rolldown/tsdown-migrate

Migrating from tsup to tsdown

Knowledge base for AI agents to migrate tsup projects to tsdown — the Rolldown-powered library bundler.

Runtime Requirement

tsdown requires Node.js 22.18.0 or higher to run (build-time only). The bundled output can still target lower Node.js versions via the target option, so a library that previously supported Node.js 18 / 20 with tsup can continue to do so after migrating.

Recommended workflow when supporting Node.js 18 / 20:

• Build with Node.js 22+ in CI, setting an explicit target such as 'node18' or 'node20'.
• Test the built output (or the packed tarball) on the lower Node.js versions you need to support.

When to Use

• Migrating a project from tsup to tsdown
• Understanding differences between tsup and tsdown options
• Reviewing or fixing post-migration configuration issues
• Advising users on tsup→tsdown compatibility

Migration Overview

Follow these steps to migrate a tsup project:

1. Rename config file: tsup.config.* → tsdown.config.*
2. Update imports: 'tsup' → 'tsdown'
3. Apply option mappings: Rename/transform options per tables below
4. Preserve tsup defaults: Explicitly set options that differ (format, clean, dts, target)
5. Update package.json: Dependencies, scripts, root config field
6. Remove unsupported options: Replace with alternatives where available
7. Test build: Run tsdown and verify output

Config File Migration

File Rename

| tsup | tsdown | |------|--------| | tsup.config.ts | tsdown.config.ts | | tsup.config.cts | tsdown.config.cts | | tsup.config.mts | tsdown.config.mts | | tsup.config.js | tsdown.config.js | | tsup.config.cjs | tsdown.config.cjs | | tsup.config.mjs | tsdown.config.mjs | | tsup.config.json | tsdown.config.json |

Import and Identifier Changes

// Before
import { defineConfig } from 'tsup'

// After
import { defineConfig } from 'tsdown'

Replace all identifiers: tsup → tsdown, TSUP → TSDOWN.

Option Mappings

Property Renames

| tsup | tsdown | Notes | |------|--------|-------| | cjsInterop | cjsDefault | CJS default export handling | | esbuildPlugins | plugins | Now uses Rolldown/Unplugin plugins | | outExtension | outExtensions | Custom output extensions |

Deprecated but Compatible

These tsup options still work in tsdown for backward compatibility, but emit deprecation warnings and will be removed in a future version. Migrate them immediately.

| tsup (deprecated) | tsdown (preferred) | Notes | |--------------------|--------------------|-------| | entryPoints | entry | Also deprecated in tsup itself | | publicDir | copy | Copy static files to output | | bundle: true | (remove) | Bundle is default behavior | | bundle: false | unbundle: true | Preserve file structure | | removeNodeProtocol: true | nodeProtocol: 'strip' | Strip node: prefix | | injectStyle: true | css: { inject: true } | CSS injection | | injectStyle: false | (remove) | Default behavior | | external: [...] | deps: { neverBundle: [...] } | Moved to deps namespace | | noExternal: [...] | deps: { alwaysBundle: [...] } | Moved to deps namespace | | skipNodeModulesBundle | deps: { skipNodeModulesBundle: true } | Moved to deps namespace |

Output Filename Differences

For IIFE builds, tsdown emits names like [name].iife.js, while tsup commonly emitted [name].global.js. outExtensions customizes extensions or suffixes, but it does not remove the built-in .iife or .umd segment. Use outputOptions.entryFileNames: '[name].global.js' to preserve old IIFE filenames.

Dependency Namespace Moves

Dependencies config moved under deps namespace. If both external and noExternal exist, merge into a single deps object:

// Before (tsup)
export default defineConfig({
  external: ['react'],
  noExternal: ['lodash-es'],
})

// After (tsdown)
export default defineConfig({
  deps: {
    neverBundle: ['react'],
    alwaysBundle: ['lodash-es'],
  },
})

tsdown also adds deps.onlyBundle (whitelist of allowed bundled packages) — no tsup equivalent.

Plugin Import Transforms

// Before (tsup - esbuild plugins)
import plugin from 'unplugin-example/esbuild'

// After (tsdown - Rolldown plugins)
import plugin from 'unplugin-example/rolldown'

All unplugin-*/esbuild imports should change to unplugin-*/rolldown.

For complete before/after examples of every transformation, see guide-option-mappings.md.

Default Value Differences

tsdown changes several defaults from tsup. When migrating, explicitly set these to preserve tsup behavior, then let the user decide which new defaults to adopt.

| Option | tsup Default | tsdown Default | Migration Action | |--------|-------------|----------------|-----------------| | format | 'cjs' | 'esm' | Set format: 'cjs' to preserve | | clean | false | true | Set clean: false to preserve | | dts | false | Auto-enabled if types/typings in package.json | Set dts: false to preserve | | target | (none) | Auto-reads from engines.node in package.json | Set target: false to preserve |

After migration, suggest the user review these — tsdown's defaults are generally better:

• ESM is the modern standard
• Cleaning output prevents stale files
• Auto DTS from package.json reduces config
• Auto target from engines.node ensures consistency

Unsupported Options

These tsup options have no direct equivalent in tsdown. Remove them and inform the user.

| tsup Option | Status | Alternative | |-------------|--------|-------------| | splitting | Always enabled | Remove — code splitting cannot be disabled in tsdown | | metafile | Not available | Suggest devtools: true for Vite DevTools bundle analysis | | swc | Not supported | Remove — tsdown uses oxc for transformation (built-in) | | experimentalDts | Not supported | Use the dts option instead | | legacyOutput | Not supported | Remove — no alternative | | plugins (tsup experimental) | Incompatible | Migrate to Rolldown plugins manually; tsup's plugin API differs from Rolldown's |

Package.json Migration

Scripts

Replace tsup and tsup-node with tsdown in all script commands:

// Before
{
  "scripts": {
    "build": "tsup src/index.ts",
    "dev": "tsup --watch"
  }
}

// After
{
  "scripts": {
    "build": "tsdown src/index.ts",
    "dev": "tsdown --watch"
  }
}

Dependencies

| Location | Action | |----------|--------| | dependencies.tsup | Rename to dependencies.tsdown | | devDependencies.tsup | Rename to devDependencies.tsdown | | optionalDependencies.tsup | Rename to optionalDependencies.tsdown | | peerDependencies.tsup | Rename to peerDependencies.tsdown | | peerDependenciesMeta.tsup | Rename to peerDependenciesMeta.tsdown |

Root Config Field

If package.json has a root-level tsup field (inline config), rename to tsdown:

// Before
{ "tsup": { "entry": ["src/index.ts"] } }

// After
{ "tsdown": { "entry": ["src/index.ts"] } }

For detailed package.json examples, see guide-package-json.md.

New tsdown Features

After migration, suggest these tsdown-exclusive features to the user:

| Feature | Config | Description | |---------|--------|-------------| | Node protocol | nodeProtocol: true \| 'strip' | Add or strip node: prefix on built-in imports | | Workspace | workspace: 'packages/*' | Build multiple packages in a monorepo | | Package exports | exports: true | Auto-generate exports field in package.json | | Package validation | publint: true, attw: true | Lint package and check type correctness | | Executable | exe: true | Bundle as Node.js standalone executable (SEA) | | DevTools | devtools: true | Vite DevTools integration for bundle analysis | | Hooks | hooks: { 'build:done': ... } | Lifecycle hooks: build:prepare, build:before, build:done | | CSS modules | css: { modules: { ... } } | Scoped class names for .module.css files | | Glob import | globImport: true | Support import.meta.glob (Vite-style) |

For detailed comparisons, see guide-differences-detailed.md.

References

| Topic | Description | Reference | |-------|-------------|-----------| | Option Mappings | Complete before/after for every option transform | guide-option-mappings | | Detailed Differences | Architecture, features, compatibility comparison | guide-differences-detailed | | Package.json | Dependency, script, and config field migration | guide-package-json |

Migration Checklist

Use this checklist when performing a migration:

- [ ] Rename tsup.config.* → tsdown.config.*
- [ ] Update import from 'tsup' to 'tsdown'
- [ ] Replace tsup/TSUP identifiers with tsdown/TSDOWN
- [ ] Apply property renames (cjsInterop→cjsDefault, esbuildPlugins→plugins, outExtension→outExtensions)
- [ ] Migrate deprecated options (publicDir→copy, bundle→unbundle, removeNodeProtocol→nodeProtocol, injectStyle→css.inject)
- [ ] Move external/noExternal/skipNodeModulesBundle into deps namespace
- [ ] Update unplugin imports from /esbuild to /rolldown
- [ ] Set explicit defaults to preserve tsup behavior (format, clean, dts, target)
- [ ] Remove unsupported options (splitting, metafile, swc, etc.)
- [ ] Update package.json scripts (tsup→tsdown)
- [ ] Update package.json dependencies
- [ ] Rename root-level tsup config field if present
- [ ] Run tsdown and verify build output
- [ ] Suggest new tsdown features to the user