[Start + Rsbuild] ssr environment has no splitChunks config → every shared module is duplicated into every server route chunk (291 MB server dist / 4-6 GB dev memory at 300 routes)

[childrentime]

Which project does this relate to?

Start

Describe the bug

With @tanstack/react-start/plugin/rsbuild, the server (ssr environment) build emits a separate copy of every shared module into every route chunk that imports it. Only the client environment gets a splitChunks config from the plugin (performance.chunkSplit: { strategy: 'custom', override: { chunks: 'async' } }); the ssr environment gets none, and Rsbuild turns optimization.splitChunks off for target: 'node' when no splitChunks config is present (rsbuild:split-chunks plugin: if (isServer) { if (splitChunks === false || Object.keys(splitChunks).length === 0) chain.optimization.splitChunks(false) }).

The RSC path already sets splitChunks: { preset: 'single-vendor' } for the server environment in createRsbuildEnvironmentPlan (start-plugin-core/src/rsbuild/planning.ts), but the non-RSC path sets nothing.

Effect on a real-sized app (same source, 300 file routes + 400 shared components, react-start 1.168.47, measured on an M5 MacBook, physical footprint):

	Rsbuild default	Rsbuild + environments.ssr.splitChunks: { chunks: 'all', minSize: 0 }	Vite (same source)
server dist (dist/server)	291 MB (311 files, each route chunk ~1 MB)	4 MB	1.4 MB
prod: memory after visiting all 300 routes	463 MB (JS heap 348 MB)	130 MB	147 MB
prod: avg first hit per route	7.2 ms	1.4 ms	2.0 ms
rsbuild build cold / peak memory	5.5 s / 1.9 GB	4.9 s / 1.4 GB	3.1 s / 0.7 GB
rsbuild dev memory (whole-session mean / after 40 HMR edits)	4.4 GB / 5.9 GB	1.4 GB / 1.7 GB	1.0 GB / 0.8 GB
dev HMR of a component imported by many routes (p50)	7.1 s	0.8 s	44 ms
dev HMR root component (p50)	938 ms	742 ms	39 ms
prod QPS	unchanged	unchanged	—

Client output is unaffected (shared modules above minSize are deduped by the client splitChunks), and module singletons still work at runtime (the webpack runtime caches modules by id, so a shared module's state is not duplicated even though its source is) — the cost is disk, parse/load time and, in dev, memory and rebuild time, all scaling with the number of routes.

Since a server bundle pays no per-request cost for extra chunks, it would make sense for the plugin to default the ssr environment to splitChunks: { chunks: 'all', minSize: 0 } (or a single server file), while still letting users override it via environments.ssr.

Rsbuild-native fix that works today:

// rsbuild.config.ts
export default defineConfig({
  plugins: [pluginReact(), tanstackStart()],
  environments: {
    ssr: { splitChunks: { chunks: 'all', minSize: 0 } },
  },
})

(chunks: 'all' alone is not enough because of the default minSize: 20000 — small shared modules are still inlined into every route chunk; preset: 'single-vendor' only dedupes node_modules.)

Complete minimal reproducer

https://github.com/childrentime/tanstack-start-rsbuild-ssr-chunk-duplication

Steps to Reproduce the Bug

1. git clone https://github.com/childrentime/tanstack-start-rsbuild-ssr-chunk-duplication && cd tanstack-start-rsbuild-ssr-chunk-duplication && pnpm install
2. pnpm build && pnpm check → Widget7 source appears in 10 server chunk(s) <-- duplicated (60 routes import 6 of 40 shared components each; the marker is a comment in src/components/Widget7.tsx)
3. pnpm build:fixed && pnpm check (same config + environments.ssr.splitChunks: { chunks: 'all', minSize: 0 }) → Widget7 source appears in 1 server chunk(s)
4. Optional: ROUTES=300 COMPONENTS=400 pnpm gen && pnpm build && du -sh dist/server to see it grow with route count.

Expected behavior

As a user, I expected the server build to emit each shared module once (like the client build and like the Vite adapter do), instead of once per route chunk that imports it — or at least a documented default for the ssr environment's splitChunks.

Screenshots or Videos

No response

Platform

• Router / Start Version: @tanstack/react-start 1.168.47 (@tanstack/start-plugin-core 1.171.37, @tanstack/react-router 1.170.30)
• OS: macOS 26.5.1 (Apple M5), Node 22.12.0
• Browser: n/a (build output)
• Browser Version: n/a
• Bundler: rsbuild
• Bundler Version: @rsbuild/core 2.1.13 (@rspack/core 2.1.10), @rsbuild/plugin-react 2.1.0

Additional context

Found while benchmarking the Vite vs Rsbuild adapters on the same Start app; happy to share the full benchmark harness/results if useful. The RSC code path (splitChunks: { preset: 'single-vendor' } for the server env) suggests configuring server chunking is already considered in scope for the plugin.

[chenjiahan]

Thanks for the feedback! It looks like we can address this directly in Rsbuild by improving the default splitChunks config for Node builds. PR for this: web-infra-dev/rsbuild#8332