deepseek-harness/docs/rfc/implemented/process/2026-06-17-ts-build-config.md

RFC: TSC-first build and one tsconfig

Status: implemented (accepted 2026-06-20)

Context

The current TypeScript build and typecheck setup had these issues:

• build used tsc to transform .ts to .d.ts files for packages under packages/<group>/<pkg> and vendor/*, and then used tsdown to transform .ts to bundled .js files. This made two tools do TypeScript transform.
• typecheck tended to validate packages, vendor source, examples, tests, and scripts through one root typecheck config.

The goal is to make build and typecheck use matching tsconfig boundaries and TypeScript resolution/transform behavior. Build should generate .js, .d.ts, .js.map, and .d.ts.map through one compiler and config, so publish output and type validation stay consistent.

Validation found several concrete technical issues and possible routes:

• tsdown uses oxc to transform TypeScript, which is not the same behavior as tsc.
  • Bundled .d.ts emitted by tsdown conflicts with Cordis' internal relative module augmentation shape.
  • The tsc output is affected by allowImportingTsExtensions, so we need to ensure that generated .js files do not import .ts files and generated .d.ts files keep explicit relative specifiers that NodeNext/Node16 accepts. Therefore, in-package relative imports use explicit .ts specifiers in TypeScript source and rewriteRelativeImportExtensions rewrites those specifiers to .js in emitted JS.
  • Bundled .js emitted by tsdown is not the same behavior as per-file .js emitted by tsc -b, such as decorator transform behavior.
• vendor/*/src, examples, tests, and scripts cannot all be plain-included in one root strict program.
  • Directly typechecking vendor/*/src under the root strict config triggers many type errors outside this project's ownership.
  • Package dependencies under packages/*/* on vendor are resolved to the vendor/*/lib for different tsconfig strictness.

Decision

In-package relative imports use explicit .ts specifiers.

pnpm run build is a two-stage build:

• Stage 1: tsc -b tsconfig.build.json emits per-module .js, declarations .d.ts, JS sourcemaps .js.map, and declaration sourcemaps .d.ts.map into each package's lib/types. This is the authoritative TypeScript compilation result. For publish we keep .d.ts / .d.ts.map and ignore .js / .js.map.
  • The build project uses the project-reference graph that tsc -b compiles. For example, root tsconfig.build.json references package and vendor tsconfigs. It validates and emits package/vendor build results.
• Stage 2: a bundler reads the emitted JS under lib/types and writes the bundled runtime entry as lib/index.js or lib/index.mjs (follow current behavior). This stage is bundling only. It must not read TypeScript source or emit declarations.

tsdown is no longer the owner of TypeScript compilation or declaration output.

pnpm run typecheck runs build mode over the root tsconfig.json.

• The root tsconfig.json is the single development/typecheck project. It typechecks examples, tests, and scripts with noEmit, and validates package/vendor source through references.
• Referenced package/vendor projects keep the same emit behavior as build, so typecheck can refresh their lib/types outputs instead of using a separate no-emit graph. Project-specific strictness changes live in the owning packages/*/*/tsconfig.json or vendor/*/tsconfig.json.
• The root no-emit project disables rewriteRelativeImportExtensions; it emits nothing and includes tests that import helpers across project-reference boundaries. Package/vendor emit projects keep the rewrite enabled.

The command orchestration shape is:

pnpm run build:
tsc -b tsconfig.build.json
tsdown

pnpm run verify-node-next-types:
tsx scripts/verify-node-next-types.ts

pnpm run typecheck:
tsc -b tsconfig.json

pnpm run demo:* still runs src directly through tsx and root paths, without a compile step.

Consequences

Build responsibilities are clearer:

• Each module under packages/<group>/<pkg> and vendor/* has one local tsconfig for build, typecheck, and tools that run source directly, such as tsx and vitest.
• The build command uses tsconfig.build.json. tsc -b owns the publishable per-module .js and .d.ts output, and the bundler owns only lib/index.*.
  • lib/types/*.d.ts and .d.ts.map are the publish declaration output.
  • lib/types/*.d.ts uses explicit .ts relative specifiers, which TypeScript's NodeNext/Node16 resolver maps to sibling .d.ts files.
  • lib/types/*.js is only a bundler input and must not be used as a runtime entry or public import target.
  • lib/index.* is the publish runtime output and is generated by the bundler, currently tsdown.
• pnpm run verify-node-next-types scans built declarations for relative specifiers without file extensions, then typechecks a temporary external ESM consumer with moduleResolution: "NodeNext" against the built types/exports surface, so declaration specifier regressions fail before publish.
• The typecheck command uses tsconfig.json. Examples, tests, and scripts are checked by the root no-emit project, while packages and vendor modules keep the same emit behavior as build. Package and vendor source stays behind project-reference boundaries.

The Cordis vendor copy now has one more type-structure divergence from upstream. During upstream sync, that divergence must be reapplied or explicitly retired.