diff --git a/MEMORY.md b/MEMORY.md
index aa22ea4..07d9485 100644
--- a/MEMORY.md
+++ b/MEMORY.md
@@ -55,3 +55,8 @@
   - 标题：`Space Grotesk`
   - 正文：`Manrope`
   - 等宽：`JetBrains Mono`
+
+## 前端联调口径（2026-04-12）
+
+- `NEXT_PUBLIC_API_BASE_URL` 若误配为 loopback（`127.0.0.1/localhost`），前端运行时会在浏览器端自动改写为“当前页面主机 + 同端口（默认 8000）”，避免公网页面触发 PNA（Private Network Access）阻断。
+- 认证请求与 WebSocket 连接均统一复用该运行时 API 基址解析逻辑。
diff --git a/memory/2026-04-12.md b/memory/2026-04-12.md
index 3169717..f63a19f 100644
--- a/memory/2026-04-12.md
+++ b/memory/2026-04-12.md
@@ -214,3 +214,20 @@
     - 注册显示 `注册`
 - 验证：
   - 复跑 `docker compose build web`，构建成功，路由静态生成完成，最终输出 `fquiz-web  Built`。
+
+## 追加修复（浏览器 PNA 阻断导致登录失败）
+
+- 触发问题：
+  - 浏览器报错：从 `http://223.109.142.84:3000` 访问 `http://127.0.0.1:8000` 被拦截，提示
+    `The request client is not a secure context and the resource is in more-private address space loopback`。
+- 根因：
+  - 前端构建时注入 `NEXT_PUBLIC_API_BASE_URL=http://127.0.0.1:8000`，线上浏览器会把 `127.0.0.1` 解释为“访问者本机回环地址”，触发 PNA 安全策略阻断。
+- 处理：
+  - 新增前端 API 基址运行时解析（`web/src/lib/api.ts`）：
+    - 浏览器端若发现配置为 loopback（`127.0.0.1/localhost`），且当前页面不在 loopback 主机，则自动改写为当前页面主机 + 原端口（默认 `8000`）。
+  - 认证请求统一走运行时 API 基址（`web/src/components/auth-provider.tsx`）。
+  - WS 连接统一走运行时 API 基址（`web/src/components/ws-provider.tsx`）。
+  - 首页 `Ping` 与 API Base 展示改为运行时解析值（`web/src/app/page.tsx`）。
+- 验证：
+  - `cd web && npx eslint src/lib/api.ts src/components/auth-provider.tsx src/components/ws-provider.tsx src/app/page.tsx` 通过。
+  - `cd web && npx tsc --noEmit` 通过。
diff --git a/web/src/app/page.tsx b/web/src/app/page.tsx
index 9213e93..67a5dbf 100644
--- a/web/src/app/page.tsx
+++ b/web/src/app/page.tsx
@@ -4,7 +4,7 @@ import Link from "next/link";
 import { FormEvent, useEffect, useState } from "react";
 
 import { useAuth } from "@/components/auth-provider";
-import { API_BASE_URL, readApiError } from "@/lib/api";
+import { API_BASE_URL, getApiBaseUrl, readApiError } from "@/lib/api";
 
 type Mode = "login" | "register";
 type PingResponse = { message: string };
@@ -27,6 +27,7 @@ export default function Home() {
   const [busy, setBusy] = useState(false);
   const [error, setError] = useState("");
   const [pingResult, setPingResult] = useState<PingResponse | null>(null);
+  const [resolvedApiBaseUrl, setResolvedApiBaseUrl] = useState(API_BASE_URL);
 
   useEffect(() => {
     try {
@@ -47,6 +48,10 @@ export default function Home() {
     }
   }, []);
 
+  useEffect(() => {
+    setResolvedApiBaseUrl(getApiBaseUrl());
+  }, []);
+
   const handleSubmit = async (event: FormEvent<HTMLFormElement>) => {
     event.preventDefault();
     setError("");
@@ -78,7 +83,7 @@ export default function Home() {
 
   const handlePing = async () => {
     setError("");
-    const response = await fetch(`${API_BASE_URL}/api/v1/ping`, {
+    const response = await fetch(`${getApiBaseUrl()}/api/v1/ping`, {
       method: "GET",
       credentials: "include",
     });
@@ -106,7 +111,7 @@ export default function Home() {
 
       <section className="surface-card">
         <p className="text-sm text-muted">API Base URL</p>
-        <p className="mt-1 font-mono text-sm">{API_BASE_URL}</p>
+        <p className="mt-1 font-mono text-sm">{resolvedApiBaseUrl}</p>
       </section>
 
       {user ? (
diff --git a/web/src/components/auth-provider.tsx b/web/src/components/auth-provider.tsx
index 83e7a78..14c1e62 100644
--- a/web/src/components/auth-provider.tsx
+++ b/web/src/components/auth-provider.tsx
@@ -10,7 +10,7 @@ import {
   useState,
 } from "react";
 
-import { API_BASE_URL, readApiError } from "@/lib/api";
+import { getApiBaseUrl, readApiError } from "@/lib/api";
 import type { AuthTokenResponse, UserPublic } from "@/types/auth";
 
 type AuthContextValue = {
@@ -36,7 +36,7 @@ function withApiPath(path: string): string {
   if (path.startsWith("http://") || path.startsWith("https://")) {
     return path;
   }
-  return `${API_BASE_URL}${path}`;
+  return `${getApiBaseUrl()}${path}`;
 }
 
 export function AuthProvider({ children }: { children: React.ReactNode }) {
diff --git a/web/src/components/ws-provider.tsx b/web/src/components/ws-provider.tsx
index b27e84e..e08ba76 100644
--- a/web/src/components/ws-provider.tsx
+++ b/web/src/components/ws-provider.tsx
@@ -12,6 +12,7 @@ import {
 } from "react";
 
 import { useAuth } from "@/components/auth-provider";
+import { getApiBaseUrl } from "@/lib/api";
 import type { WsEventEnvelope, WsServerMessage, WsTicketResponse } from "@/types/ws";
 
 type TopicHandler = (event: WsEventEnvelope) => void;
@@ -25,7 +26,7 @@ type WSContextValue = {
 const WSContext = createContext<WSContextValue | undefined>(undefined);
 
 function toWebSocketUrl(path: string): string {
-  const base = process.env.NEXT_PUBLIC_API_BASE_URL ?? "http://localhost:8000";
+  const base = getApiBaseUrl();
   const url = new URL(path, base);
   url.protocol = url.protocol === "https:" ? "wss:" : "ws:";
   return url.toString();
diff --git a/web/src/lib/api.ts b/web/src/lib/api.ts
index 020967b..e2d4dff 100644
--- a/web/src/lib/api.ts
+++ b/web/src/lib/api.ts
@@ -1,5 +1,41 @@
-export const API_BASE_URL =
-  process.env.NEXT_PUBLIC_API_BASE_URL ?? "http://127.0.0.1:8000";
+const LOOPBACK_HOSTS = new Set(["localhost", "127.0.0.1", "::1"]);
+
+function trimTrailingSlash(value: string): string {
+  return value.replace(/\/+$/, "");
+}
+
+function isLoopbackHost(hostname: string): boolean {
+  return LOOPBACK_HOSTS.has(hostname.toLowerCase());
+}
+
+export function getApiBaseUrl(): string {
+  const configured =
+    process.env.NEXT_PUBLIC_API_BASE_URL?.trim() ?? "http://127.0.0.1:8000";
+
+  if (typeof window === "undefined") {
+    return trimTrailingSlash(configured);
+  }
+
+  let parsed: URL;
+  try {
+    parsed = new URL(configured);
+  } catch {
+    return trimTrailingSlash(configured);
+  }
+
+  const browserHost = window.location.hostname;
+  const shouldRewriteLoopback =
+    isLoopbackHost(parsed.hostname) && !isLoopbackHost(browserHost);
+  if (!shouldRewriteLoopback) {
+    return trimTrailingSlash(configured);
+  }
+
+  const port = parsed.port || "8000";
+  const pathname = parsed.pathname === "/" ? "" : parsed.pathname.replace(/\/+$/, "");
+  return `${parsed.protocol}//${browserHost}:${port}${pathname}`;
+}
+
+export const API_BASE_URL = getApiBaseUrl();
 
 export async function readApiError(response: Response): Promise<string> {
   try {