SessionToken sometimes missing if the callback isn't specified

What is the improvement or update you wish to see?

I was trying to fetch the session client side to get the sessionToken. The sessionToken would only sometimes be available in the session (e.g. when I refresh the page). To resolve this, I needed to add

 async session({ session }) {
      return session;
    },

in the session callbacks to force NextAuth to return the entire session each time. If this is the intended behaviour, it would be good to include this in the documentation.

Is there any context that might help us understand?

My setup:

1. On the top level, I have everything wrapped in the following (note I need a custom basePath):

<SessionProvider basePath="/auth" session={session}></SessionProvider>

2. I am using useQuery to make a call to my custom NestJS backend and need to pass it the session token in the authorisation headers.
3. When calling useSession, the sessionToken would only be present sometimes (I am assuming that there must be some caching)
4. Adding the above forced it to always return the entire session object
   Note: I am using a Resend provider and a PrismaAdapter

I don't know if this is the intended behaviour but would be great to include it in the docs if so 🙏
Thank you!

Does the docs page already exist? Please link to it.

No response

Start with the SessionProvider configured with basePath="/auth" and the useSession call, then compare behavior with and without the session callback shown in the report. Verify whether sessionToken is intentionally omitted or intermittently unavailable, and document the confirmed callback or configuration behavior if that is the intended result.