version: "1.0.1" name: tsdown-migrate description: Migrate TypeScript library projects from tsup to tsdown. Provides complete option mappings, config transformation rules, default value differences, and unsupported option alternatives so AI agents can intelligently perform migrations.

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

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

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.

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.

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.

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

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:

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

References

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