Turbopack Error: esmExternals = "loose" is not supported

[0xtz]

Turbopack version: 664110c1

Error message:

esmExternals = "loose" is not supported

Debug info:
- Execution of get_entrypoints_with_issues_operation failed
- Execution of EntrypointsOperation::new failed
- Execution of Project::entrypoints failed
- Execution of Project::middleware_endpoint failed
- Execution of Project::middleware_context failed
- Execution of Project::edge_middleware_context failed
- Execution of AppProject::edge_ecmascript_client_reference_transition failed
- Execution of AppProject::edge_ssr_transition failed
- Execution of AppProject::edge_ssr_module_context failed
- Execution of AppProject::edge_shared_transition failed
- Execution of get_server_module_options_context failed
- Execution of NextConfig::import_externals failed
- esmExternals = "loose" is not supported

[weyniis]

I see u're running into this error when trying to use the esmExternals = "loose" setting with Turbopack:

"esmExternals = 'loose' is not supported"

u're not alone — this has confused a lot of people transitioning from Webpack to Turbopack in Next.js.

---

🧠 Explanation:

Turbopack is the new Rust-based bundler introduced in Next.js (as a replacement for Webpack), and it's still under active development. Many Webpack options are not yet supported — including esmExternals, especially the "loose" mode.

The esmExternals: "loose" config was used in Webpack mode to allow certain CommonJS packages to be treated as ESModules, improving compatibility and reducing bundle size. However, Turbopack does not currently support that flag, which is why u're seeing this failure.

---

📌 How to fix it:

✅ Option 1: Remove esmExternals: "loose" from your config

In your next.config.js, change or remove this setting:

// ❌ Don't use this with Turbopack
experimental: {
  esmExternals: "loose",
}

If u're switching to Turbopack (the default in some newer Next.js versions), just remove the esmExternals field entirely, or set it to false:

// ✅ Compatible with Turbopack
experimental: {
  esmExternals: false
}

 Option 2: Switch back to Webpack (if u rely on esmExternals)

If u're using packages that require "loose" mode to work, and u’re not ready to migrate to Turbopack, u can opt back into Webpack by explicitly disabling Turbopack:

// next.config.js
const nextConfig = {
  // Explicitly use Webpack instead of Turbopack
  experimental: {
    turbo: false,
    esmExternals: "loose"
  }
};

module.exports = nextConfig;

Tips

Some packages that depend on CommonJS or mixing ESM/CJS may still break in Turbopack.

u can monitor Turbopack’s progress here: https://turbo.build/pack/docs

If u need more stable support for complex package trees, sticking with Webpack (for now) is safer.

Summary

esmExternals: "loose" is not supported in Turbopack.

u can either remove it, or fallback to Webpack if your project depends on it.

Turbopack is still evolving — avoid using unsupported Webpack flags.