Build failure: CSS modules + PostCSS in the RSC environment

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.

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Part of #1328.