🔧 Next.js App Router Caching: What Actually Gets Cached (and What Doesn't)

You deployed a Next.js App Router app, called revalidatePath() after a mutation, and the page still shows stale data. Or you disabled caching on a fetch call and the page is somehow still cached. Or you migrated from Next.js 14 to 15 and everything that was working broke silently.

All of these happen because the App Router has four separate...