cloudflare/vinext

Build failure: `esmExternals` config breaks production build

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#1,507 opened on May 22, 2026

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Description

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.

Part of #1328.

Contributor guide

Research direction
Reproduce the issue by setting `experimental.esmExternals: true` in a small Next.js fixture using vinext, then trace the build failure to understand how externals are handled in vinext's Rolldown based build. Compare with Next.js behavior and implement a fix or no op.
Tech stack
typescriptnextjsvite
Domain
build systemtooling
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
TypeScriptGit
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60