npm error ERESOLVE: unable to resolve dependency tree

What causes this error

Starting with npm 7, peer dependencies are resolved and validated strictly by default. If package A needs react@^18 as a peer and package B needs react@^19, npm can't satisfy both simultaneously in a flat tree and stops rather than silently installing a version that could break one of them.

The Fix

# 1. Read the actual conflict npm reports — it names both packages and
#    the exact version ranges that don't overlap

# 2. Best fix: upgrade the outdated package to a version that supports
#    the newer peer dependency
npm install some-outdated-package@latest

# 3. Last resort — bypass strict peer resolution (npm picks one,
#    which may cause subtle runtime bugs if the versions aren't actually compatible)
npm install --legacy-peer-deps

Common causes / variations

• Upgrading a major framework version (e.g. React 18 → 19) before all your UI library dependencies have caught up
• A stale, unmaintained package pinning an old peer dependency range that's now genuinely incompatible
• Installing a new package into a project that already has conflicting transitive peer requirements

Related errors

See also: Module not found tailwindcss after Vercel build, @prisma/client did not initialize.