diff --git a/2026/board-advanced/completed/public/extra.js b/2026/board-advanced/completed/public/extra.js
index 94998966..241099b9 100644
--- a/2026/board-advanced/completed/public/extra.js
+++ b/2026/board-advanced/completed/public/extra.js
@@ -3,6 +3,36 @@
 // script.js のあとに読み込むので、script.js の source や posts をそのまま使える。
 // このファイルが途中で止まっても、script.js は動き続ける。
 
+// 2章の応用課題: 見ていない間に届いた件数をタブに出す
+const title = document.title;
+let unread = 0;
+
+function countUnread() {
+  if (!document.hidden) return;
+  unread = unread + 1;
+  document.title = `(${unread}) ${title}`;
+}
+function resetTitle() {
+  if (document.hidden) return;
+  unread = 0;
+  document.title = title;
+}
+
+source.addEventListener('message', countUnread);
+document.addEventListener('visibilitychange', resetTitle);
+
+// 3章の応用課題: 名前を呼ばれたら知らせる
+function notifyMention(e) {
+  const post = JSON.parse(e.data);
+  const me = document.getElementById('name-input').value;
+  if (me === '' || post.name === me) return;
+  if (!post.text.includes(me)) return;
+  alert(`${post.name}さんに呼ばれました: ${post.text}`);
+}
+
+source.addEventListener('message', notifyMention);
+
+// 4章の応用課題: 接続状態を画面に出す
 const status = document.getElementById('status');
 
 function showOnline() {
@@ -16,3 +46,10 @@ function showOffline() {
 
 source.addEventListener('open', showOnline);
 source.addEventListener('error', showOffline);
+
+// 5章の応用課題3: 接続数を画面に出す
+function showCount(e) {
+  document.getElementById('count').textContent = `${e.data}人が接続中`;
+}
+
+source.addEventListener('count', showCount);
diff --git a/2026/board-advanced/completed/public/index.html b/2026/board-advanced/completed/public/index.html
index 3cba6187..e7cf62d5 100644
--- a/2026/board-advanced/completed/public/index.html
+++ b/2026/board-advanced/completed/public/index.html
@@ -18,6 +18,7 @@ <h1>みんなの掲示板</h1>
 
       <div class="list-header">
         <h2>みんなの投稿</h2>
+        <span id="count"></span>
         <span id="status" class="status">—</span>
         <button id="reload-btn">更新する</button>
       </div>
diff --git a/2026/board-advanced/completed/public/script.js b/2026/board-advanced/completed/public/script.js
index b2c92058..db23df49 100644
--- a/2026/board-advanced/completed/public/script.js
+++ b/2026/board-advanced/completed/public/script.js
@@ -36,18 +36,12 @@ async function addPost() {
   const name = document.getElementById('name-input').value;
   const text = document.getElementById('text-input').value;
 
-  const res = await fetch(`${API}/posts`, {
+  await fetch(`${API}/posts`, {
     method: 'POST',
     headers: { 'Content-Type': 'application/json' },
     body: JSON.stringify({ name: name, text: text }),
   });
 
-  if (!res.ok) {
-    const { message } = await res.json();
-    alert(message);
-    return;
-  }
-
   document.getElementById('text-input').value = '';
 }
 
diff --git a/2026/board-advanced/completed/server.js b/2026/board-advanced/completed/server.js
index 6315d0c9..08c33caf 100644
--- a/2026/board-advanced/completed/server.js
+++ b/2026/board-advanced/completed/server.js
@@ -17,6 +17,21 @@ const posts = [];
 // 現在の接続の一覧。接続した順に並ぶ。
 const connections = [];
 
+// 15 秒ごとにコメント行を送り、データが流れない接続が切られないようにする
+function sendPing() {
+  for (let i = 0; i < connections.length; i++) {
+    connections[i].write(': ping\n\n');
+  }
+}
+setInterval(sendPing, 15000);
+
+// いまの接続数を、接続している全員に送る
+function sendCount() {
+  for (let i = 0; i < connections.length; i++) {
+    connections[i].write(`event: count\ndata: ${connections.length}\n\n`);
+  }
+}
+
 const server = createServer(async (req, res) => {
   const url = new URL(req.url, `http://${req.headers.host}`);
 
@@ -33,10 +48,12 @@ const server = createServer(async (req, res) => {
     res.flushHeaders();
     connections.push(res);
     console.log(`接続数: ${connections.length}`);
+    sendCount();
 
     function removeConnection() {
       connections.splice(connections.indexOf(res), 1);
       console.log(`接続数: ${connections.length}`);
+      sendCount();
     }
     req.on('close', removeConnection);
     return;
@@ -45,6 +62,11 @@ const server = createServer(async (req, res) => {
   // 投稿を 1 件受け取る
   if (req.method === 'POST' && url.pathname === '/posts') {
     const post = await readPost(req);
+    if (post.text === '') {
+      res.writeHead(400, { 'Content-Type': 'application/json; charset=utf-8' });
+      res.end(JSON.stringify({ message: 'text が空です' }));
+      return;
+    }
     posts.push(post);
     if (posts.length > MAX_POSTS) posts.shift();