diff --git a/apps/api/src/auth/auth.controller.spec.ts b/apps/api/src/auth/auth.controller.spec.ts
index 86d66fa..4ce8095 100644
--- a/apps/api/src/auth/auth.controller.spec.ts
+++ b/apps/api/src/auth/auth.controller.spec.ts
@@ -1,15 +1,18 @@
 import { describe, it, expect, beforeEach, vi } from "vitest";
 import { Test, TestingModule } from "@nestjs/testing";
 import type { AuthUser, AuthSession } from "@mosaic/shared";
+import type { Request as ExpressRequest, Response as ExpressResponse } from "express";
 import { AuthController } from "./auth.controller";
 import { AuthService } from "./auth.service";
 
 describe("AuthController", () => {
   let controller: AuthController;
-  let authService: AuthService;
+
+  const mockNodeHandler = vi.fn().mockResolvedValue(undefined);
 
   const mockAuthService = {
     getAuth: vi.fn(),
+    getNodeHandler: vi.fn().mockReturnValue(mockNodeHandler),
   };
 
   beforeEach(async () => {
@@ -24,25 +27,30 @@ describe("AuthController", () => {
     }).compile();
 
     controller = module.get<AuthController>(AuthController);
-    authService = module.get<AuthService>(AuthService);
 
     vi.clearAllMocks();
+
+    // Restore mock implementations after clearAllMocks
+    mockAuthService.getNodeHandler.mockReturnValue(mockNodeHandler);
+    mockNodeHandler.mockResolvedValue(undefined);
   });
 
   describe("handleAuth", () => {
-    it("should call BetterAuth handler", async () => {
-      const mockHandler = vi.fn().mockResolvedValue({ status: 200 });
-      mockAuthService.getAuth.mockReturnValue({ handler: mockHandler });
-
+    it("should delegate to BetterAuth node handler with Express req/res", async () => {
       const mockRequest = {
         method: "GET",
         url: "/auth/session",
-      };
+        headers: {},
+        ip: "127.0.0.1",
+        socket: { remoteAddress: "127.0.0.1" },
+      } as unknown as ExpressRequest;
 
-      await controller.handleAuth(mockRequest as unknown as Request);
+      const mockResponse = {} as unknown as ExpressResponse;
 
-      expect(mockAuthService.getAuth).toHaveBeenCalled();
-      expect(mockHandler).toHaveBeenCalledWith(mockRequest);
+      await controller.handleAuth(mockRequest, mockResponse);
+
+      expect(mockAuthService.getNodeHandler).toHaveBeenCalled();
+      expect(mockNodeHandler).toHaveBeenCalledWith(mockRequest, mockResponse);
     });
   });
 
diff --git a/apps/api/src/auth/auth.controller.ts b/apps/api/src/auth/auth.controller.ts
index 8b8f8d9..c632bbc 100644
--- a/apps/api/src/auth/auth.controller.ts
+++ b/apps/api/src/auth/auth.controller.ts
@@ -1,5 +1,6 @@
-import { Controller, All, Req, Get, UseGuards, Request, Logger } from "@nestjs/common";
+import { Controller, All, Req, Res, Get, UseGuards, Request, Logger } from "@nestjs/common";
 import { Throttle } from "@nestjs/throttler";
+import type { Request as ExpressRequest, Response as ExpressResponse } from "express";
 import type { AuthUser, AuthSession } from "@mosaic/shared";
 import { AuthService } from "./auth.service";
 import { AuthGuard } from "./guards/auth.guard";
@@ -88,37 +89,29 @@ export class AuthController {
    */
   @All("*")
   @Throttle({ strict: { limit: 10, ttl: 60000 } })
-  async handleAuth(@Req() req: Request): Promise<unknown> {
+  async handleAuth(@Req() req: ExpressRequest, @Res() res: ExpressResponse): Promise<void> {
     // Extract client IP for logging
     const clientIp = this.getClientIp(req);
-    const requestPath = (req as unknown as { url?: string }).url ?? "unknown";
-    const method = (req as unknown as { method?: string }).method ?? "UNKNOWN";
 
     // Log auth catch-all hits for monitoring and debugging
-    this.logger.debug(`Auth catch-all: ${method} ${requestPath} from ${clientIp}`);
+    this.logger.debug(`Auth catch-all: ${req.method} ${req.url} from ${clientIp}`);
 
-    const auth = this.authService.getAuth();
-    return auth.handler(req);
+    const handler = this.authService.getNodeHandler();
+    return handler(req, res);
   }
 
   /**
    * Extract client IP from request, handling proxies
    */
-  private getClientIp(req: Request): string {
-    const reqWithHeaders = req as unknown as {
-      headers?: Record<string, string | string[] | undefined>;
-      ip?: string;
-      socket?: { remoteAddress?: string };
-    };
-
+  private getClientIp(req: ExpressRequest): string {
     // Check X-Forwarded-For header (for reverse proxy setups)
-    const forwardedFor = reqWithHeaders.headers?.["x-forwarded-for"];
+    const forwardedFor = req.headers["x-forwarded-for"];
     if (forwardedFor) {
       const ips = Array.isArray(forwardedFor) ? forwardedFor[0] : forwardedFor;
       return ips?.split(",")[0]?.trim() ?? "unknown";
     }
 
     // Fall back to direct IP
-    return reqWithHeaders.ip ?? reqWithHeaders.socket?.remoteAddress ?? "unknown";
+    return req.ip ?? req.socket.remoteAddress ?? "unknown";
   }
 }
diff --git a/apps/api/src/auth/auth.rate-limit.spec.ts b/apps/api/src/auth/auth.rate-limit.spec.ts
index 89da36f..07bafb1 100644
--- a/apps/api/src/auth/auth.rate-limit.spec.ts
+++ b/apps/api/src/auth/auth.rate-limit.spec.ts
@@ -23,10 +23,17 @@ describe("AuthController - Rate Limiting", () => {
   let app: INestApplication;
   let loggerSpy: ReturnType<typeof vi.spyOn>;
 
+  const mockNodeHandler = vi.fn(
+    (_req: unknown, res: { statusCode: number; end: (body: string) => void }) => {
+      res.statusCode = 200;
+      res.end(JSON.stringify({}));
+      return Promise.resolve();
+    }
+  );
+
   const mockAuthService = {
-    getAuth: vi.fn().mockReturnValue({
-      handler: vi.fn().mockResolvedValue({ status: 200, body: {} }),
-    }),
+    getAuth: vi.fn(),
+    getNodeHandler: vi.fn().mockReturnValue(mockNodeHandler),
   };
 
   beforeEach(async () => {
@@ -76,7 +83,7 @@ describe("AuthController - Rate Limiting", () => {
         expect(response.status).not.toBe(HttpStatus.TOO_MANY_REQUESTS);
       }
 
-      expect(mockAuthService.getAuth).toHaveBeenCalledTimes(3);
+      expect(mockAuthService.getNodeHandler).toHaveBeenCalledTimes(3);
     });
 
     it("should return 429 when rate limit is exceeded", async () => {
diff --git a/apps/api/src/auth/auth.service.ts b/apps/api/src/auth/auth.service.ts
index c960766..d97553f 100644
--- a/apps/api/src/auth/auth.service.ts
+++ b/apps/api/src/auth/auth.service.ts
@@ -1,5 +1,7 @@
 import { Injectable, Logger } from "@nestjs/common";
 import type { PrismaClient } from "@prisma/client";
+import type { IncomingMessage, ServerResponse } from "http";
+import { toNodeHandler } from "better-auth/node";
 import { PrismaService } from "../prisma/prisma.service";
 import { createAuth, type Auth } from "./auth.config";
 
@@ -7,11 +9,13 @@ import { createAuth, type Auth } from "./auth.config";
 export class AuthService {
   private readonly logger = new Logger(AuthService.name);
   private readonly auth: Auth;
+  private readonly nodeHandler: (req: IncomingMessage, res: ServerResponse) => Promise<void>;
 
   constructor(private readonly prisma: PrismaService) {
     // PrismaService extends PrismaClient and is compatible with BetterAuth's adapter
     // Cast is safe as PrismaService provides all required PrismaClient methods
     this.auth = createAuth(this.prisma as unknown as PrismaClient);
+    this.nodeHandler = toNodeHandler(this.auth);
   }
 
   /**
@@ -21,6 +25,14 @@ export class AuthService {
     return this.auth;
   }
 
+  /**
+   * Get Node.js-compatible request handler for BetterAuth.
+   * Wraps BetterAuth's Web API handler to work with Express/Node.js req/res.
+   */
+  getNodeHandler(): (req: IncomingMessage, res: ServerResponse) => Promise<void> {
+    return this.nodeHandler;
+  }
+
   /**
    * Get user by ID
    */
diff --git a/apps/api/vitest.config.ts b/apps/api/vitest.config.ts
index 83f3f78..0e687b2 100644
--- a/apps/api/vitest.config.ts
+++ b/apps/api/vitest.config.ts
@@ -26,7 +26,7 @@ export default defineConfig({
   },
   plugins: [
     swc.vite({
-      module: { type: "es6" },
+      tsconfigFile: path.resolve(__dirname, "tsconfig.json"),
     }),
   ],
 });