refactor(build): migrate tsup → tsdown#2513
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.nvmrc was 22.15.0 → 24.16.0 and the CI surface moved to a Node 22.x/24.x unit-test matrix some time ago; CLAUDE.md still claimed 22.15.0 + Node 18/20, which is wrong on both counts.
tsup 8.5.1's DTS plugin unconditionally injects `baseUrl: "."` into the compiler options it hands to rollup-plugin-dts. TypeScript 6.0 rejects that with TS5101, blocking the root TS bump (see #2491). The workaround in tests.yml only patched the test-types matrix and not the build job — piling more workarounds on a deprecated bundler is the wrong direction. tsdown is rolldown-based, uses rolldown-plugin-dts (no baseUrl injection), declares peer typescript ^5 || ^6, and tsup's own README recommends it. What changes: - tsup.config.base.ts → tsdown.config.base.mjs (+ matching .d.mts). JS + JSDoc rather than TS so the per-package tsdown.config.ts files load it through the native ESM loader; pnpm's strict isolation collides with tsdown's tsx loader. - All 12 per-package tsup.config.ts → tsdown.config.ts. - Each package's build/dev scripts swap tsup → tsdown and pre-clean dist/ (tsdown's clean defaults differ). - Drop the now-dead test-types '6.0' jq patch + comment block. The six-bundle layout (cjs dev + prod-min and esm legacy + modern + modern.dev + modern.prod-min) is preserved byte-for-byte at the file listing level — packages' public `exports` fields are unchanged. Known behaviour difference: for the modern.mjs entry (the one carrying .d.mts), rolldown extracts its `__exportAll` runtime helper into a sibling dist/chunk.mjs that codeSplitting:false does not suppress in tsdown 0.22 / rolldown 1.0. The chunk ships via each package's `files: ["dist/**/*"]` and is resolved as a relative import from the entry, so consumers are unaffected; are-the-types-wrong is green across all resolution modes for the produced tarballs. Refs: egoist/tsup#1388.
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Why
tsup8.5.1's DTS plugin unconditionally injectsbaseUrl: "."into the compiler options it hands torollup-plugin-dts. TypeScript 6.0 rejects that with TS5101 ("Option 'baseUrl' is deprecated"), which is what blocked #2491. The existingignoreDeprecationspatch intests.ymlonly covers thetest-typesmatrix entry — thebuildjob has none. Stacking more workarounds on a deprecated bundler is the wrong direction.tsdownis rolldown-based, usesrolldown-plugin-dts(nobaseUrlinjection — verified against the plugin source), declares peertypescript: "^5.0.0 || ^6.0.0", and tsup's own README recommends the migration. Our Node floor already clears tsdown 0.22's^22.18.0 || >=24.0.0requirement, so no Node bump rides along.Upstream tracking: egoist/tsup#1388.
Known behaviour difference
For each package's
modern.mjsentry (the one carrying the canonical.d.mts), rolldown extracts its__exportAllruntime helper into a siblingdist/chunk.mjs.codeSplitting: falsedoes not suppress this in tsdown 0.22 / rolldown 1.0. The chunk ships via each package'sfiles: ["dist/**/*"]and is resolved as a relative import from the entry, so consumers are unaffected —arethetypeswrongis green across all four resolution modes (node10,node16 from CJS,node16 from ESM,bundler) for the produced tarballs.