Next.js · build error

npm ci: package.json and package-lock.json are not in sync

npm error `npm ci` can only install packages when your package.json and package-lock.json or npm-shrinkwrap.json are in sync. Please update your lock file with `npm install` before continuing.

What it means

Someone (often an agent fixing something else) changed the next version in package.json without updating package-lock.json, so a clean install refuses to run. The lockfile is what pins the exact version you tested: a range like ^16.4.3 in package.json allows any later 16.x, so without a committed lockfile each install can build against a different Next.js.

Common causes

• An agent edited the next, react or eslint-config-next version in package.json, or ran npm install next@latest, while fixing something else
• package-lock.json is in .gitignore, wasn't committed, or was deleted to "fix" an install error
• next, eslint-config-next and react changed in different commits

How to fix it

1. See what's installed: npm ls next react react-dom
2. Pin the exact version you last tested (use yours): npm pkg set dependencies.next=16.4.3 devDependencies.eslint-config-next=16.4.3, run npm install to update package-lock.json, then commit both files together
3. Use npm ci in CI and deploy builds so installs match the lockfile exactly
4. Upgrade on purpose in its own branch: npx @next/codemod@canary upgrade latest, then run the build and tests

npm ls next react react-dom
npm pkg set dependencies.next=16.4.3 devDependencies.eslint-config-next=16.4.3
npm install
git add package.json package-lock.json
git commit -m "Pin next to 16.4.3"
npm ci && npm run build

Prompt for Cursor or Claude Code

npm ci fails because package.json and package-lock.json are out of sync on next. Don't upgrade anything. Find which commit changed the next/react/eslint-config-next versions, restore the last version the app built with, pin it as an exact version (no ^), regenerate the lockfile with npm install, and confirm npm ci && npm run build pass. List every version you changed.

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