Tsdown Agent Skill

tsdown - The Elegant Library Bundler

Blazing-fast bundler for TypeScript/JavaScript libraries powered by Rolldown and Oxc.

When to Use

• Building TypeScript/JavaScript libraries for npm
• Generating TypeScript declaration files (.d.ts)
• Bundling for multiple formats (ESM, CJS, IIFE, UMD)
• Optimizing bundles with tree shaking and minification
• Migrating from tsup with minimal changes
• Building React, Vue, Solid, or Svelte component libraries

Quick Start

# Install
pnpm add -D tsdown

# Basic usage
npx tsdown

# With config file
npx tsdown --config tsdown.config.ts

# Watch mode
npx tsdown --watch

# Migrate from tsup
npx tsdown-migrate

Basic Configuration

import {defineConfig} from 'tsdown'

export default defineConfig({
  entry: ['./src/index.ts'],
  format: ['esm', 'cjs'],
  dts: true,
  clean: true,
})

Core References

| Topic | Description | Reference |

| ------------------ | ------------------------------------------------- | -------------------------------------------------------------------- |

| Getting Started | Installation, first bundle, CLI basics | guide-getting-started |

| Configuration File | Config file formats, multiple configs, workspace | option-config-file |

| CLI Reference | All CLI commands and options | reference-cli |

| Migrate from tsup | Migration guide and compatibility notes | guide-migrate-from-tsup |

| Plugins | Rolldown, Rollup, Unplugin support | advanced-plugins |

| Hooks | Lifecycle hooks for custom logic | advanced-hooks |

| Programmatic API | Build from Node.js scripts | advanced-programmatic |

| Rolldown Options | Pass options directly to Rolldown | advanced-rolldown-options |

| CI Environment | CI detection, 'ci-only' / 'local-only' values | advanced-ci |

Build Options

| Option | Usage | Reference |

| ------------------ | ------------------------------------------------------- | ---------------------------------------------------------------- |

| Entry points | entry: 'src/*.ts', '!**/*.test.ts'] | [option-entry |

| Output formats | format: 'esm', 'cjs', 'iife', 'umd'] | [option-output-format |

| Output directory | outDir: 'dist', outExtensions | option-output-directory |

| Type declarations | dts: true, dts: { sourcemap, compilerOptions, vue } | option-dts |

| Target environment | target: 'es2020', target: 'esnext' | option-target |

| Platform | platform: 'node', platform: 'browser' | option-platform |

| Tree shaking | treeshake: true, custom options | option-tree-shaking |

| Minification | minify: true, minify: 'dce-only' | option-minification |

| Source maps | sourcemap: true, 'inline', 'hidden' | option-sourcemap |

| Watch mode | watch: true, watch options | option-watch-mode |

| Cleaning | clean: true, clean patterns | option-cleaning |

| Log level | logLevel: 'silent', failOnWarn: false | option-log-level |

Dependency Handling

| Feature | Usage | Reference |

| ----------------- | ---------------------------------------------------- | -------------------------------------------------------- |

| Never bundle | deps: { neverBundle: 'react', /^@myorg\//] } | [option-dependencies |

| Always bundle | deps: { alwaysBundle: 'dep-to-bundle'] } | [option-dependencies |

| Only bundle | deps: { onlyBundle: 'cac', 'bumpp'] } - Whitelist | [option-dependencies |

| Skip node_modules | deps: { skipNodeModulesBundle: true } | option-dependencies |

| Auto external | Automatic peer/dependency externalization | option-dependencies |

Output Enhancement

| Feature | Usage | Reference |

| ------------------ | ------------------------------------------------------------------------------------------------------------------------------------ | -------------------------------------------------------------- |

| Shims | shims: true - Add ESM/CJS compatibility | option-shims |

| CJS default | cjsDefault: true (default) / false | option-cjs-default |

| Package exports | exports: true - Auto-generate exports field | option-package-exports |

| CSS handling | experimental] css: { ... } — full pipeline with preprocessors, Lightning CSS, PostCSS, code splitting; requires @tsdown/css | [option-css |

| CSS inject | css: { inject: true } — preserve CSS imports in JS output | option-css |

| Unbundle mode | unbundle: true - Preserve directory structure | option-unbundle |

| Root directory | root: 'src' - Control output directory mapping | option-root |

| Executable | experimental] exe: true - Bundle as standalone executable, cross-platform via @tsdown/exe | [option-exe |

| Package validation | publint: true, attw: true - Validate package | option-lint |

Framework & Runtime Support

| Framework | Guide | Reference |

| --------- | ------------------------------------------------------------ | -------------------------------------------- |

| React | JSX transform, React Compiler | recipe-react |

| Vue | SFC support, JSX | recipe-vue |

| Solid | SolidJS JSX transform | recipe-solid |

| Svelte | Svelte component libraries (source distribution recommended) | recipe-svelte |

| WASM | WebAssembly modules via rolldown-plugin-wasm | recipe-wasm |

Common Patterns

Basic Library Bundle

export default defineConfig({
  entry: ['src/index.ts'],
  format: ['esm', 'cjs'],
  dts: true,
  clean: true,
})

Multiple Entry Points

export default defineConfig({
  entry: {
    index: 'src/index.ts',
    utils: 'src/utils.ts',
    cli: 'src/cli.ts',
  },
  format: ['esm', 'cjs'],
  dts: true,
})

Browser Library (IIFE/UMD)

export default defineConfig({
  entry: ['src/index.ts'],
  format: ['iife'],
  globalName: 'MyLib',
  platform: 'browser',
  minify: true,
})

React Component Library

export default defineConfig({
  entry: ['src/index.tsx'],
  format: ['esm', 'cjs'],
  dts: true,
  deps: {
    neverBundle: ['react', 'react-dom'],
  },
  inputOptions: {
    jsx: {runtime: 'automatic'},
  },
})

Preserve Directory Structure

export default defineConfig({
  entry: ['src/**/*.ts', '!**/*.test.ts'],
  unbundle: true, // Preserve file structure
  format: ['esm'],
  dts: true,
})

CI-Aware Configuration

export default defineConfig({
  entry: ['src/index.ts'],
  format: ['esm', 'cjs'],
  dts: true,
  failOnWarn: 'ci-only', // opt-in: fail on warnings in CI
  publint: 'ci-only',
  attw: 'ci-only',
})

WASM Support

import {wasm} from 'rolldown-plugin-wasm'
import {defineConfig} from 'tsdown'

export default defineConfig({
  entry: ['src/index.ts'],
  plugins: [wasm()],
})

Library with CSS and Sass

export default defineConfig({
  entry: ['src/index.ts'],
  format: ['esm', 'cjs'],
  dts: true,
  target: 'chrome100',
  css: {
    preprocessorOptions: {
      scss: {
        additionalData: `@use "src/styles/variables" as *;`,
      },
    },
  },
})

Standalone Executable

export default defineConfig({
  entry: ['src/cli.ts'],
  exe: true,
})

Cross-Platform Executable (requires @tsdown/exe)

export default defineConfig({
  entry: ['src/cli.ts'],
  exe: {
    targets: [
      {platform: 'linux', arch: 'x64', nodeVersion: '25.7.0'},
      {platform: 'darwin', arch: 'arm64', nodeVersion: '25.7.0'},
      {platform: 'win', arch: 'x64', nodeVersion: '25.7.0'},
    ],
  },
})

Advanced with Hooks

export default defineConfig({
  entry: ['src/index.ts'],
  format: ['esm', 'cjs'],
  dts: true,
  hooks: {
    'build:before': async (context) => {
      console.log('Building...')
    },
    'build:done': async (context) => {
      console.log('Build complete!')
    },
  },
})

Configuration Features

Multiple Configs

Export an array for multiple build configurations:

export default defineConfig([
  {
    entry: ['src/index.ts'],
    format: ['esm', 'cjs'],
    dts: true,
  },
  {
    entry: ['src/cli.ts'],
    format: ['esm'],
    platform: 'node',
  },
])

Conditional Config

Use functions for dynamic configuration:

export default defineConfig((options) => {
  const isDev = options.watch
  return {
    entry: ['src/index.ts'],
    format: ['esm', 'cjs'],
    minify: !isDev,
    sourcemap: isDev,
  }
})

Workspace/Monorepo

Use glob patterns to build multiple packages:

export default defineConfig({
  workspace: 'packages/*',
  entry: ['src/index.ts'],
  format: ['esm', 'cjs'],
  dts: true,
})

CLI Quick Reference

# Basic commands
tsdown                          # Build once
tsdown --watch                  # Watch mode
tsdown --config custom.ts       # Custom config
npx tsdown-migrate              # Migrate from tsup

# Output options
tsdown --format esm,cjs        # Multiple formats
tsdown -d lib                  # Custom output directory (--out-dir)
tsdown --minify                # Enable minification
tsdown --dts                   # Generate declarations
tsdown --exe                   # Bundle as standalone executable
tsdown --unbundle              # Bundleless mode

# Entry options
tsdown src/index.ts            # Single entry
tsdown src/*.ts                # Glob patterns
tsdown src/a.ts src/b.ts       # Multiple entries

# Workspace / Monorepo
tsdown -W                      # Enable workspace mode
tsdown -W -F my-package        # Filter specific package
tsdown --filter /^pkg-/        # Filter by regex

# Development
tsdown --watch                 # Watch mode
tsdown --sourcemap             # Generate source maps
tsdown --clean                 # Clean output directory
tsdown --from-vite             # Reuse Vite config
tsdown --tsconfig tsconfig.build.json  # Custom tsconfig

Best Practices

• Always generate type declarations for TypeScript libraries:

   {
     dts: true
   }

• Externalize dependencies to avoid bundling unnecessary code:

   {
     deps: {
       neverBundle: [/^react/, /^@myorg\//]
     }
   }

• Use tree shaking for optimal bundle size:

   {
     treeshake: true
   }

• Enable minification for production builds:

   {
     minify: true
   }

• Add shims for better ESM/CJS compatibility:

   {
     shims: true
   } // Adds __dirname, __filename, etc.

• Auto-generate package.json exports:

   {
     exports: true
   } // Creates proper exports field

• Use watch mode during development:

   tsdown --watch

• Preserve structure for utilities with many files:

   {
     unbundle: true
   } // Keep directory structure

• Validate packages in CI before publishing:

   { publint: 'ci-only', attw: 'ci-only' }

Resources

• Documentation: https://tsdown.dev
• GitHub: https://github.com/rolldown/tsdown
• Rolldown: https://rolldown.rs
• Migration Guide: https://tsdown.dev/guide/migrate-from-tsup