A request passes through five stages, and no single tool reaches all of them.
The overlap is where the cost lives. A page module is a plain export, so its synchronous parts are cheap to test; its asynchronous parts drag in the request scope, and the further a unit test pushes toward the server, the more of Next.js you end up mocking.
| Artifact | Best tested by | Why |
|---|---|---|
| Validators, formatters, data helpers | Vitest 102,989 or Jest 62,362 , no DOM | Ordinary modules |
| Client and synchronous Server Components | Vitest or Jest + Testing Library 221,825 | jsdom 21,691 is enough |
| Async Server Components | Playwright 26,280 , or await the function | No runner renders them as JSX |
| Server Action bodies | Vitest or Jest, next/cache mocked | Import and call directly |
| proxy.ts, caching, streaming | Playwright | Only the real server runs them |
Two rules follow. Push behavior you want covered quickly into plain modules a route merely calls, and budget for a real end-to-end run: caching, prefetching, streaming and proxy.ts have no stand-in.