tsdown-migrate — Claude Code skill: move a library from tsup to tsdown

---
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.

## 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**:

1. **Stage 1 — migrate on `[email protected]`**: Install `[email protected]`, 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**.
2. **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`](../tsdown/references/option-target.md) 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 (Stage 1: `[email protected]`), scripts, root config field
6. **Remove unsupported options**: Replace with alternatives where available
7. **Test build on 0.22.14**: Run `tsdown` and resolve every deprecation warning — zero warnings required
8. **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

```ts
// 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:

```ts
// 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

```ts
// 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](references/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:

```json
// 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`:

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

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

For detailed package.json examples, see [guide-package-json.md](references/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/guide-differences-detailed.md).

## References

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

## 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: [email protected])
- [ ] 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
```