You upgrade from Next.js 14 to 15. The app compiles, the tests pass, nothing visibly breaks. A week later your database bill is higher, your pages feel slower, and your Time to First Byte graph has a step in it that lines up exactly with the deploy.

Nothing is broken. Something that used to happen for free stopped happening.

What Changed

In Next.js 14, data fetching was cached by default . A fetch in a Server Component ran once, the result was stored, and later requests reused it. You had to opt out of caching.

In Next.js 15, the default flipped. Per the upgrade notes:

fetch requests are no longer cached by default.

GET route handlers are no longer cached by default.

The client-side router cache no longer holds page segments between navigations by default.

So code you wrote once and never touched now behaves differently:

// app/products/page.tsx export default async function ProductsPage () { const res = await fetch ( ' https://api.example.com/products ' ); const products = await res .…