Hey!
Thank you for the fast reply and the explanations.
I was actually using the 'Middleware types' page for reference, but without the adjustments I couldn't get it to work for me. A friend of mine suggested me to use these additional annotations and it helped to solve the issue. I'll ask him about it again :)

Hi, I'm that friend! I can confirm that I cannot get this to work without the declare global bit in a project of mine as well. The interesting thing is that it does work in a minimal reproduction. Not entirely sure what the issue is here, I'll keep digging.

Edit: It was the imports messing everything up. I think we should mention more clearly in the docs that normal imports can't be used in this file, as I feel like newcomers will just try to import types using the top-level import syntax. A quick note like
"If you want to use a type from another file, make sure to use inline imports. Otherwise, your types won't be applied."

Thanks for confirming! 😄 I updated this example, so I was surprised that it didn't work. Yeah, top-level imports/exports are probably always the culprit when it doesn't work.

But this is a Typescript thing, not Astro, so we generally don't document this kind of behavior. Unfortunately, even the Typescript documentation is not very clear on this (ambient/augmentation) and this question comes back often. So, I agree, there is probably something we can do here to make it more clear!

Because we use .env.d.ts in many pages, my suggestion would be an explanation on the Typescript guide that we could link from other pages to avoid repeating the same things everywhere.

The biggest issue is really the bit about inline imports. I've been writing TS for quite a while now and didn't know about that, so I feel like it might catch a lot of people off-guard! The other things (like the triple slash reference), even though unnecessary, don't really make a difference anyway. So while they should be removed from examples, it's not a big issue as it doesn't break anything.
Perhaps this is something we can discuss in Talking & Doc'ing to a greater extent?

Alright, we'll put it on the stack for Thursday!

For context, if someone has opinion on this (following T&D), we had this issue #10945 referring to both Middleware and Environment variables pages (and there is additional PR/issues linked in that issue). And I know we also have some examples in third party guides.
So, it would probably be helpful to have this information in the Typescript guide to be able to link to that page directly instead of repeating the same explanation everywhere.

What if we didn't recommend using a separate ambient module at all and instead recommended declaring the middleware types in the middleware file itself, which would avoid the entire confusion?

People can use declare global {} wrapping the namespace on the file itself, no need to introduce an extra concept that is quite confusing even for TS ninjas.

This is not a bad idea, but the problem with this solution is that not everyone encounters this behavior when using Middleware specifically. Some people notice this when using environment variables (#11243 for example), and some others could already have a env.d.ts file because they follow some other guides (e.g. Supabase & Astro) and decide to reuse that file. We use env.d.ts in different pages of the docs.

So, I think it could help reduce the number of confused people, but I don't think this would solve the issue entirely.

Perhaps this is something to discuss on Talking & Doc'ing again?