Migrating from tsup to tsdown Skill

Migrating from tsup to tsdown

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

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:

Rename config file: tsup.config.* → tsdown.config.*
Update imports: 'tsup' → 'tsdown'
Apply option mappings: Rename/transform options per tables below
Preserve tsup defaults: Explicitly set options that differ (format, clean, dts, target)
Update package.json: Dependencies, scripts, root config field
Remove unsupported options: Replace with alternatives where available
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 |

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 |

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)
- [ ] 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

Agent Skills: Migrating from tsup to tsdown

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