Engineering
How A/B tests run inside a Next.js app: the proxy, caching, flicker, page speed and where data lives.
The anti-flicker tax: what hiding your page does to LCP
Client-side A/B tools either flash the original page before swapping in the variant, or hide the page until their script is ready. Both cost you. Here's what the defaults are, what the published measurements show, and how server-side assignment on Next.js avoids the choice.
Two ways to serve an A/B variant in the Next.js App Router
Static mode gives every variant its own cached render. Dynamic mode renders per request and fits apps that are already dynamic. How each works in Next.js 16, when init picks which, and what cacheComponents changes.
Four numbers per arm: running A/B tests without sending visitor data to a vendor
evolveAB computes Bayesian results and bandit weights without ever seeing a single visitor. The trick is that the statistics only need four sums per variant, and your own database can produce them.
