How to fix npm ERR! code E406 npm ERR! 406 Not Acceptable in npm

First, check which registry npm is currently configured to use:

npm config get registry

If you see a custom registry URL (not https://registry.npmjs.org/), that's likely where the issue is. Take note of the full URL for the next steps.

Ensure your registry URL follows the correct format:

For Artifactory:

npm config set registry https://artifacts.company.com/artifactory/api/npm/npm-remote/

For Nexus 3:

npm config set registry https://nexus.company.com/repository/npm/

For GitLab:

npm config set registry https://gitlab.company.com/api/v4/packages/npm/

The key is including the /api/ or /repository/ path that points to the npm proxy/repository.

Clear any cached data that might be causing issues:

npm cache clean --force

Then attempt your install again:

npm install

This removes stale cache entries that might be interfering with content negotiation.

Temporarily switch to the official npm registry to isolate the problem:

npm config set registry https://registry.npmjs.org/
npm install

If this succeeds, the issue is with your custom registry configuration. If it fails, the problem is elsewhere (network, authentication, etc.). Once confirmed, switch back to your custom registry and check the URL format from Step 2.

For GitHub dependencies in package.json, use the git:// protocol instead of https://:

{
  "dependencies": {
    "my-package": "git://github.com/user/repo.git#branch-name"
  }
}

For private registries requiring authentication, ensure your .npmrc has the correct credentials:

npm login --registry=https://artifacts.company.com/artifactory/api/npm/npm-remote/

Or add to .npmrc:

@myorg:registry=https://artifacts.company.com/artifactory/api/npm/npm-remote/
//artifacts.company.com/artifactory/api/npm/npm-remote/:_authToken=YOUR_TOKEN_HERE