If I’m reading the code right, it support their hybrid model where your code can run in three places: the user’s browser, Vercel’s edge, and an actual server. It looks like the idea was for when code in the edge context to be able to call the server faster but it was not protected to keep anyone else from calling it directly.

If I he for that right, this is a security review failure since people perennially try that optimization and have it end poorly for reasons like this. It’s safer, and almost always less work, to treat all calls equally and optimize if needed rather than having to support an “internal” call type over the same interface.

As I understand it, the middleware runs before a request hits a page or API route.. so to avoid infinite loops from internal subrequests (URL rewrites, etc), Next.js tags them with the x-middleware-subrequest header. This tells the runtime to skip middleware for those requests and proceed directly to the target. Unfortunately this also works externally.