Add replacement: `tsup` → `tsdown`

[sxzz]

Following up from the discussion in e18e/ecosystem-issues#170, I'd like to propose adding tsup → tsdown to the module-replacements dataset.

Basic info

• Module to replace: tsup
• Replacement: tsdown
• Manifest: preferred
• Replacement type: documented

Why it should be replaced

• Upstream recommends it. tsup's own README has recommended migrating to tsdown since August 2025. The project is effectively in maintenance mode.
• Performance. tsdown is built on Rolldown (Rust-based) and is significantly faster than tsup for typical library builds, while producing comparable or smaller output.
• Size. tsdown's install footprint is now comparable to tsup (~22–26MB vs ~25–35MB) and shrinking further as the dts plugin is integrated natively.
• Significant adoption. tsup has tens of millions of weekly downloads. tsdown already powers builds in major projects including Vite, Slidev, RSSHub, Rolldown itself, unplugin, and 600+ other projects with >100 stars.
• Drop-in compatible. tsdown is designed as a tsup successor with a compatible config surface; most projects migrate with little to no config change.

Availability

• tsdown supports Node.js ≥ 20.19 (current active LTS line), aligning with modern tooling expectations.
• Available on npm, no native/system dependencies beyond what Rolldown ships.

Migration example

Most tsup.config.ts files map 1:1 to tsdown.config.ts:

// tsup.config.ts
import { defineConfig } from 'tsup'

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

// tsdown.config.ts
import { defineConfig } from 'tsdown'

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

Tooling is also available to automate this:

• CLI: npx tsdown-migrate
• AI skill: npx skills add rolldown/tsdown --skill tsdown-migrate

Scope

To echo the concern raised by @43081j in the linked discussion: this replacement is intended only for existing tsup users. It should not be read as a recommendation for projects on other bundlers, nor for projects that don't need a bundler at all.

Happy to open the PR once the direction here is agreed upon.

[SuperchupuDev]

tsdown supports Node.js ≥ 20.19 (current active LTS line)

super minor correction that's not relevant to the issue itself but fyi node 20 went eol last week :^)

[sxzz]

Support for Node 20 was dropped in v0.22.0-beta.1, but this change hasn't been released in a stable version yet.

[ghostdevv]

I imagine we'll want to add replacements for other building tools, and in that case usually what we do is create a documented replacement that shows all of the available replacements. E.g. there are multiple packages to fetch stuff that all point to the fetch replacement which at the time of writing lists native fetch, oftech, and ky as replacements.

So I guess, for now we can just add this tsup -> tsdown but then in future we can update it to also mention other replacements. For example, if you don't need dual builds then you could just use tsc directly (or something like obuild if it's ready).

Let's wait to see what @43081j thinks too 🙏