User object disappears on redirect and not available for callbacks

[h1sashin]

Hello, I've a little problem with my next-auth config i guess. Didn't see problem like that along the internet so I'm asking here. So my callbacks don't have access to user object generated by authorize function. Especially jwt. I got a custom backend that returns just my JWT token, so I want to store it in session object but I think I'm doing something wrong, like I don't fully understand this library. This is my [...nextauth].ts file

import { Chain } from '@/zeus';
import { NextApiRequest, NextApiResponse } from 'next';
import NextAuth from 'next-auth';
import CredentialsProvider from 'next-auth/providers/credentials';

export default (req: NextApiRequest, res: NextApiResponse) =>
    NextAuth(req, res, {
        providers: [
            CredentialsProvider({
                id: 'Credentials',
                name: 'Credentials',
                type: 'credentials',
                credentials: {
                    username: { label: 'Username', type: 'string' },
                    password: { label: 'Password', type: 'string' },
                },
                authorize: async (params) => {
                    const resp = await Chain(process.env.NEXT_PUBLIC_HOST as string)('mutation')({
                        Login: [{ payload: { username: params!.username, password: params!.password } }, true],
                    }).catch((err) => {
                        throw new Error('Error has occured: ' + err);
                    });
                    return { token: resp.Login };
                },
            }),
        ],
        callbacks: {
            jwt: async ({ token, user }) => {
                if (!user?.token) throw new Error('You are not logged in');
                const resp = await Chain(process.env.NEXT_PUBLIC_HOST as string)('query')({
                    UserQuery: {
                        me: [
                            { token: user?.token },
                            {
                                email: true,
                                username: true,
                                _id: true,
                            },
                        ],
                    },
                });
                token = {
                    email: resp.UserQuery?.me.email,
                    name: resp.UserQuery?.me.username,
                };
                return token;
            },
            session: async ({ session, user, token }) => {
                session.accessToken = user.token;
                session.user = token;
                return session;
            },
            redirect: async ({ baseUrl }) => {
                return baseUrl;
            },
        },
        secret: process.env.NEXTAUTH_SECRET,
        pages: {
            signIn: '/login',
        },
    });

And there in api routes session is always null

import { NextApiRequest, NextApiResponse } from 'next';
import { getSession } from 'next-auth/react';
const checkSession = async (req: NextApiRequest, res: NextApiResponse) => {
    const session = await getSession({ req });
    res.json(session);
};

export default checkSession;

What's wrong there? I can't find any useful tutorial how to do it correctly.