Sentry

Sentry is the default error-tracking tool for modern web apps and has earned that position by being genuinely good at what it does. The Next.js SDK captures client-side and server-side errors with a single install, groups errors intelligently by stack trace and release, and integrates release tracking via git SHA. Session Replay lets you watch exactly what the user did before the error. The most valuable feature for small teams is that the free tier is generous enough to cover real traffic while you figure out if you need a paid plan. Pair it with a single alert routed to Slack or PagerDuty and you have the error half of your observability stack on day one.