# Prompt to Migrate a Next.js 14 App to Next.js 16

> Structured AI agent prompt to migrate a Next.js 14 App Router project to Next.js 16 with breaking-change handling and incremental steps.

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

---

Use this prompt to drive a careful, incremental Next.js 14 to 16 migration — handling
the async `params`/`searchParams` API change, the React 19 peer dep, and deprecated
`next/headers` patterns before upgrading.

## Main Prompt

```txt title="Main Prompt"
You are migrating an existing Next.js 14 App Router project to Next.js 16.

Do this in the following ordered steps — do not skip steps or combine them.

Step 1 — Audit (read-only):
  - List every file that uses `params` or `searchParams` as synchronous props.
  - List every file that calls `cookies()`, `headers()`, or `draftMode()` without `await`.
  - List every `next/image` usage with a deprecated prop (e.g., `layout`, `objectFit`).
  - List the current `peerDependencies` for React and React DOM.
  Output the audit as a plain list. Stop and wait for my review.

Step 2 — Update deps:
  - Run: `bun add next@16 react@19 react-dom@19 @types/react@19 @types/react-dom@19`
  - Do NOT change any source files in this step.

Step 3 — Fix async params:
  - For every Page, Layout, and `generateMetadata` function that receives `params` or
    `searchParams`, make the function `async` and `await` the prop before use.
  - Type the props as `Promise<{ slug: string }>` (or the relevant shape), not `{ slug: string }`.

Step 4 — Fix async dynamic APIs:
  - Add `await` before every call to `cookies()`, `headers()`, and `draftMode()`.
  - If the call is inside a non-async function, make that function async.

Step 5 — Fix deprecated Image props:
  - Remove `layout="fill"` and replace with `fill` (boolean).
  - Remove `objectFit` and `objectPosition` props and move them to a `className` or `style`.

Step 6 — Verify:
  - Run `bun run typecheck` and fix all remaining TypeScript errors.
  - Run `bun run build` and confirm exit 0.

After each step, show the diff and wait for confirmation before proceeding.
```

## Implementation Notes

- In Next.js 15+, `params` and `searchParams` are `Promise`-wrapped. Accessing them synchronously
  will compile but throw at runtime — the audit in Step 1 is essential before upgrading.
- `cookies()` and `headers()` are async in Next.js 15+; the most common failure mode is calling
  them inside a function that is not marked `async`.
- React 19 removes some legacy APIs (`defaultProps` on function components, string refs). Run
  `bun run typecheck` between each step to surface these early.

## Expected File Changes

```txt
package.json                          (edited — next, react, react-dom versions)
bun.lock / package-lock.json          (edited automatically)
src/app/**/page.tsx                   (edited — async params)
src/app/**/layout.tsx                 (edited — async params)
src/app/**/generateMetadata           (edited — async params)
src/components/**/*.tsx               (edited — deprecated Image props)
```

## Acceptance Criteria

- `bun run typecheck` exits 0 after all steps.
- `bun run build` exits 0 with no warnings about deprecated APIs.
- All dynamic routes render correctly in `bun run dev`.
- No `params` or `searchParams` prop is accessed synchronously.

## Test Commands

```bash
bun run typecheck
bun run build
bun run dev
# visit several dynamic routes and confirm no runtime errors
grep -r "layout=\"fill\"" src/ && echo "FAIL: deprecated Image prop found"
grep -rn "cookies()\|headers()\|draftMode()" src/ | grep -v "await" && echo "WARN: unawaited dynamic API"
```

## Common AI Mistakes

- Skipping the audit step and making all changes in one pass, causing hard-to-trace errors.
- Typing async params as `{ params: { slug: string } }` instead of `{ params: Promise<{ slug: string }> }`.
- Forgetting to upgrade `@types/react` alongside `react`, leaving type mismatches.
- Not awaiting `cookies()` inside a nested helper function that itself is not async.

## Fix Prompt

```txt title="Fix Prompt"
The build fails with type errors on params or a runtime error about cookies/headers.
Fix in order:
1. For any page or layout receiving `params`, update the prop type to `Promise<{ slug: string }>` and
   add `const { slug } = await params;` at the top of the function body.
2. Find every call to `cookies()`, `headers()`, `draftMode()` not preceded by `await` and add it.
   Make the containing function async if it isn't already.
3. Run `bun run typecheck` and show me only the remaining errors.
```