Fix: npm install failing due to peer dependency conflicts (Vercel)

You’ve added a new library to your project, it installed fine on your machine, and you’re ready to deploy. You push to GitHub, Vercel starts the build, and then... BAM. A giant red wall of text appears in the logs: "ERESOLVE: could not resolve dependency node_modules."

The build fails before it even starts. The culprit? Peer Dependencies.

This is the "Strict Registry Wall." Local development environments are often "dirty" or use permissive settings that ignore minor version conflicts. But Vercel starts every build with a clean slate and a strict adherence to the npm versioning contract. If two of your packages are fighting over which version of React or Radix UI they want, Vercel will shut down the build rather than risk a broken production site.

For "Vibe Coders" using AI to manage their packages, this is a top-tier deployment blocker. The AI suggests npm install, but it doesn't know how to handle the complex graph of a package-lock.json file.

In this guide, we’ll explain why your build works locally but fails on Vercel, identify the "Legacy Fix" that gets you unstuck today, and show you the "Sustainable Path" to healthy dependencies.

What are Peer Dependencies? (The Version Contract)

To fix the wall, you have to understand the contract.

A Peer Dependency is a package that a library expects you to already have installed. For example, a "React Calendar" library doesn't ship with its own copy of React—it expects you to provide a specific version of React.

If you have react@19.0.0 installed, but the calendar library only supports react@^18.0.0, you have a Peer Dependency Conflict.

Why it works locally but fails on Vercel

If it’s broken, why did it work on your computer?

1. npm v6 vs npm v7+: Older versions of npm ignored these conflicts. Modern versions treat them as fatal errors. You might be running an old version locally while Vercel uses the latest.
2. Dirty Node Modules: You might have remnants of an older version in your local node_modules that are masking the conflict.
3. The --force Reflex: You might have used npm install --force months ago and forgotten about it. Vercel doesn't use --force unless you tell it to.

The Temporary Fix: --legacy-peer-deps

If you are on a deadline and just need the site to go live, you can tell Vercel to "chill out" and use the old, permissive resolution strategy.

How to fix it:

Create a file named .npmrc in your project root and add this single line:

legacy-peer-deps=true

When Vercel runs npm install, it will read this file and ignore the peer dependency conflicts.

Warning: This is a band-aid. It can lead to "Double React" errors or subtle UI bugs if the version conflicts are truly incompatible. Use this only as a temporary measure.

The Permanent Fix: Using .npmrc for Environment Consistency

If you want to handle conflicts more cleanly, use the Overrides field in your package.json. This is the "Nuclear Option" where you tell npm: "I don't care what the library wants; force it to use THIS version."

{
  "name": "my-app",
  "overrides": {
    "react": "$react",
    "react-dom": "$react-dom"
  }
}

This ensures that every package in your tree uses the same version of React that your main project is using. For more on resolving UI-specific version hell, check our Radix UI dependency guide.

Auditing Your Dependencies: Identifying the Conflicting Package

If you want to solve the problem for real, you have to find the "bad actor."

Read the Vercel logs carefully. Look for the lines that say: Conflicting peer dependency: react@^18.0.0 Required by: some-old-library@1.2.3

Once you find the package that is holding your project back, you should either:

1. Update it: npm install some-old-library@latest
2. Replace it: Find a modern alternative that supports your version of React/Next.js.
3. Patch it: Use a tool like patch-package if you absolutely must keep the library.

FlowQL: Dependency Tree Management for Scaling Apps

Dependency hell is the "last 20%" of SaaS development. AI assistants are great at adding packages, but they are remarkably bad at Dependency Pruning and Lockfile Auditing. They will suggest adding package after package until your node_modules is a 2GB nightmare that is impossible to deploy.

At FlowQL, we help developers manage their technical debt. We provide the senior technical oversight to audit your package.json, untangle your version conflicts, and optimize your build pipeline for speed and stability.

If your "Vibe" is being killed by a red wall of npm errors, it's time for a professional audit.

Conclusion

The ERESOLVE error on Vercel is a sign that your dependency tree has become inconsistent. By using a .npmrc file for quick fixes and the overrides field for long-term stability, you can ensure that your app deploys perfectly every time.

Your Action Plan:

1. Add legacy-peer-deps=true to your .npmrc to unblock the build.
2. Audit the Vercel logs to find the conflicting package.
3. Update or override the conflicting version.
4. Remove the .npmrc once the tree is healthy.

Don't let a version mismatch stop your ship. [Book a session with FlowQL] and let’s get your dependency tree perfectly balanced.

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FAQ: Vercel npm Install Errors

Q: Should I use --force on Vercel? A: No. Using --force is more dangerous than --legacy-peer-deps because it can overwrite files and lead to unpredictable build failures. Stick to .npmrc.

Q: Does this happen with pnpm? A: pnpm is much stricter than npm. If you are using pnpm, you must use the pnpm.overrides field in package.json to resolve these issues.

Q: Why does my build timeout during npm install? A: This usually happens if you have a massive amount of dependencies or if you are using a slow private registry. Check our Vercel 504 Timeout guide for performance tips.