Question 1

Middleware in Next 15 — what breaks from the 14 upgrade?

Accepted Answer

Next 15 middleware keeps Edge by default but now supports `runtime: 'nodejs'`, async `NextRequest` helpers, and `instrumentation` sharing — update matchers and await `request` APIs.

Question 2

Caching defaults in Next 15 — what is no longer cached by default?

Accepted Answer

Next 15 makes `fetch` and `GET` Route Handlers dynamic by default — add `cache: 'force-cache'` / `'use cache'` or `export const dynamic = 'force-static'` to re-cache.

Question 3

What breaks when you upgrade to Next.js 16, and how do you migrate?

Accepted Answer

**Target Audience:** Frontend engineers preparing for Next.js interviews — useful for anyone who has to plan a 15 to 16 upgrade.
**Difficulty:** Medium

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:`. Every behaviour claim was checked against the Next.js 16.4.2 upgrade guide shipped in `node_modules/next/dist/docs/` and, in the **Verified** section, against real `next build` and `next start` output from a 16.4.2 lab app written i

Question 4

How do Next.js Devtools MCP and AGENTS.md change AI-assisted debugging?

Accepted Answer

**Target Audience:** Frontend engineers preparing for Next.js interviews — no prior knowledge of MCP needed.
**Difficulty:** Medium

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:`. Every behaviour claim was checked against the Next.js 16.4.2 docs (the AI agents and MCP guides, `agentRules`, `agentFeedback`) and, in the **Verified** section, against a 16.4.2 `next dev` server: its terminal, the files it wrote, and JSON-RPC calls t

Question 5

How do you migrate from the Pages Router to the App Router in Next.js?

Accepted Answer

**Target Audience:** Frontend engineers preparing for Next.js interviews — assumes basic React and App Router familiarity.
**Difficulty:** Hard

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:`. Every behaviour claim was checked against the Next.js docs shipped with the package (written on 16.3.4, re-reviewed against 16.4.2) and, in the **Verified** section, against real output from a build or a running server; each Verified block

Question 6

What changed about caching defaults in Next.js 15?

Accepted Answer