Next 15 made upgrades feel like product work

Next.js 15 made a framework upgrade look like product work.

Async request APIs, changed caching defaults, and route behavior affect freshness, loading, metadata, and access control. The dependency diff is not the user-facing diff.

The first checks should be route-level:

• dynamic params resolve the right page
• query strings reach the intended component
• metadata matches the page
• protected routes still block correctly
• cached pages are cached deliberately
• live pages remain live deliberately

The async request change exposes assumptions quickly:

type Params = Promise<{ slug: string }>

export default async function Page({ params }: { params: Params }) {
  const { slug } = await params
  return <Article slug={slug} />
}

The caching change asks the same product question in another form. A dashboard should not behave like a blog post. A pricing page should not behave like a private workspace. The upgrade forces those choices into code.

Framework upgrades do not need ceremony for its own sake. They need smoke tests, a preview rollout, and a rollback path that does not require guessing which behavior changed. The test is whether the product still tells the truth after the framework moves underneath it.