[...nextauth].ts

//...
providers: [
  Twitch({
    clientId: process.env.TWITCH_ID,
    clientSecret: process.env.TWITCH_SECRET,
    authorization: { params: { scope: "openid user:read:email moderation:read" } },
    async profile(profile, tokens) {
      const id = profile.sub
      const res = await fetch(`https://api.twitch.tv/helix/users?id=${id}}`, {
      headers: { Authorization: `Bearer ${tokens.access_token}`, "Client-Id": process.env.TWITCH_ID },
    })
    const { data: [user] } = await res.json()
    return {
      id,
      name: profile.preferred_username,
      email: profile.email,
      image: profile.picture,
      description: user.description,
    }
  },
  }),
],
callbacks: {
  jwt({ token, user }) {
    token.description ??= user.description
    return token
  },
  session({ session, token }) {
    session.user.description = token.description
    return session
  },
}

// ...
declare module "next-auth/jwt" {
  interface JWT { description: string }
}
declare module "next-auth" {
  interface User { description: string }
  interface Session {user: User }
}
// ...