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.

Target Version: Two-Stage Migration

tsdown v0.23 removed all previously-deprecated tsup compatibility options — bundle, outExtension, publicDir, removeNodeProtocol, injectStyle, and skipNodeModulesBundle are no longer recognized. They fail TypeScript type checking and are silently ignored at runtime, so a missed mapping on v0.23+ produces wrong output without any error. Migrating directly to v0.23+ is therefore unsafe. tsdown v0.22.14 is the last version that still accepts these options and flags each one with a deprecation warning, making it the safe migration checkpoint. Migrate in two stages:

Stage 1 — migrate on tsdown@0.22.14: Install tsdown@0.22.14, migrate the config per the tables below, run a build, and resolve every deprecation warning by mapping each flagged tsup option to its real tsdown equivalent. The warnings are the completeness check — the migration is not done until the build produces zero warnings.
Stage 2 — upgrade to the latest tsdown (^0.23.0 or newer): Only after a warning-free build on 0.22.14. Since the config no longer uses any removed compat options, the silent-ignore behavior of v0.23+ is no longer a risk.

Runtime Requirement

tsdown requires Node.js ^22.18.0 || ^24.11.0 || >=26.0.0 to run (build-time only) — that is, Node.js 22.18+, 24.11+, or 26+. Odd-numbered and EOL release lines (e.g. Node.js 23, 25) are not supported. 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:

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 (Stage 1: tsdown@0.22.14), scripts, root config field
Remove unsupported options: Replace with alternatives where available
Test build on 0.22.14: Run tsdown and resolve every deprecation warning — zero warnings required
Upgrade to latest tsdown (^0.23.0 or newer) and verify the build again

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 | |------|--------|-------| | entryPoints | entry | Also deprecated in tsup itself | | cjsInterop | cjsDefault | CJS default export handling | | esbuildPlugins | plugins | Now uses Rolldown/Unplugin plugins | | outExtension | outExtensions | Custom output extensions | | 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 | | skipNodeModulesBundle | deps: { neverBundle: true } | Externalize all dependencies |

None of the old names are recognized by tsdown v0.23+ — always emit the new names. The compatibility options (outExtension, skipNodeModulesBundle, publicDir, bundle, removeNodeProtocol, injectStyle) were accepted with deprecation warnings up to v0.22.14 and removed entirely in v0.23; on v0.23+ leftovers are silently ignored, not errors, so builds misbehave without warning. This is why Stage 1 runs on v0.22.14, where every leftover is flagged.

Deprecated but Still Accepted

external and noExternal are the only tsup option names tsdown v0.23 still accepts. They emit deprecation warnings, will be removed in a future version, and cannot be combined with their replacements (mixing external with deps.neverBundle, or noExternal with deps.alwaysBundle, throws an error). Always emit the replacements.

| tsup (deprecated) | tsdown (preferred) | Notes | |--------------------|--------------------|-------| | external: [...] | deps: { neverBundle: [...] } | Moved to deps namespace | | noExternal: [...] | deps: { alwaysBundle: [...] } | 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.

Dependency handling defaults in tsdown v0.23+:

dependencies, peerDependencies, and optionalDependencies are all externalized by default (tsup only externalizes dependencies and peerDependencies, so optionalDependencies may switch from bundled to external after migration).
deps.resolveDepSubpath (resolving subpath imports of externalized packages without an exports field) is disabled by default.

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, publicDir→copy, bundle→unbundle, removeNodeProtocol→nodeProtocol, injectStyle→css.inject)
- [ ] Move external/noExternal into deps namespace, replace skipNodeModulesBundle with `deps.neverBundle: true`
- [ ] 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 (Stage 1: tsdown@0.22.14)
- [ ] Rename root-level tsup config field if present
- [ ] Run tsdown on 0.22.14 and resolve every deprecation warning (zero warnings required)
- [ ] Upgrade to the latest tsdown (^0.23.0 or newer) and verify the build again
- [ ] Suggest new tsdown features to the user

Agent Skills: Migrating from tsup to tsdown

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