Node.js 'Error [ERR_PACKAGE_PATH_NOT_EXPORTED]: Package subpath './lib/x' is not defined by "exports"' (or 'No "exports" main defined'). Why can't I import that file anymore?

Cause: a package's package.json can declare an "exports" field that lists exactly which entry points other code may import. Once a package uses "exports", Node refuses imports of any path not listed there, including deep imports into internal files (pkg/lib/x.js) that used to work. The error usually appears after upgrading a dependency that added or tightened "exports", or when the package exports only an ESM ('import') condition and you require() it (or the reverse: 'No "exports" main defined'). Fix: 1. Import the public entry point instead of the internal file: check the package's package.json "exports" map (node_modules/<pkg>/package.json) and its docs for the supported subpaths, and use one of those (e.g. import x from 'pkg/feature' if './feature' is exported). 2. If you used a deep import because something wasn't exposed, look for the documented replacement in the package's changelog or open an issue asking the maintainers to export it. 3. If the error mentions conditions ('import', 'require', 'default'), match the module system: use import (ESM) for packages that only export an 'import' condition, or a dynamic import() from CommonJS; on Node 20.19+ / 22.12+, require() of ESM also works for synchronous modules. 4. Temporarily pin the previous version of the dependency if you need time to migrate. 5. Avoid patching the package's exports in node_modules; it breaks on the next install. Tools like patch-package are a last resort. 6. In your own packages, list every public subpath in "exports" (and types conditions for TypeScript), since consumers can't reach anything else. Verify: node -e "import('pkg/feature').then(()=>console.log('ok'))" (or require.resolve for CommonJS) succeeds for the path you now use. Sources: https://nodejs.org/api/packages.html https://nodejs.org/api/errors.html