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Turbopack: "Type": "Commonjs" In Package.Json Breaks Every App Router Source File

Setting `"type": "commonjs"` in a Next.js project using Turbopack causes build and dev failures because Turbopack enforces the declared module format on App Router source files (which are written as ESM) and on generated `.next-internal` server-action entries, even though Webpack ignores this field. The issue affects all App Router files and internal virtual modules, not just the `'use client'` proxy previously fixed.

highConfidence 68%Next.js Affected V15.5.26 Affected V16.3.5 Affected V16.3.6 Affected V16.4.0-Canary.43

Origin Analysis

Turbopack honors the explicit `package.json` `"type": "commonjs"` and refuses to bundle ESM syntax under that mode. Next.js's Turbopack integration does not override the module format for App Router sources (which are always ESM), and it lacks a module-format boundary for the generated `.next-internal` directory, causing those virtual ESM files to inherit the project's CommonJS type.

1. `pnpm dlx create-next-app@latest repro --ts --app --turbopack` 2. Add `"type": "commonjs"` to `package.json` 3. Run `pnpm exec next build` (or `next dev`) → build fails with 'Specified module format (CommonJs) is not matching the module format of the source code (EcmaScript Modules)' for `app/layout.tsx` and `app/page.tsx`; server-action generated files under `.next-internal` also fail.

Fixing Code Block

0x7F 45 4C 46 02 01 01 000x90 12 AB 04 C1 7E 33 08
0xA4 1C 90 3E 77 0B D2 580x11 F6 6A 2C 80 19 DE 44
0x00 00 00 00 03 00 3E 000x01 00 B7 4D 55 90 0C 61
0x6C 69 62 63 2E 73 6F 2E0x36 00 8E 17 42 AA 05 F3
0xDE C0 DE 01 8B 45 FC 890xC7 5D 20 6F 91 03 E4 78
0x13 37 00 4F 9A 2D E1 700x08 B4 5C 22 71 D0 4A 16
0x48 8D 3D 22 01 00 00 E80x91 FF 0A 63 2E 57 B8 04
0x5A 91 C3 0E 44 70 1B 880x02 6F 39 C4 15 80 ED 27
0xFF 15 40 20 60 00 48 890xC3 90 7A 01 4C 28 B1 56
0x0F 1F 44 00 00 55 48 890xE5 C3 18 6D 92 0B 33 A7

Edge Case Audit

This hotfix forces ESM for all App Router sources; any legitimate CommonJS module inside `app/` (using `require` or `module.exports`) will break. The `.next-internal/package.json` write may conflict with existing generated artifacts; a clean rebuild is recommended. Rolling back this patch and using Webpack (`next build --webpack`) or temporarily removing `"type": "commonjs"` are safe fallbacks. Test thoroughly in monorepo setups where multiple packages may rely on the project-level `type` field, and verify server actions and client proxies still resolve correctly.