cloudflare/vinext

Build failure: CSS modules + PostCSS in the RSC environment

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#1,509 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

When an app combines CSS modules with PostCSS configuration in the RSC environment, the build fails with Custom deploy script failed: undefined (1). Likely a CSS pipeline interaction between @vitejs/plugin-rsc and PostCSS that does not handle CSS module class hashing through PostCSS-processed files.

CSS modules + PostCSS in RSC environment fails to build

Estimated Impact

~1 test failures across the deploy suite.

Affected Test Suites

  • test/e2e/app-dir/css-modules-rsc-postcss/css-modules-rsc-postcss.test.ts

Recommendation

  1. Reproduce first in vinext's own test suite. Add a fixture combining a *.module.css import with a custom postcss.config.js in the RSC environment.

  2. Run PostCSS before CSS-module class hashing. Make sure @vitejs/plugin-rsc runs PostCSS first, then the CSS-module class hashing pass.

Part of #1328.

Contributor guide

Research direction
Examine the failing test file and trace the CSS handling pipeline, focusing on the interaction between PostCSS and CSS module class hashing in the RSC environment.
Tech stack
typescriptreactvite
Domain
frontend
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.
Half day
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
TypeScriptReactVite
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60