If you need to upgrade Next.js 15 to 16 on a site that pulls content from Sanity, the good news is that the Sanity side is usually the quiet part. The risk sits in caching, revalidation, middleware and third-party packages that have not caught up. This post covers what to check, what tends to break, what it costs to put the upgrade off, and how much effort to expect.

I will only describe changes I am confident about. Always read the official Next.js 16 upgrade guide for the exact list, because it is the source of truth and I would rather you trust it than my memory of a single flag.

What changed in Next.js 16, at a practical level

Three themes matter for a Sanity-backed site.

Turbopack as the default bundler. Dev and build now go through Turbopack by default. If you have a custom webpack config, for example for SVG handling or aliasing, that is the first place a build can fail. Either port it or opt out explicitly while you plan the migration.

A more explicit caching model.…