# Prompt-to-PR: Migrate Next.js 14 to 16

> Step-by-step SOP for migrating a Next.js 14 App Router project to Next.js 16 — Turbopack, React 19, async APIs, and caching changes covered.

**Type:** Playbook  
**Tools:** Cursor, Claude Code, Codex, Windsurf  
**Stack:** Next.js, TypeScript  
**Difficulty:** hard  
**Updated:** 2026-06-08

---

Next.js 16 ships React 19, async `params` and `searchParams`, overhauled fetch caching, and Turbopack as the default dev bundler. This playbook keeps the agent focused on breaking changes and prevents it from rewriting working code.

## 1. Requirement

Upgrade `next` from 14.x to 16.x (and `react`/`react-dom` to 19.x) with zero functionality regressions. Address every breaking change surfaced by the official codemod and any residual issues not caught by it.

## 2. First Prompt

```txt title="First Prompt"
Migrate this project from Next.js 14 to Next.js 16. Follow these steps exactly.

Step 1 — run the official codemod:
  npx @next/codemod@latest upgrade latest --yes

Step 2 — manual fixes the codemod does not cover:
  a. `params` and `searchParams` in page.tsx/layout.tsx files are now Promises.
     Await them: `const { id } = await params;`
     Do NOT destructure params in the function signature.
  b. `cookies()`, `headers()`, `draftMode()` are now async.
     Add `await` before every call.
  c. `fetch()` is no longer cached by default in Route Handlers.
     Where caching is intentional, add `{ next: { revalidate: N } }`.
  d. Remove any `export const dynamic = 'force-dynamic'` that is now the default.
  e. Replace `<Image>` with the new `onLoad` prop signature if present.

Step 3 — update package.json:
  "next": "^16.0.0"
  "react": "^19.0.0"
  "react-dom": "^19.0.0"
  "@types/react": "^19.0.0"
  "@types/react-dom": "^19.0.0"

Do not change any business logic, UI, or database code.
List every file you changed and why.
```

## 3. Expected File Changes

```txt
package.json                          (next, react, react-dom, @types/react*)
src/app/**/page.tsx                   (await params / searchParams)
src/app/**/layout.tsx                 (await params where used)
src/app/api/**/route.ts               (await cookies/headers; fetch cache opts)
src/middleware.ts                     (if it uses deprecated config keys)
next.config.ts                        (remove deprecated options)
```

## 4. Review Checklist

- `package.json` pins `next` to `^16.0.0` and React to `^19.0.0`.
- Every page/layout that destructures `params` now awaits it first.
- No synchronous `cookies()` or `headers()` calls remain in Server Components.
- `fetch` calls that previously relied on implicit caching have an explicit `revalidate` or `no-store` option.
- `next.config.ts` has no deprecated `experimental.appDir` or `swcMinify` keys.
- TypeScript compiles with `bun tsc --noEmit` — zero errors.
- The agent did not rewrite any UI components or business logic.

## 5. Test Commands

```bash
# Install updated deps
bun install

# Type-check
bun tsc --noEmit

# Dev build (Turbopack default in Next.js 16)
bun dev

# Production build — catches async-params errors at compile time
bun run build

# Run existing test suite
bun test
```

## 6. Common Failures

- **`params.id` used before await** — TypeScript error: `Property 'id' does not exist on type 'Promise<...>'`. The agent sometimes awaits `params` in one file and not another.
- **`cookies()` not awaited** — runtime error: `cookies() was called outside of a Server Component`. Add `await`.
- **Turbopack incompatible webpack plugin** — `next dev` errors on a custom `webpack()` config. Turbopack ignores webpack plugins; migrate or conditionally gate them.
- **`@types/react` version conflict** — peer-dep mismatch between old component libraries and React 19 types. Pin `@types/react` to 19 and override if needed.
- **`useFormState` removed** — replaced by `useActionState` from `react`. The codemod usually catches this but confirm.

## 7. Fix Prompt

```txt title="Fix Prompt"
TypeScript reports: "Property 'slug' does not exist on type
'Promise<{ slug: string }>'" in src/app/blog/[slug]/page.tsx.

The page function signature must be:
  export default async function Page({ params }: { params: Promise<{ slug: string }> })

Then at the top of the function body:
  const { slug } = await params;

Apply the same fix to every other page or layout that destructures params
without awaiting. List every file changed.
```

## 8. PR Description

```md title="PR description"
## Chore: Migrate Next.js 14 → 16 + React 19

**Breaking changes addressed**:
- `params` / `searchParams` are now `Promise`s — awaited in all pages/layouts
- `cookies()` / `headers()` are now async — awaited in all Server Components
- Fetch caching defaults changed — explicit `revalidate` added where needed
- Removed deprecated `next.config.ts` keys (`swcMinify`, `experimental.appDir`)

**Tooling**: dev server now uses Turbopack by default (`next dev --turbopack`).

Run `bun run build` to confirm zero type errors before merging.
```