Build failure: `esmExternals` config breaks production build

This issue was created by an agent analysing CI failures from the Next.js Deploy Suite (vinext main vs Next.js v16.2.6, 2026-05-22).

Problem

Test apps that enable esmExternals (or experimental.esmExternals) in next.config.js fail the production build with Custom deploy script failed: undefined (1). This blocks the entire esm-externals suite (10 tests).

Build fails when esmExternals is configured

Estimated Impact

~10 test failures across the deploy suite.

Affected Test Suites

• test/e2e/esm-externals/esm-externals.test.ts (10 failures)

Recommendation

1. Reproduce first in vinext's own test suite. Set experimental.esmExternals: true in next.config.js of a small fixture and confirm the build fails.

2. Handle the option or no-op it cleanly. Either implement the documented semantics (treat externals as ESM) or no-op the option to unblock the suite.

3. Investigate Rolldown ESM externals semantics. May involve adjusting how externals are externalised in each environment.

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Part of #1328.