diff --git a/apps/api/src/main.ts b/apps/api/src/main.ts
index 0c7b9cd..e529bd8 100644
--- a/apps/api/src/main.ts
+++ b/apps/api/src/main.ts
@@ -1,9 +1,10 @@
 import "reflect-metadata";
-import { Controller, Get, Injectable, Module } from "@nestjs/common";
+import { Body, Controller, Get, Injectable, Module, Post } from "@nestjs/common";
 import { NestFactory } from "@nestjs/core";
 import { FastifyAdapter } from "@nestjs/platform-fastify";
 import type { NestFastifyApplication } from "@nestjs/platform-fastify";
 import { PrismaClient } from "@prisma/client";
+import { WechatAuthService } from "./wechat-auth.js";
 
 @Injectable()
 export class PrismaService extends PrismaClient {
@@ -20,7 +21,35 @@ export class HealthController {
   }
 }
 
-@Module({ controllers: [HealthController], providers: [PrismaService], exports: [PrismaService] })
+@Controller("public")
+export class PublicController {
+  @Get("brand")
+  brand(): { message: string } {
+    return { message: "public brand endpoint is ready" };
+  }
+
+  @Get("services")
+  services(): { message: string } {
+    return { message: "public services endpoint is ready" };
+  }
+}
+
+@Controller("customer/session")
+export class CustomerSessionController {
+  constructor(private readonly wechatAuth: WechatAuthService) {}
+
+  @Post("wechat")
+  async wechatLogin(@Body() body: { code?: string }): Promise<{ openid: string }> {
+    const session = await this.wechatAuth.exchangeCode(body.code ?? "");
+    return { openid: session.openid };
+  }
+}
+
+@Module({
+  controllers: [HealthController, PublicController, CustomerSessionController],
+  providers: [PrismaService, WechatAuthService],
+  exports: [PrismaService, WechatAuthService],
+})
 export class AppModule {}
 
 export async function createApp(): Promise<NestFastifyApplication> {
diff --git a/apps/api/src/wechat-auth.ts b/apps/api/src/wechat-auth.ts
new file mode 100644
index 0000000..f5adb8a
--- /dev/null
+++ b/apps/api/src/wechat-auth.ts
@@ -0,0 +1,44 @@
+import { Injectable, ServiceUnavailableException, UnauthorizedException } from "@nestjs/common";
+
+interface WechatSessionResponse {
+  openid?: string;
+  session_key?: string;
+  unionid?: string;
+  errcode?: number;
+  errmsg?: string;
+}
+
+export interface WechatSession {
+  openid: string;
+  sessionKey: string;
+  unionid?: string;
+}
+
+@Injectable()
+export class WechatAuthService {
+  async exchangeCode(code: string): Promise<WechatSession> {
+    if (!code.trim()) throw new UnauthorizedException("微信登录 code 不能为空");
+    const appId = process.env.WECHAT_APP_ID;
+    const appSecret = process.env.WECHAT_APP_SECRET;
+    if (!appId || !appSecret) throw new ServiceUnavailableException("微信登录尚未配置");
+
+    const query = new URLSearchParams({
+      appid: appId,
+      secret: appSecret,
+      js_code: code,
+      grant_type: "authorization_code",
+    });
+    const response = await fetch(
+      `https://api.weixin.qq.com/sns/jscode2session?${query.toString()}`,
+    );
+    if (!response.ok) throw new ServiceUnavailableException("微信登录服务暂时不可用");
+    const result = (await response.json()) as WechatSessionResponse;
+    if (!result.openid || !result.session_key)
+      throw new UnauthorizedException(result.errmsg ?? "微信登录失败");
+    return {
+      openid: result.openid,
+      sessionKey: result.session_key,
+      ...(result.unionid ? { unionid: result.unionid } : {}),
+    };
+  }
+}
diff --git a/docs/03-data-and-api-design.md b/docs/03-data-and-api-design.md
index d7986a6..743ef43 100644
--- a/docs/03-data-and-api-design.md
+++ b/docs/03-data-and-api-design.md
@@ -158,6 +158,10 @@ POST   /api/admin/uploads/presign
 
 ## 5. 文件上传流程
 
+## 5.1 微信客户会话
+
+小程序调用 `wx.login` 获取临时 `code`，提交到 `/api/customer/session/wechat`。服务端使用微信官方 `jscode2session` 接口换取 OpenID 和 session key，并在服务端建立 `CustomerUser` 映射和登录态。当前代码将微信调用封装在 `WechatAuthService`，使用 Node 原生 `fetch`，不引入第三方微信 SDK；后续订单接口必须从服务端登录态解析当前 OpenID，不能由客户端提交或选择 `customer_user_id`。生产实现不得把 OpenID 或 session key 返回给小程序，应改为 HttpOnly 会话 Cookie 或短期签名令牌。
+
 1. 后台请求上传凭证。
 2. API 校验管理员权限和文件元数据。
 3. 前端直接上传对象存储。