Vitest vs Jest in 2026: I Migrated My AI SaaS and Here's What Changed

Jest has been the default JavaScript testing framework for years. I used it without thinking. Then I migrated my AI SaaS to Vitest and the difference was immediate. Here's what actually changed. The Setup My project: a Next.js 15 app with Claude API integration, Drizzle ORM, and a test suite of ~140 unit and integration tests. The pain point: Jest was taking 28-34 seconds for a full suite run in CI. Not catastrophic, but noticeable enough that I'd started skipping test runs local...