Why the object form stopped working
Rollup’s output.manualChunks accepted two shapes: a function that gets called once per module and returns the chunk name it belongs to, or a plain object mapping chunk names to arrays of module IDs, a common shorthand for a simple vendor/app split. Vite’s migration guide is direct about what changed: “The object form output.manualChunks option is not supported anymore. The function form output.manualChunks is deprecated.”
Rolldown only implements the function form. The object-map shorthand (arguably the more commonly used of the two, since it’s the one most copy-pasted vendor-chunk snippets use) is rejected at build time, not silently ignored:
Warning: Invalid output options (1 issue found) - For the 'manualChunks'. Invalid type: Expected Function but received Object.That exact error comes from a real migration tool’s own config validator (voidzero-dev/vite-plus#900, opened 2026-03-15), hit while trying to convert a project’s Rollup-era config to Rolldown. The same shape of error appears in a plain Vite 8 build with an object-form manualChunks, not just inside that specific tool.
Fix it: rewrite as a function
Before: the object form
export default defineConfig({
build: {
rollupOptions: {
output: {
manualChunks: {
vendor: ["react", "react-dom"], // BROKEN in Vite 8: object form rejected
},
},
},
},
});After: the same split, as a function
export default defineConfig({
build: {
rollupOptions: {
output: {
manualChunks(id) {
// FIXED: same vendor-chunk logic, called per module
if (
id.includes("node_modules/react") ||
id.includes("node_modules/react-dom")
) {
return "vendor";
}
},
},
},
},
});The function receives each module’s resolved ID and runs once per module during the build. Returning a string assigns that module to a named chunk; returning nothing leaves Rolldown’s default chunking in place for it, the same contract the function form already had under Rollup.
Consider skipping manualChunks entirely
If your object form was a simple vendor/app split rather than something tuned
for a specific loading strategy, removing manualChunks and letting
Rolldown’s own chunking handle it automatically is a real option, not just a
fallback. Rolldown also exposes a codeSplitting option built for this exact
case, which is worth checking before porting a complex object map line by line
into a function.
Confirmed version range
Confirmed via Rolldown’s real-world error output, cited above, against rolldown@1.0.0-rc.9 and vite@8.0.0. Documented as an intentional, permanent removal in Vite’s own current migration guide, not a temporary regression. This site’s own astro.config.mjs doesn’t customize manualChunks at all, so this specific error wasn’t independently reproducible against this repo’s own build. Same key rename to check while you’re here: Fix Vite 8’s build.rollupOptions Deprecation - manualChunks is the single most common thing people configure inside that renamed key. Browse more posts like this in the Guides & Fixes archive.
