Firebase auth Problem in Custom API Routes

I used firebase authentication’s method signInWithEmailAndPassword in login api routes and user authenticated with this. Then I want to get user information with get-user api routes. I used auth in get-user api route. When user authenticates with login api routes auth has currentUser but when I want to get currentUser with auth in get-user api route I can’t see currentUser in auth so I want to fetch with get-user api route I always see user as null

/api/auth/login

// import { signInFB } from '@/app/libs/user';
import { NextResponse } from 'next/server';
import { serialize } from 'cookie';
import { SignJWT } from 'jose';
import { auth } from '@/app/libs/firebase';
import { signInWithEmailAndPassword } from 'firebase/auth';

const JWT_TOKEN = new TextEncoder().encode(process.env.JWT_SECRET!);

export const POST = async (req: Request) => {
  if (req.method !== 'POST') return NextResponse.json({ message: 'Invalid method' }, { status: 401 });

  try {
    const { email, password } = await req.json();

    const userCredential = await signInWithEmailAndPassword(auth, email, password);
    const user = userCredential.user;

    if (!user) return NextResponse.json({ message: 'giriş başarılı' }, { status: 401 });

    const payload = {
      id: user.uid,
      email: user.email,
    };

    const token = await new SignJWT(payload)
      .setProtectedHeader({ alg: 'HS256' })
      .setExpirationTime('1h')
      .sign(JWT_TOKEN);

    const response = NextResponse.json({ message: 'giriş başarılı', user }, { status: 200 });

    const cookie = serialize('authToken', token, {
      httpOnly: true,
      secure: process.env.NODE_ENV === 'production',
      sameSite: 'strict',
      path: '/',
      maxAge: 60 * 60,
    });

    response.headers.append('Set-Cookie', cookie);

    return response;
  } catch (error) {
    console.log('error:', error);
    return NextResponse.json({ message: 'giriş başarılı' }, { status: 500 });
  }
};

export const runtime = 'nodejs';

/api/auth/get-user

import { auth } from '@/app/libs/firebase';
import { getUserFromFS } from '@/app/libs/user';
import { NextRequest, NextResponse } from 'next/server';

export const GET = async (req: NextRequest) => {
  if (req.method !== 'GET') return NextResponse.json({ message: 'Invalid method' }, { status: 401 });

  const user = auth.currentUser;

  if (user) {
    const fetchedUser = await getUserFromFS(user.uid);

    return NextResponse.json({ fetchedUser }, { status: 200 });
  }

  return NextResponse.json({ message: 'No user!' }, { status: 401 });
};

getUserFromFS function:

export const getUserFromFS = async (uid: string) => {
  try {
    const userDocRef = doc(db, 'users', uid);
    const userRef = await getDoc(userDocRef);

    if (userRef.exists()) {
      return userRef.data() as userProfile;
    } else {
      return null;
    }
  } catch (error) {
    if (error instanceof FirebaseError) {
      // Hata türü FirebaseError ise hatayı olduğu gibi fırlatıyoruz
      throw error;
    } else {
      // FirebaseError değilse genel bir hata olarak fırlatıyoruz
      throw new Error('An unexpected error occurred');
    }
  }
};

I am expecting getting user information