Investigate migrating from unmaintained tsup to tsdown

Background

The CLI and TUI clients (and test-servers) currently bundle with tsup, which is effectively unmaintained. tsdown is the actively-maintained, Rolldown-powered successor positioned as a drop-in replacement.

Goal

Investigate why we're on tsup and evaluate migrating to tsdown.

Tasks

Inventory every place tsup is used (clients/cli, clients/tui, test-servers, any others) and the config/options each relies on.
Assess tsdown compatibility with those configs (entry points, formats, dts, externals, the @inspector/core alias, watch mode).
Prototype the migration for one client and confirm builds + tests pass.
Document findings and recommend whether/how to migrate.

Acceptance criteria

A clear recommendation on migrating to tsdown, with the migration path and any blockers identified.

Triage: Priority = Medium

Scored 6/16 with the priority rubric added to AGENTS.md in #1891. This card is already approved and sitting in Todo — only Priority was set here, its Status is unchanged.

Axis	Score	Reasoning
Severity / impact	2/5	Build tooling hygiene. `tsup` is effectively unmaintained but nothing is broken; this is an investigation whose outcome may legitimately be 'stay'.
Urgency / staleness	3/5	v2.2.0 milestone. No forcing function beyond the maintenance risk of sitting on an unmaintained bundler.
Signal bonuses	+1	milestone (v2.2.0) (+1)
Total	6	Lands in the Medium band (6-8).

Related issues. One connected issue:

chore(deps): untangle the vitest/Storybook exact-pin knot to clear GHSA-p63j-vcc4-9vmv #1839 — same theme of dev-dependency maintenance debt, though a different tool and a different reason

Scores are a starting point, not a verdict — the rubric is meant to be overruled when it's plainly wrong, provided the reason is written down.

I tried this locally on v2/main (d0714d5) with tsdown 0.23.1 (Rolldown 1.2.13), migrating all four tsup builds: clients/cli, clients/tui, clients/mcpdo (two entries) and clients/web/tsup.runner.config.ts. All four build and --help works on each binary. The cli, mcpdo and tui test suites, scripts/verify-bundle-externals and the smoke:* scripts pass, and every package emits the same set of bare imports as with tsup. I didn't run the full web suite, pack:verify or local:gate.

The config mapping is mostly mechanical. Common shape:

export default defineConfig({
  entry: { index: "src/index.ts" },
  format: ["esm"], platform: "node", target: "node22",
  outDir: "build", clean: true, sourcemap: false, dts: false,
  fixedExtension: false,
  alias: { "@inspector/core": path.join(repoRoot, "core") },
  deps: {
    alwaysBundle: [/^@inspector\/core/],
    neverBundle: EXTERNALS.flatMap((name) => [name, new RegExp(`^${name}(?:/|$)`)]),
  },
  outputOptions: { comments: { jsdoc: false } },
});

The esbuild alias callbacks become a plain alias. noExternal/external become deps.alwaysBundle/deps.neverBundle (the old names still work but are deprecated). The TUI also needs inputOptions: { transform: { jsx: { runtime: "automatic" } } } and ink-form/ink-scroll-view in alwaysBundle. Shebangs are kept and tsdown sets the exec bit itself, so no banner is needed.

Gotchas, none of which fail the build:

A plain string in neverBundle only matches the exact id, so subpaths like zod/v4, @modelcontextprotocol/client/stdio and react/jsx-runtime got inlined (the CLI grew to 1.9 MB, the TUI to 2.1 MB). That's why the config adds a regex per name. tsdown prints "Detected dependencies in bundle", but the build still succeeds.
Output defaults to .mjs. fixedExtension: false keeps index.js/mcpdod.js, which e2e.test.ts, mcpdo/src/daemon/ensure.ts and pack-and-verify.mjs expect.
Rolldown keeps JSDoc comments, so the CLI bundle came out at 1.18 MB vs 877 KB with tsup. With comments: { jsdoc: false } it's 658 KB (I only checked that on the CLI).
scripts/verify-bundle-externals.mjs depends on tsup's output and config. It looks for esbuild's // path banners, but Rolldown writes //#region path, so it fails with "has no esbuild module banners". It also parses external: [ from the config text, and its test discovers tsup*.config.ts. clients/tui/__tests__/tsupConfig.test.ts imports the tsup config and its Options type. All of these need small updates, as do the config names in the tsconfig includes and in scripts/sdk-watch.mjs.

I'd migrate. The per-client change is small, and the externals guard catches the one subtle part once it understands Rolldown's region comments. Suggested order: CLI first (single entry, with an e2e test on the built binary), then the web runner and mcpdo, then the TUI (JSX plus the React externals policy test).

Shipped in #2803 (merged to v2/main as c621b99). Recommendation: migrate. The CLI is on tsdown now; the remaining builds are tracked in #2797.

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Investigate migrating from unmaintained tsup to tsdown #1492

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Triage: Priority = Medium

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Investigate migrating from unmaintained tsup to tsdown #1492

Description

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cliffhall commented on Aug 1, 2026

Triage: Priority = Medium

7 remaining items

FelixFangX commented on Oct 20, 2026

cliffhall commented on Oct 25, 2026

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