+import type { EditorView } from "@codemirror/view"
+import { Codemirror } from "vue-codemirror"
+import { oneDark } from "../themes/oneDark"
+import { smoothy } from "../themes/smoothy"
+import { styleTheme } from "shared/extensions/baseTheme"
+import { useCollabDoc } from "../composables/collabDoc"
+import { useCollabStore } from "shared/store/collab"
+
+const isDark = useDark()
+const collabStore = useCollabStore()
+const { start, stop, getInitialExtension } = useCollabDoc()
+
+const code = ref("")
+// shallowRef，理由见 SyncCodeEditor.vue：EditorView 是类实例，ref() 的深度
+// UnwrapRef 会把它拆成一个丢了原型方法的假类型，vue-tsc 报莫名其妙的类型错。
+const editorView = shallowRef<EditorView | null>(null)
+
+// 教师端只在自己发起接单时开。学生端的协作在 SyncCodeEditor 里
+const show = computed({
+  get: () => collabStore.isTeacher && collabStore.room !== null,
+  set: (value: boolean) => {
+    if (!value) collabStore.leave()
+  },
+})
+
+const extensions = computed(() => [
+  styleTheme,
+  cpp(),
+  bracketMatching(),
+  closeBrackets(),
+  isDark.value ? oneDark : smoothy,
+  getInitialExtension(),
+])
+
+const handleEditorReady = (payload: { view: EditorView }) => {
+  editorView.value = payload.view
+}
+
+watch(
+  () => collabStore.room,
+  async (room) => {
+    if (room && collabStore.isTeacher) {
+      await nextTick()
+      if (!editorView.value) return
+      // ★ 教师端 seedContent 必须是 null —— 内容全部来自学生端
+      start({ editorView: editorView.value as EditorView, seedContent: null })
+    } else {
+      stop()
+    }
+  },
+)
+
+onUnmounted(stop)
+</script>
+
+<template>
+  <n-modal
+    v-model:show="show"
+    preset="card"
+    :style="{ width: '80vw', maxWidth: '1100px' }"
+    :title="`正在帮 ${collabStore.room?.peerName ?? ''} · ${collabStore.room?.problemId ?? ''}`"
+  >
+    <template #header-extra>
+      <n-button
+        text
+        tag="a"
+        target="_blank"
+        :href="`/problem/${collabStore.room?.problemId}`"
+      >
+        打开题面
+      </n-button>
+    </template>
+
+    <Codemirror
+      v-model="code"
+      indentWithTab
+      :extensions="extensions"
+      :tab-size="4"
+      style="height: 60vh; font-size: 18px"
+      @ready="handleEditorReady"
+    />
+
+    <template #footer>
+      <n-flex justify="end">
+        <n-button type="primary" @click="collabStore.leave()">结束协作</n-button>
+      </n-flex>
+    </template>
+  </n-modal>
+</template>
diff --git a/apps/web/src/shared/components/Header.vue b/apps/web/src/shared/components/Header.vue
index 3a9d6c2..4924de5 100644
--- a/apps/web/src/shared/components/Header.vue
+++ b/apps/web/src/shared/components/Header.vue
@@ -6,6 +6,7 @@ import { useLearnProgress } from "shared/composables/learnProgress"
 import { useAuthModalStore } from "shared/store/authModal"
 import { useScreenModeStore } from "shared/store/screenMode"
 import { logout } from "../api"
+import CollabModal from "./CollabModal.vue"
 import HelpRequestList from "./HelpRequestList.vue"
 import { useConfigStore } from "../store/config"
 import { useUserStore } from "../store/user"
@@ -363,6 +364,14 @@ function handleMenuSelect(key: string) {
         </template>
       </n-button>
     </n-flex>
+    <!--
+      挂在根 n-flex 内部而不是同级：Header.vue 一旦变成多根 fragment，
+      default.vue 里 `<Header class="header" />` 那个 class 就没有任何单一
+      根节点可以落地（Vue 会报 "Extraneous non-props attributes" 警告并把它
+      整个丢弃），header 行随之丢掉 `max-width: 2000px` 那条居中样式。
+      n-modal 默认 teleport 到 body，塞在这里不影响它的实际渲染位置。
+    -->
+    <CollabModal v-if="userStore.isTeacherOrAbove" />
   </n-flex>
 </template>
 
diff --git a/apps/web/src/shared/components/SyncCodeEditor.vue b/apps/web/src/shared/components/SyncCodeEditor.vue
index 752888c..8c34441 100644
--- a/apps/web/src/shared/components/SyncCodeEditor.vue
+++ b/apps/web/src/shared/components/SyncCodeEditor.vue
@@ -10,13 +10,18 @@ import {
   completeAnyWord,
 } from "@codemirror/autocomplete"
 import type { Extension } from "@codemirror/state"
+import type { EditorView } from "@codemirror/view"
 import type { LANGUAGE } from "utils/types"
 import { oneDark } from "../themes/oneDark"
 import { smoothy } from "../themes/smoothy"
 import { styleTheme } from "shared/extensions/baseTheme"
 import { enhanceCompletion } from "shared/extensions/autocompletion"
+import { useCollabDoc } from "../composables/collabDoc"
+import { useCollabStore } from "shared/store/collab"
 
 const isDark = useDark()
+const collabStore = useCollabStore()
+const { start, stop, getInitialExtension } = useCollabDoc()
 
 interface Props {
   problem: string
@@ -51,8 +56,46 @@ const extensions = computed(() => [
   autocompletion({
     override: [enhanceCompletion(language), completeAnyWord],
   }),
+  getInitialExtension(),
 ])
 
+interface EditorReadyPayload {
+  view: EditorView
+}
+
+// shallowRef，不是 ref：CodeMirror 的 EditorView 是带 getter 的类实例，
+// Vue 的 UnwrapRef 深度展开会把它结构化成一个丢了原型方法的假类型，
+// vue-tsc 会报 "missing dispatchTransactions/_root/..." 这类莫名其妙的错。
+// 项目里旧的 sync.ts 用的是裸变量，同一个道理，这里换成 shallowRef 规避。
+const editorView = shallowRef<EditorView | null>(null)
+
+const handleEditorReady = (payload: EditorReadyPayload) => {
+  editorView.value = payload.view
+}
+
+// 房间开了才建文档。学生点求助时什么都不做 —— 老师没来之前不该动他的编辑器。
+// 只在学生端启用：这个组件挂在每个用户的题目页上（教师也不例外，
+// ProblemEditor.vue 只按语言是不是 Flowchart 分支，不看角色），
+// 教师接单同样会让 collabStore.room 非空 —— 如果这里不按角色收窄，
+// 教师自己停在某道题的页面上接单时，这个组件会把**教师自己的编辑器内容**
+// 当成种子插入文档，还会跟 CollabModal 抢 setBinaryHandler 这个单例槽位，
+// 两者谁后调用谁把对方顶掉。教师端的协作只归 CollabModal 管。
+watch(
+  () => collabStore.room,
+  (room) => {
+    if (room && !collabStore.isTeacher && editorView.value) {
+      // 学生端：当前编辑器内容就是内容源
+      start({
+        editorView: editorView.value,
+        seedContent: editorView.value.state.doc.toString(),
+      })
+    } else {
+      stop()
+    }
+  },
+)
+
+onUnmounted(stop)
 </script>
 
 <template>
@@ -64,5 +107,6 @@ const extensions = computed(() => [
     :tab-size="4"
     :placeholder="placeholder"
     :style="{ height, fontSize: `${fontSize}px` }"
+    @ready="handleEditorReady"
   />
 </template>
diff --git a/apps/web/src/shared/composables/collabDoc.ts b/apps/web/src/shared/composables/collabDoc.ts
new file mode 100644
index 0000000..27be8a3
--- /dev/null
+++ b/apps/web/src/shared/composables/collabDoc.ts
@@ -0,0 +1,159 @@
+import { Compartment } from "@codemirror/state"
+import type { EditorView } from "@codemirror/view"
+import { useCollabStore } from "shared/store/collab"
+import { useUserStore } from "shared/store/user"
+
+/** y-websocket 那套消息头，服务端不解析，只有两端认 */
+const MESSAGE_SYNC = 0
+const MESSAGE_AWARENESS = 1
+
+const TEACHER_COLOR = "#ff6b6b"
+const STUDENT_COLOR = "#4dabf7"
+
+interface StartOptions {
+  editorView: EditorView
+  /**
+   * 文档的初始内容。
+   *
+   * **学生端传当前编辑器内容，教师端必须传 null。** 这是硬规则：
+   * 求助是学生发起的，学生的代码是唯一内容源。老师端插入任何初始内容都会
+   * 与学生的内容合并，结果就是两份代码拼在一起 —— 老实现的竞态就是这么来的。
+   */
+  seedContent: string | null
+}
+
+export function useCollabDoc() {
+  const collabStore = useCollabStore()
+  const userStore = useUserStore()
+  const compartment = new Compartment()
+
+  let doc: any = null
+  let awareness: any = null
+  let view: EditorView | null = null
+  let detachDocUpdate: (() => void) | null = null
+  let detachAwarenessUpdate: (() => void) | null = null
+
+  async function start({ editorView, seedContent }: StartOptions) {
+    const [Y, awarenessProtocol, syncProtocol, encoding, decoding, { yCollab }] =
+      await Promise.all([
+        import("yjs"),
+        import("y-protocols/awareness"),
+        import("y-protocols/sync"),
+        import("lib0/encoding"),
+        import("lib0/decoding"),
+        import("y-codemirror.next"),
+      ])
+
+    view = editorView
+    doc = new Y.Doc()
+    const ytext = doc.getText("codemirror")
+    awareness = new awarenessProtocol.Awareness(doc)
+
+    // ★ 顺序不能反：先把内容写进 ytext，再挂 yCollab。
+    // yCollab 挂上去时会用 ytext 覆盖编辑器内容，先挂就会把学生的代码清空。
+    if (seedContent) ytext.insert(0, seedContent)
+
+    const send = (build: (encoder: any) => void) => {
+      const encoder = encoding.createEncoder()
+      build(encoder)
+      collabStore.sendBinary(encoding.toUint8Array(encoder))
+    }
+
+    collabStore.setBinaryHandler((data) => {
+      const decoder = decoding.createDecoder(new Uint8Array(data))
+      const messageType = decoding.readVarUint(decoder)
+      if (messageType === MESSAGE_SYNC) {
+        const encoder = encoding.createEncoder()
+        encoding.writeVarUint(encoder, MESSAGE_SYNC)
+        syncProtocol.readSyncMessage(decoder, encoder, doc, "remote")
+        // 只有需要回话时才发（readSyncMessage 可能什么都没写）
+        if (encoding.length(encoder) > 1) {
+          collabStore.sendBinary(encoding.toUint8Array(encoder))
+        }
+      } else if (messageType === MESSAGE_AWARENESS) {
+        awarenessProtocol.applyAwarenessUpdate(
+          awareness,
+          decoding.readVarUint8Array(decoder),
+          "remote",
+        )
+      }
+    })
+
+    const onDocUpdate = (update: Uint8Array, origin: any) => {
+      if (origin === "remote") return
+      send((encoder) => {
+        encoding.writeVarUint(encoder, MESSAGE_SYNC)
+        syncProtocol.writeUpdate(encoder, update)
+      })
+    }
+    doc.on("update", onDocUpdate)
+    detachDocUpdate = () => doc?.off("update", onDocUpdate)
+
+    const onAwarenessUpdate = (
+      { added, updated, removed }: { added: number[]; updated: number[]; removed: number[] },
+      origin: any,
+    ) => {
+      if (origin === "remote") return
+      const changed = added.concat(updated, removed)
+      send((encoder) => {
+        encoding.writeVarUint(encoder, MESSAGE_AWARENESS)
+        encoding.writeVarUint8Array(
+          encoder,
+          awarenessProtocol.encodeAwarenessUpdate(awareness, changed),
+        )
+      })
+    }
+    awareness.on("update", onAwarenessUpdate)
+    detachAwarenessUpdate = () => awareness?.off("update", onAwarenessUpdate)
+
+    awareness.setLocalStateField("user", {
+      name: userStore.user?.username ?? "匿名",
+      color: userStore.isTeacherOrAbove ? TEACHER_COLOR : STUDENT_COLOR,
+    })
+
+    editorView.dispatch({
+      effects: compartment.reconfigure(yCollab(ytext, awareness)),
+    })
+
+    // 握手：双方都发 SyncStep1，各自回 Step2，两边收敛。
+    // 服务端是哑转发，不参与同步，所以这一步必须由两端对称完成
+    send((encoder) => {
+      encoding.writeVarUint(encoder, MESSAGE_SYNC)
+      syncProtocol.writeSyncStep1(encoder, doc)
+    })
+    send((encoder) => {
+      encoding.writeVarUint(encoder, MESSAGE_AWARENESS)
+      encoding.writeVarUint8Array(
+        encoder,
+        awarenessProtocol.encodeAwarenessUpdate(awareness, [doc.clientID]),
+      )
+    })
+  }
+
+  function stop() {
+    collabStore.setBinaryHandler(null)
+    detachDocUpdate?.()
+    detachAwarenessUpdate?.()
+    detachDocUpdate = null
+    detachAwarenessUpdate = null
+
+    if (view) {
+      try {
+        view.dispatch({ effects: compartment.reconfigure([]) })
+      } catch (error) {
+        console.warn("移除协同编辑扩展失败:", error)
+      }
+      view = null
+    }
+    awareness?.destroy()
+    doc?.destroy()
+    awareness = null
+    doc = null
+  }
+
+  function getInitialExtension() {
+    return compartment.of([])
+  }
+
+  return { start, stop, getInitialExtension }
+}
diff --git a/bun.lock b/bun.lock
index fcde3ef..a53939a 100644
--- a/bun.lock
+++ b/bun.lock
@@ -80,6 +80,7 @@
         "vue-codemirror": "^6.1.1",
         "vue-router": "^5.2.0",
         "y-codemirror.next": "^0.3.6",
+        "y-protocols": "1.0.7",
         "y-webrtc": "^10.3.0",
         "yjs": "^13.6.32",
       },
@@ -1366,7 +1367,7 @@
 
     "tree-sitter-c": ["tree-sitter-c@0.24.1", "https://registry.npmjs.com/tree-sitter-c/-/tree-sitter-c-0.24.1.tgz", { "dependencies": { "node-addon-api": "^8.3.1", "node-gyp-build": "^4.8.4" }, "peerDependencies": { "tree-sitter": "^0.22.4" }, "optionalPeers": ["tree-sitter"] }, "sha512-lkYwWN3SRecpvaeqmFKkuPNR3ZbtnvHU+4XAEEkJdrp3JfSp2pBrhXOtvfsENUneye76g889Y0ddF2DM0gEDpA=="],
 
-    "tree-sitter-cpp": ["tree-sitter-cpp@0.23.4", "", { "dependencies": { "node-addon-api": "^8.2.1", "node-gyp-build": "^4.8.2", "tree-sitter-c": "^0.23.1" }, "peerDependencies": { "tree-sitter": "^0.21.1" }, "optionalPeers": ["tree-sitter"] }, "sha512-qR5qUDyhZ5jJ6V8/umiBxokRbe89bCGmcq/dk94wI4kN86qfdV8k0GHIUEKaqWgcu42wKal5E97LKpLeVW8sKw=="],
+    "tree-sitter-cpp": ["tree-sitter-cpp@0.23.4", "https://registry.npmjs.com/tree-sitter-cpp/-/tree-sitter-cpp-0.23.4.tgz", { "dependencies": { "node-addon-api": "^8.2.1", "node-gyp-build": "^4.8.2", "tree-sitter-c": "^0.23.1" }, "peerDependencies": { "tree-sitter": "^0.21.1" }, "optionalPeers": ["tree-sitter"] }, "sha512-qR5qUDyhZ5jJ6V8/umiBxokRbe89bCGmcq/dk94wI4kN86qfdV8k0GHIUEKaqWgcu42wKal5E97LKpLeVW8sKw=="],
 
     "tree-sitter-python": ["tree-sitter-python@0.25.0", "https://registry.npmjs.com/tree-sitter-python/-/tree-sitter-python-0.25.0.tgz", { "dependencies": { "node-addon-api": "^8.5.0", "node-gyp-build": "^4.8.4" }, "peerDependencies": { "tree-sitter": "^0.25.0" }, "optionalPeers": ["tree-sitter"] }, "sha512-eCmJx6zQa35GxaCtQD+wXHOhYqBxEL+bp71W/s3fcDMu06MrtzkVXR437dRrCrbrDbyLuUDJpAgycs7ncngLXw=="],
 
@@ -1540,7 +1541,7 @@
 
     "strip-literal/js-tokens": ["js-tokens@10.0.0", "https://registry.npmjs.com/js-tokens/-/js-tokens-10.0.0.tgz", {}, "sha512-lM/UBzQmfJRo9ABXbPWemivdCW8V2G8FHaHdypQaIy523snUjog0W71ayWXTjiR+ixeMyVHN2XcpnTd/liPg/Q=="],
 
-    "tree-sitter-cpp/tree-sitter-c": ["tree-sitter-c@0.23.6", "", { "dependencies": { "node-addon-api": "^8.3.0", "node-gyp-build": "^4.8.4" }, "peerDependencies": { "tree-sitter": "^0.22.1" }, "optionalPeers": ["tree-sitter"] }, "sha512-0dxXKznVyUA0s6PjNolJNs2yF87O5aL538A/eR6njA5oqX3C3vH4vnx3QdOKwuUdpKEcFdHuiDpRKLLCA/tjvQ=="],
+    "tree-sitter-cpp/tree-sitter-c": ["tree-sitter-c@0.23.6", "https://registry.npmjs.com/tree-sitter-c/-/tree-sitter-c-0.23.6.tgz", { "dependencies": { "node-addon-api": "^8.3.0", "node-gyp-build": "^4.8.4" }, "peerDependencies": { "tree-sitter": "^0.22.1" }, "optionalPeers": ["tree-sitter"] }, "sha512-0dxXKznVyUA0s6PjNolJNs2yF87O5aL538A/eR6njA5oqX3C3vH4vnx3QdOKwuUdpKEcFdHuiDpRKLLCA/tjvQ=="],
 
     "tsx/esbuild": ["esbuild@0.28.1", "https://registry.npmjs.com/esbuild/-/esbuild-0.28.1.tgz", { "optionalDependencies": { "@esbuild/aix-ppc64": "0.28.1", "@esbuild/android-arm": "0.28.1", "@esbuild/android-arm64": "0.28.1", "@esbuild/android-x64": "0.28.1", "@esbuild/darwin-arm64": "0.28.1", "@esbuild/darwin-x64": "0.28.1", "@esbuild/freebsd-arm64": "0.28.1", "@esbuild/freebsd-x64": "0.28.1", "@esbuild/linux-arm": "0.28.1", "@esbuild/linux-arm64": "0.28.1", "@esbuild/linux-ia32": "0.28.1", "@esbuild/linux-loong64": "0.28.1", "@esbuild/linux-mips64el": "0.28.1", "@esbuild/linux-ppc64": "0.28.1", "@esbuild/linux-riscv64": "0.28.1", "@esbuild/linux-s390x": "0.28.1", "@esbuild/linux-x64": "0.28.1", "@esbuild/netbsd-arm64": "0.28.1", "@esbuild/netbsd-x64": "0.28.1", "@esbuild/openbsd-arm64": "0.28.1", "@esbuild/openbsd-x64": "0.28.1", "@esbuild/openharmony-arm64": "0.28.1", "@esbuild/sunos-x64": "0.28.1", "@esbuild/win32-arm64": "0.28.1", "@esbuild/win32-ia32": "0.28.1", "@esbuild/win32-x64": "0.28.1" }, "bin": { "esbuild": "bin/esbuild" } }, "sha512-HrJrvZv5ayxBzPfwphOoNzkzOIIlifzk0KJrGK2c8R4+LKpMtpYLQeUdjnwjWv/LZlkH2laZk+4w78pi99D4Vw=="],