Redis authed session not available

import vercel from "@astrojs/vercel";
import { defineConfig, envField } from "astro/config";
import { loadEnv } from "vite";

const { BASE_URL, REDIS_URL } = loadEnv(process.env.NODE_ENV, process.cwd(), "");

// https://astro.build/config
export default defineConfig({
    output: "server",
    adapter: vercel(),
    /**
     * @link https://docs.astro.build/en/guides/sessions/#configuring-sessions
     * @link https://docs.astro.build/en/guides/integrations-guide/vercel/#sessions
     * @todo Fix session config by figuring out correct driver / config. Redis seems to be the only option, but it doesn't work with the provided config.
     */
    session: {
        /** @link https://unstorage.unjs.io/drivers/vercel */
        driver: "redis",
        options: {
            url: REDIS_URL
        },
        ttl: 60 * 60 * 24 // Results in 1 day
    },
    /**
     * @link https://docs.astro.build/en/guides/environment-variables/#type-safe-environment-variables
     * @link https://docs.astro.build/en/guides/environment-variables/#use-variables-from-your-schema
     */
    env: {
        schema: {
            DATABASE_URL: envField.string({
                context: "server",
                access: "secret",
                url: true,
                optional: true
            }),
            DATABASE_TOKEN: envField.string({
                context: "server",
                access: "secret",
                optional: true
            }),
            DATABASE_FILE: envField.string({
                context: "server",
                access: "secret",
                optional: true,
                default: "database.db"
            }),
            BETTER_AUTH_SECRET: envField.string({
                context: "server",
                access: "secret"
            }),
            BETTER_AUTH_URL: envField.string({
                context: "server",
                access: "secret",
                url: true,
                optional: true,
                startsWith: "http",
                default: BASE_URL
            }),
            REDIS_URL: envField.string({
                context: "server",
                access: "secret",
                url: true,
                default: "redis://default:"
            })
            // ... rest of code
        }
    }
});

import { auth, authRoot, loginPath } from "@lib/auth";
import type { APIContext } from "astro";
import { defineMiddleware } from "astro:middleware";

export function isAuthed(context: APIContext) {
    return auth.api.getSession({
        headers: context.request.headers,
    });
}

export const onRequest = defineMiddleware(async (context, next) => {
    const { pathname } = context.url;
    const isLogin = pathname === loginPath;
    if ((!isLogin && pathname.startsWith(authRoot)) || pathname.startsWith("/_server-islands/")) {
        return next();
    }
    const authed = await isAuthed(context);
    console.log("Session check result:", authed);
    if (authed) {
        context.locals.user = authed.user;
        context.locals.session = authed.session;
    } else {
        context.locals.user = null;
        context.locals.session = null;
        if (!isLogin) return context.redirect(loginPath);
    }
    return next();
});