vitejs/vite · error · Error

Multiple entry points are not supported when output formats

Error message

Multiple entry points are not supported when output formats include "umd" or "iife".

What it means

In resolveBuildOutputs (build.ts:1060), UMD and IIFE formats bundle everything into a single self-contained file and therefore cannot represent multiple entry points. If libOptions.entry is an object with more than one value and the formats include 'umd' or 'iife', Vite throws.

Source

Thrown at packages/vite/src/node/build.ts:1061

export function resolveBuildOutputs(
  outputs: OutputOptions | OutputOptions[] | undefined,
  libOptions: LibraryOptions | false,
  logger: Logger,
): OutputOptions | OutputOptions[] | undefined {
  if (libOptions) {
    const libHasMultipleEntries =
      typeof libOptions.entry !== 'string' &&
      libOptions.entry &&
      Object.values(libOptions.entry).length > 1
    const libFormats =
      libOptions.formats ||
      (libHasMultipleEntries ? ['es', 'cjs'] : ['es', 'umd'])

    if (!Array.isArray(outputs)) {
      if (libFormats.includes('umd') || libFormats.includes('iife')) {
        if (libHasMultipleEntries) {
          throw new Error(
            'Multiple entry points are not supported when output formats include "umd" or "iife".',
          )
        }

        if (!libOptions.name) {
          throw new Error(
            'Option "build.lib.name" is required when output formats include "umd" or "iife".',
          )
        }
      }

      return libFormats.map((format) => ({ ...outputs, format }))
    }

    // By this point, we know "outputs" is an Array.
    if (libOptions.formats) {
      logger.warn(
        colors.yellow(

View on GitHub (pinned to 89620f09af)

Solutions

1. Restrict formats to 'es' and/or 'cjs' when you have multiple entries.
2. Or reduce to a single string entry if you must output UMD/IIFE.
3. Split into separate builds, one per entry, each producing its own UMD bundle.

Example fix

// before
build: { lib: { entry: { a: 'src/a.ts', b: 'src/b.ts' }, formats: ['es', 'umd'] } }
// after
build: { lib: { entry: { a: 'src/a.ts', b: 'src/b.ts' }, formats: ['es', 'cjs'] } }

const lib = config.build?.lib
const multi = lib && typeof lib.entry !== 'string' && lib.entry && Object.values(lib.entry).length > 1
const fmts = lib?.formats ?? (multi ? ['es', 'cjs'] : ['es', 'umd'])
if (multi && (fmts.includes('umd') || fmts.includes('iife'))) {
  throw new Error('Multiple entries cannot use umd/iife formats')
}

function isSingleEntry(lib: any): boolean {
  return typeof lib.entry === 'string' ||
    (lib.entry && Object.values(lib.entry).length <= 1)
}

When it happens

Trigger: Configuring build.lib with an object entry map of 2+ entries and formats containing 'umd' or 'iife' (or relying on the default formats which include 'umd' for single, but for multi default to es/cjs).

Common situations: Multi-entry library configs where someone adds 'umd' to formats; expecting UMD output for each entry.

Related errors

• Entries in "build.rolldownOptions.output" must specify "name
• Name in package.json is required if option "build.lib.fileNa
• Option "build.lib.name" is required when output formats incl
• Either "build.lib.entry" or the top-level "input" option is
• Vite does not support "rolldownOptions.output.file". Please

AI-assisted analysis of vitejs/vite@89620f09af (2026-08-03). Data as JSON: /data/errors/01ffd8cdffca89cd.json. Report an issue: GitHub.