Build fails at "Applying modifyConfig from Vercel" — TypeError: path argument undefined

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1

Symptom

Every Next.js 16 build on my Hobby project has failed since ~16:21 UTC today (2026-05-22) with this exact error during the “Applying modifyConfig from Vercel” step — before any user code compiles:

Detected Next.js version: 16.2.6
Running "pnpm run build"
> next build
Applying modifyConfig from Vercel
> Build error occurred
TypeError: The "path" argument must be of type string. Received undefined
at ignore-listed frames {
  code: 'ERR_INVALID_ARG_TYPE'
}
ELIFECYCLE  Command failed with exit code 1.
Error: Command "pnpm run build" exited with 1

Repro is platform-side, not code-side

  • Local pnpm run build passes cleanly on every commit
  • The exact same commit c1a6351 deployed successfully at 16:01 UTC and now fails at 16:33 UTC — no code changes, no env changes, no lockfile changes
  • The build worker’s pnpm version flipped from pnpm 9 → pnpm 10.28.0 between the two builds; Vercel CLI on worker also bumped from 54.2.0 → 54.3.0

What I tried

  1. Removed path.resolve(__dirname) from next.config.ts (in case __dirname was the culprit under ESM) — no change
  2. Pinned "packageManager": "pnpm@9.15.9" in package.json to force pnpm 9 — no change
  3. vercel deploy --force — no change

Affected

  • Project: mustwemuse-2641s-projects/jackiej-events (Hobby plan)
  • Framework: Next.js 16.2.6
  • Last successful prod build: dpl_kqkwxexu2… 16:01 UTC
  • First failure: dpl_84988ie0b… 16:21 UTC
  • Sample failing deploy IDs (all same error): 1kyngeyyz, 17gm92lpc, 1kft8mysi

Production is still serving the 2h-old build — nothing user-facing is broken — but I can’t ship any new commits until this is resolved. Looks like a regression in either Vercel CLI 54.3.0 or the build-worker config wrapper. Anyone else seeing this?