Bug tracking for Next.js — session replay, console + HAR (2026)

Next.js 15 ships with the App Router, React Server Components, Turbopack, and PPR (Partial Pre-Rendering) as defaults. Each unlocks performance — and each introduces a class of bug your old Pages Router playbook does not cover. Hydration boundaries are now everywhere, edge runtime errors look different from Node errors, and the "use client" / "use server" boundary is a frequent source of mystery payloads.

A bug report from a Next.js app needs the route segment (was it server-rendered, streamed, edge-cached?), the request waterfall (which RSCs ran, what they fetched), and the hydration state. BugMojo captures the DOM + console + network HAR end-to-end, so you can see exactly where the server response ended and client interactivity began.

This guide covers the most common Next.js 15 bug classes, how to capture them cleanly, and how to wire BugMojo into a Next.js team's Jira/Linear workflow.

Framework-specific failure modes engineers actually ship through. Each is hard to spot in a screenshot and obvious in a session replay.

Real Next.js bug patterns — the symptom you will see in a report and the fix that actually works.

Hydration mismatch on dates rendered by server

Symptom: Date text flickers on first paint; console warning about hydration mismatch.

Fix: Render the date inside a client component with `suppressHydrationWarning`, or pass the timestamp as a number and format in `useEffect`. Never call `toLocaleString()` directly in a server component if it depends on user locale.

Server action throws but UI shows success

Symptom: Form submits, toast says "Saved", database has no row.

Fix: A throw inside a server action without explicit error handling silently resolves the action. Wrap in try/catch and return `{ ok: false, error }`. BugMojo's network HAR shows the 200 response with an error payload.

Cookies set in a server component never reach the browser

Symptom: User logs in, redirect happens, but next request is unauthenticated.

Fix: Cookies can only be set in Route Handlers, Server Actions, or middleware — not inside a Server Component render. Move the cookie-set to the action that triggered the login.

For source-mapped errors with App Router, upload your `.next/static/chunks/*.map` to BugMojo or wire it to your Vercel build hooks.

The honest comparison — where BugMojo wins, and where another tool might serve you better.