WTF? I filled the template, don't understand how did it ended like that

If you think about it, if you don't need session information, hence const sessionId = context.cookies.get( "mycookiename" ) isn't needed.

Yes, but if I add const sessionId = context.cookies.get( "mycookiename" ) to middleware.ts, it gets run for every page, even the (prerender = true) ones where it doesn't function and it isn't needed, correct?

So the only ways to avoid the warning would be A) use prerender = false for all pages, regardless of whether they need session information, or B) Only call const sessionId = context.cookies.get( "mycookiename" ) from prerender = false pages (instead of calling it from middleware.ts). Am I understanding correctly?

Oooh, I see! That makes sense now! I'll think about it

In the future it might be nice to have 2 separate hooks, e.g. defineMiddlewarePrerenderTrue() and defineMiddlewarePrerenderFalse()

In my case I have refactored my code so that all of the session cookie retrieval happens inside my SessionData class. For my specific situation this is probably more natural, because the SessionData class was already needed for precisely those pages that require session access 🙂

That's a good idea and after tinkering a bit on that idea, I was able to create the defineMiddleWarePrerenderFalse like this which essentially is a wrapper and seems to be working for me.

const defineMiddlewarePrerenderFalse: typeof defineMiddleware = (fn) => {
  return defineMiddleware((context, next) => {
    if (context.isPrerendered) {
      return next();
    }

    return fn(context, next);
  });
};

// which works in the same style as the `defineMiddleware`
export const onRequest = defineMiddlewarePrerenderFalse(async (context, next) => {
  // but anything here will be ignored if prerendered
});

I am sure you come from a different background, so I understand what you mean. However, we document its behaviour in the documentation, so there's no unexpected things: https://docs.astro.build/en/guides/middleware/

Middleware allows you to intercept requests and responses and inject behaviors dynamically every time a page or endpoint is about to be rendered. This rendering occurs at build time for all prerendered pages, but occurs when the route is requested for pages rendered on demand.

Things like redacting, authenticating server-side tools that are per page, any shared logic between routes, populating the Astro.locals with information that needs to be accessible by components deep in the page without prop drilling, etc.

I'm pretty sure that was not available, or at least not documented when I opened the issue one year ago.
Is it documented now?

Very possibly! Sorry, should maybe have phrased that as “is now possible” — wasn’t paying attention to the original post date.

isPrerendered was added in Astro v5 and is documented here: https://docs.astro.build/en/reference/api-reference/#isprerendered

Ah, wonderful. Many thanks for the reference. Indeed this was 4.x era, so this counts as "done" or "implemented".
Thanks again.