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npm ERR! ERESOLVE unable to resolve dependency tree

Exact error

npm ERR! code ERESOLVE
npm ERR! ERESOLVE unable to resolve dependency tree

What it means

npm (v7+) found conflicting peer dependency requirements and refuses to install.

Likely causes

• A package requires a different major version of React/Next than you have.
• Lockfile created with a different package manager or npm version.

How to fix it

1. Read the conflict lines: they name the package and the peer version it wants. Upgrade or downgrade to a compatible version.

2. If you accept the risk, install ignoring peer conflicts and commit the lockfile:

   npm install --legacy-peer-deps

3. To make Vercel use the same flag, add an .npmrc:

   # .npmrc
   legacy-peer-deps=true

Prevent this next time

• Add a CI job that runs `npm ci && npm run build` on every push, mirroring a clean Vercel build.
• Pin your Node.js version in package.json `engines` and commit your lockfile.

Guide: catch outages before your users do.

Related errors

• Command "npm run build" exited with 1