diff --git a/static/chat.html b/static/chat.html
index 3a95cc6..424852d 100644
--- a/static/chat.html
+++ b/static/chat.html
@@ -4,13 +4,42 @@
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no">
     <title>AI Support</title>
-    <link rel="stylesheet" href="/static/css/style.css">
-    <script src="/static/js/theme.js"></script>
+    <script>
+    (function () {
+        /* spec 2026-10-19 §4:路径前缀模式(/t/<slug>/)按当前地址推导基础路径;
+           Host 模式(根路径 "/")推导为空串——全部引用零变化。
+           拼接安全:match 命中即字符集白名单([a-z0-9-]),base 只做 URL 属性
+           赋值,绝不拼 HTML(防线 §2/§3);找不到标记(如 /admin 域)同样得空串。 */
+        var m = location.pathname.match(/^\/t\/[a-z0-9-]+\/$/);
+        var base = m ? location.pathname.slice(0, -1) : "";
+        window.__BASE_PATH__ = base;
+        function css(href) {
+            var l = document.createElement("link");
+            l.rel = "stylesheet"; l.href = base + href;
+            document.head.appendChild(l);
+        }
+        function js(src) {
+            var s = document.createElement("script");
+            s.src = base + src; s.async = false; /* 保序:theme 先于 chat */
+            document.head.appendChild(s);
+        }
+        css("/static/css/style.css");
+        document.addEventListener("DOMContentLoaded", function () {
+            /* js 装载放 DOMContentLoaded:head 动态插入的 async=false 脚本与
+               解析进度无形式化顺序保证,且 chat.js 顶层有同步 DOM 查询——
+               DOM 就绪后装载与原 body 末尾形态等价且保序 */
+            js("/static/js/theme.js");
+            js("/static/js/chat.js");
+            var img = document.querySelector(".avatar-img");
+            if (img) img.src = base + "/api/public/avatar";
+        });
+    })();
+    </script>
 </head>
 <body>
     <div class="chat-container">
         <div class="chat-header">
-            <div class="avatar"><span class="avatar-emoji">🤖</span><img class="avatar-img" alt="" src="/api/public/avatar"></div>
+            <div class="avatar"><span class="avatar-emoji">🤖</span><img class="avatar-img" alt=""></div>
             <div class="info">
                 <h1>AI Support</h1>
                 <div class="availability">Online now - Usually replies in minutes</div>
@@ -76,7 +105,5 @@
         </div>
         <div class="chat-footer"><span class="footer-left">By chatting, you agree to our <a target="_blank" rel="noopener">privacy policy</a>.</span></div>
     </div>
-
-    <script src="/static/js/chat.js"></script>
 </body>
 </html>
diff --git a/static/js/chat.js b/static/js/chat.js
index 98aa98b..0a22244 100644
--- a/static/js/chat.js
+++ b/static/js/chat.js
@@ -1,8 +1,12 @@
+// ---- basePath(spec 2026-10-19 §4):chat.html 引导脚本按 location.pathname ----
+// ---- 推导并挂载 window.__BASE_PATH__(路径模式 /t/<slug>,Host 模式空串)。 ----
+var B = window.__BASE_PATH__ || "";
+
 // ---- Task 10: brand 拉取(同源 /api/public/brand,下方两个 IIFE 共用) ----
 // 失败/超时(3s) → resolve(null) → 中性兜底,页面照常可用不白屏。
 var chatBrandReady = (function () {
     try {
-        var p = fetch("/api/public/brand", { cache: "no-store" }).then(function (r) {
+        var p = fetch(B + "/api/public/brand", { cache: "no-store" }).then(function (r) {
             return r.ok ? r.json() : null;
         }).catch(function () {
             return null;
@@ -129,7 +133,7 @@ var chatBrandReady = (function () {
     function getWsUrl() {
         var loc = window.location;
         var proto = loc.protocol === "https:" ? "wss:" : "ws:";
-        return proto + "//" + loc.host + "/ws/chat";
+        return proto + "//" + loc.host + B + "/ws/chat";
     }
 
     function connect(sessionKey) {
diff --git a/tests/test_chat_static.py b/tests/test_chat_static.py
new file mode 100644
index 0000000..f5270fa
--- /dev/null
+++ b/tests/test_chat_static.py
@@ -0,0 +1,43 @@
+"""chat.html 静态面前缀守卫(spec 2026-10-19 §4):页面级 __BASE_PATH__ 引导。
+
+chat 是页面而非嵌入脚本(与 widget.js 的脚本地址自推导不同),由 chat.html
+head 内联引导脚本按 location.pathname 推导 basePath 并动态装载 css/js/
+avatar;Host 模式(根路径 "/")推导为空串,所有引用零变化。守卫四点:
+引导脚本在场且走正则白名单 match、静态标签不再绝对引用、引导块零
+innerHTML(拼接安全防线 §2/§3)、chat.js 端点构造全部经 __BASE_PATH__。
+"""
+from pathlib import Path
+
+STATIC = Path(__file__).resolve().parent.parent / "static"
+
+
+def read(rel):
+    return (STATIC / rel).read_text(encoding="utf-8")
+
+
+def test_引导脚本存在且定义_BASE_PATH():
+    src = read("chat.html")
+    assert "__BASE_PATH__" in src
+    # 代码锚(非注释):base 推导必须走正则白名单 match
+    assert "location.pathname.match(" in src
+
+
+def test_不再有静态绝对引用标签():
+    src = read("chat.html")
+    assert 'href="/static/' not in src
+    assert 'src="/static/js/' not in src
+    assert 'src="/api/public/avatar"' not in src
+
+
+def test_引导脚本零innerHTML_拼接安全防线():
+    # 安全防线 §2/§3:base 只做 URL 属性赋值,绝不拼 HTML——
+    # 断言首个内联 <script>...</script> 块(引导脚本)内无 innerHTML
+    src = read("chat.html")
+    boot = src.split("<script>", 1)[1].split("</script>", 1)[0]
+    assert "innerHTML" not in boot
+
+
+def test_chatjs_端点全部走_BASE_PATH():
+    src = read("js/chat.js")
+    assert "__BASE_PATH__" in src
+    assert 'fetch("/api/' not in src