did you try using `context.waitUntil()`?

    if (!valid) {
        console.log('🛑 error ', model)
        const headers = new Headers();
        await new Promise((resolve) => setTimeout(resolve, 8000)); // Is it billable?
        headers.set('Set-Cookie', `app=default; Path=/; HttpOnly; Secure; Domain=domain.com; Max-Age=${60 * 60}`);
        return new Response('Overload', {
            status: 400,
            headers: headers
        });
    }