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Rspack Compatibility with React Server Components

Status: Supported as a GA path as of React on Rails Pro 17.0.0. Rspack is the default bundler for fresh installs, the generator scaffolds the native RSCRspackPlugin, and RSC-on-Rspack is covered end-to-end by a CI gate — the dummy-app-rspack-rsc-runtime-gate job in .github/workflows/pro-integration-tests.yml, which builds the client/server/RSC bundles with Rspack, boots Rails plus the Pro Node renderer, and runs the RSC Playwright suite so RSC routes are proven to render and hydrate under Rspack. The native plugin delivery tracked in issue #3488 is complete. The remaining production CSS/FOUC simplification follow-up is tracked in issue #4557. The remaining known limitations below (no official React endorsement of the react-server-dom-webpack runtime under Rspack; production-posture streamed-CSS coverage) are genuine and still apply.

This page documents the compatibility status of Rspack with React on Rails Pro's React Server Components (RSC) implementation.

Overview​

React on Rails Pro's RSC implementation uses a three-bundle architecture (client, server, RSC). The generator supports Rspack — when assets_bundler: rspack is detected in shakapacker.yml, all RSC config files are created in config/rspack/ instead of config/webpack/, and the server/client configs are scaffolded with the native RSCRspackPlugin instead of RSCWebpackPlugin.

The RSC implementation depends on the react-on-rails-rsc npm package, which provides bundler-specific manifest plugins plus a shared loader:

  • WebpackPlugin (react-on-rails-rsc/WebpackPlugin) — generates client/server component manifest files under webpack.
  • RspackPlugin (react-on-rails-rsc/RspackPlugin) — the rspack-native equivalent (RSCRspackPlugin). It emits the same manifest JSON schema using only standard rspack public APIs, so the RSC runtime resolves client references identically. Exported by stable react-on-rails-rsc 19.2.1 and later on the 19.2.x package line.
  • WebpackLoader (react-on-rails-rsc/WebpackLoader) — transforms 'use client' files into client reference proxies in the RSC bundle. Works under both webpack and rspack.

React and Package Version Policy​

RSC apps generated by React on Rails Pro 17.2 and later use React 19.3.x: react@~19.3.0 and react-dom@~19.3.0. Apps generated by React on Rails Pro 17.0 use react@~19.2.7, react-dom@~19.2.7, and [email protected]; that set stays supported and needs no change. Apps generated by 17.1.0 pin the superseded [email protected] and must move to stable 19.3.0 with React 19.2.x at 19.2.8 or later, or to 19.3.1 with React 19.3.x. React 19.0.x is no longer a supported Pro RSC runtime line in v17 because the generator, peer metadata, and node-renderer startup check target the coordinated React 19.2 and 19.3 package lines.

The React on Rails Pro 17.2 generator pins stable [email protected] for both webpack and rspack projects. Keep React, React DOM, and react-on-rails-rsc upgraded as a coordinated set.

Compatibility Matrix​

ComponentRspack CompatibleNotes
RSC bundle config (rscWebpackConfig.js)YesDoes not use WebpackPlugin; only uses loader + resolve settings
WebpackLoader (react-on-rails-rsc/WebpackLoader)YesStandard loader interface (this.resourcePath, source transform)
conditionNames: ['react-server', '...']YesRspack supports conditional exports resolution
React server file aliasesYesRspack supports exact aliases that keep React's RSC dispatcher shared
resolve.alias (react-dom/server: false)YesRspack supports alias to false
LimitChunkCountPluginYesGenerated configs use bundler-agnostic bundler.optimize.LimitChunkCountPlugin
Loader chain (SWC + Babel)YesGenerated config handles both function and Array rule.use styles
Manifest plugin (RSCRspackPlugin)YesNative rspack plugin; emits the same manifest schema (see below)
WebpackPlugin (react-on-rails-rsc/WebpackPlugin)Not used on rspackReplaced by RSCRspackPlugin under rspack; remains the webpack-only plugin
Three-bundle build (RSC_BUNDLE_ONLY, SERVER_BUNDLE_ONLY)YesEnvironment variable routing is bundler-agnostic

Manifest Plugin: Native RSCRspackPlugin​

The manifest plugin is the critical compatibility question. It generates react-client-manifest.json and react-server-client-manifest.json, which map client component file paths to their chunk IDs and bundle filenames. Without these manifests, the RSC runtime cannot resolve 'use client' component references during streaming.

Under webpack, this is RSCWebpackPlugin, which wraps React's react-server-dom-webpack/plugin and depends on webpack-internal APIs (webpack/lib/dependencies/*, webpack.AsyncDependenciesBlock, compilation.chunkGraph, the processAssets/thisCompilation/make hooks, etc.).

Under rspack, the generator instead scaffolds the native RSCRspackPlugin (react-on-rails-rsc/RspackPlugin). Rather than rely on Rspack's webpack-compatibility layer for those internal APIs, the native plugin emits the same manifest JSON schema using only standard rspack public APIs. It discovers 'use client' modules with a tagging loader, injects them as named async chunks, and walks compilation.chunkGroups at processAssets. Because the output schema is identical, the RSC runtime (buildServerRenderer / buildClientRenderer) works unchanged regardless of bundler. The two plugins share the same { isServer, clientReferences } options.

note

Why native instead of the webpack plugin under Rspack's compat layer? A controlled A/B on a real app showed the webpack-plugin path producing valid-looking manifests that still failed ~7/11 RSC routes at runtime under Rspack, while the native RSCRspackPlugin rendered and hydrated every route. The native plugin is therefore the supported Rspack path. That route-hydration coverage is now wired into this repo's CI as the dummy-app-rspack-rsc-runtime-gate job (it builds the three bundles with Rspack, boots Rails plus the Pro Node renderer, and runs the RSC Playwright suite on every qualifying change). The native-plugin delivery completed in issue #3488, which superseded the abandoned manifest-helper approach in PR #3385. The live production CSS/FOUC simplification follow-up is issue #4557.

How the RSC Bundle Avoids the Plugin​

The RSC bundle config (rscWebpackConfig.js) calls serverWebpackConfig(true), which skips adding the manifest plugin (RSCRspackPlugin under rspack, RSCWebpackPlugin under webpack). The RSC bundle only uses:

  1. The WebpackLoader to transform 'use client' files into client reference proxies
  2. conditionNames: ['react-server', '...'] to resolve React's server entry points
  3. React server file aliases to keep the RSC renderer and app Server Components on one React server package instance
  4. Aliases to exclude react-dom/server from the RSC bundle

The manifest plugin is only added to the server and client bundles.

Testing with Rspack​

To test RSC with Rspack in your project:

  1. Ensure assets_bundler: rspack is set in config/shakapacker.yml
  2. Run the RSC generator: rails generate react_on_rails:rsc
  3. Verify configs are in config/rspack/
  4. Build all three bundles and check for:
    • rsc-bundle.js in the output
    • react-client-manifest.json and react-server-client-manifest.json (from RSCRspackPlugin)
    • No Rspack compilation errors

Known Limitations​

  1. React on Rails does not use Rspack's native RSC system: As of Rspack v2, Rspack ships its own built-in RSC support. The Rspack 2.0 announcement calls it experimental low-level build support, and the API is namespaced under experiments.rsc and rspackExperiments.reactServerComponents, with builtin:swc-loader applying the directive transform. It renders through the react-server-dom-rspack runtime, which is built from a fork of React and published outside React's own releases. React on Rails Pro does not use that path. Its RSC integration is built on the react-on-rails-rsc package: manifest generation uses the native RSCRspackPlugin (standard rspack public APIs, not Rspack's webpack-compatibility layer), while the RSC bundle still uses the react-server-dom-webpack loader and runtime.

  2. The RSC bundle still uses the react-server-dom-webpack loader: Only the manifest plugin is bundler-native. The RSC bundle transforms 'use client' files with react-on-rails-rsc/WebpackLoader (which wraps React's react-server-dom-webpack node loader) under both bundlers. This loader is compatible with rspack.

  3. No official React Rspack support: The React team has not officially tested or endorsed the react-server-dom-webpack runtime with Rspack. React does not publish an Rspack binding either: react-server-dom-rspack, used by Rspack's own RSC path, is a pre-1.0 package built outside React's releases. Because React on Rails Pro uses React's official react-server-dom-webpack runtime, React's security fixes ship in that package rather than in a fork. An app is protected only once its lockfile resolves a patched version, including the copy react-on-rails-rsc depends on. For example, the October 2026 RSC advisory is fixed in react-server-dom-webpack 19.0.9, 19.1.10, 19.2.9, and 19.3.1. Check with your package manager (for example pnpm why react-server-dom-webpack), and update or override that transitive dependency if it still resolves a vulnerable release. The native RSCRspackPlugin is maintained by ShakaCode in react-on-rails-rsc.

CSS / FOUC Status​

The CSS and Styling guide documents the streamed-CSS (FOUC prevention) pipeline in full. The Rspack-specific facts, verified against [email protected] and a production dummy build (2026-07-09):

  • Manifest css arrays require the 19.2.1+ line. The RSCRspackPlugin in the 19.2.0 packages has no CSS collection at all. Without css arrays there are no stylesheet hints — no preload head start and no browser-side gating.
  • output.publicPath must be a concrete string. With 'auto' or a function, CSS files are omitted from the manifest (the build emits a compilation warning but succeeds).
  • CSS must be extracted via CssExtractRspackPlugin (css/mini-extract module type). Rspack's native CSS support (experiments.css) is not seen by the plugin's CSS-dependency recovery; split-chunks configs that move CSS outside the client reference's chunk group can also drop entries.
  • The reviewed default production builds still left the streamed reveal ungated — this is bundler-independent, not Rspack-specific: numeric chunk ids disable Path B, while Path A then depends on a manifest-listed preload href that arrives before the reveal. The 2026-07-09 production dummy capture under both bundlers still emitted only a non-blocking preload, so a streamed component can paint unstyled until its CSS applies. See production builds are the primary silent-no-op case for details and status.

The end-to-end FOUC e2e gate currently runs only against dev-mode (NODE_ENV=test) builds; production-posture coverage is tracked in issue #4557.