Repro: Cannot find module 'react' in Nitro SSR output (pnpm monorepo)

Summary

When building a TanStack Start + Nitro app in a pnpm monorepo where a workspace package uses use-sync-external-store/shim alongside @auth0/auth0-react, Nitro's SSR bundle leaves an unbundled __require("react") call in a CJS shim factory. The runner stage has no node_modules, so this call fails on every SSR request with Cannot find module 'react'.

This is not Alpine/musl-specific — the same error occurs on Debian/glibc images. It only works on macOS local dev because node_modules is present in the project tree, allowing Node.js to resolve react via normal module resolution.

Root Cause

Rolldown splits the SSR bundle into multiple chunks. When @auth0/auth0-react is present, Auth0 and React end up bundled together in _libs/auth0__auth0-react+react.mjs which exports an inlined require_react. However, use-sync-external-store/shim (from a separate workspace package) ends up in a different SSR chunk (_ssr/app-*.mjs) that does not have access to the inlined React. Instead it uses:

var require_use_sync_external_store_shim_production = /* @__PURE__ */ __commonJSMin(((exports) => {
    var React = __require("react"); // ← __require = createRequire(import.meta.url)
    ...
}));

__require is createRequire(import.meta.url) — a real Node.js require(). Because the Nitro runner stage only copies .output/ (not node_modules), require("react") has nothing to resolve against and throws.

Environment

	Version
Node.js	24
pnpm	10.28.0
react	19.2.3
nitro	3.0.260311-beta
@tanstack/react-start	1.167.9
@auth0/auth0-react	^2.16.0
use-sync-external-store	^1.5.0
vite	^8.0.3 (Rolldown)

Project structure

packages/
  ui/               ← workspace package with use-sync-external-store/shim + Radix UI
  web/              ← TanStack Start app, depends on @repro/ui + @auth0/auth0-react
Dockerfile          ← Alpine runner (reproduces)
Dockerfile.glibc    ← Debian runner (also reproduces — not Alpine-specific)

Steps to Reproduce

git clone <this-repo>
cd nitro-rolldown-musl-repro

# Build and run (Alpine — fails)
docker build -t repro .
docker run -p 3000:3000 repro
curl http://localhost:3000/

Actual Behavior

Server starts but crashes on the first SSR request:

Error: Cannot find module 'react'
Require stack:
- /app/.output/server/_ssr/app-<hash>.mjs
    at Module._resolveFilename (node:internal/modules/cjs/loader:1456:15)
    ...
  code: 'MODULE_NOT_FOUND',
  requireStack: [ '/app/.output/server/_ssr/app-<hash>.mjs' ]

Expected Behavior

Nitro's server bundle should be fully self-contained. CJS shim packages like use-sync-external-store/shim that transitively require react should have React bundled inline — not left as an external require() that relies on node_modules being present at runtime.

Workaround

After pnpm run build, copy React into the server output before the runner stage:

node -e "
  const fs = require('fs'), path = require('path');
  const r = path.dirname(require.resolve('react/package.json'));
  fs.cpSync(r, '.output/server/node_modules/react', { recursive: true, dereference: true });
"

In a Dockerfile, add this immediately after RUN pnpm run build.