Investigation: TypeScript 7 Upgrade Feasibility

I bumped TypeScript to 7.0.2 (latest stable, released July 2026) and tested compilation and tooling. Here are the findings.

Current State

Component	Version	Notes
TypeScript	^5.9.3	Dev dep in ~70 packages
ts-node	^10.9.2	Used in ~63 packages + root scripts (`fix-project-references`, `syncInstructions`)
@swc/core	^1.15.46	Jest transform + ts-node --swc in ~70 packages
@swc/jest	^0.2.39	Used in all jest configs (no ts-jest)
@swc-node/register	^1.10.9	1 package (server-side-rendering)
Next.js	^16.2.12	5 packages
typescript-plugin-css-modules	^5.1.0	1 package (server-side-rendering)
Yarn	4.17.1 (PnP)	No `node_modules` directory

Installed Successfully

yarn install with "typescript": "7.0.2" completed without errors
The Go native tsc binary is available and executes

What Breaks

1. No JS Compiler API (BLOCKER)

TS 7.0 ships with no programmatic API. require('typescript') returns an object where createProgram, transpileModule, and sys are all undefined. This is confirmed by the TS 7 announcement — the API is planned for 7.1.

Broken tools:

ts-node: Crashes immediately with TypeError: Cannot read properties of undefined (reading 'fileExists') because ts.sys is undefined. This impacts root scripts (fix-project-references, syncInstructions) and the template-ts scripts in ~30+ template packages.
@swc-node/register: Uses the TS API internally. The latest version (1.12.1) explicitly declares "typescript": ">= 4.3 < 7" — it excludes TS 7. The current version (1.10.9) has >= 4.3 but will fail at runtime.
typescript-plugin-css-modules: Uses the TS language service plugin API — will not work.

Tools that are OK:

@swc/core & @swc/jest: SWC has its own parser and does NOT use the TS compiler API. Jest tests should work after config fixes.
Next.js: Uses SWC for transpilation internally, does not rely on the TS API for builds. However, next build may attempt type-checking if typescript.ignoreBuildErrors is not explicitly set.

2. Config Deprecations Become Hard Errors (226 errors)

yarn tsc --build produces 349 errors across 4 categories:

Error	Count	Issue
TS5102 `baseUrl` removed	116	~58 tsconfig files use `baseUrl: "."`. Must use `paths: {"": ["./"]}` instead
TS5108 `moduleResolution=node` removed	6	Next.js/SSR packages use `moduleResolution: "node"`. Must switch to `"bundler"`
TS5108 `target=ES5` removed	1	Root `tsconfig.json` has `"target": "es5"`. Must change to at least `ES2015`
TS5011 `rootDir` must be explicit	1	Root tsconfig with `include: ["./scripts/*"]` needs explicit `rootDir`

3. Yarn PnP + Native `tsc` Type Resolution (226 errors, POTENTIAL BLOCKER)

TS2688: Cannot find type definition file for 'jest' and 'node'. This appears because the Go native tsc cannot resolve @types/ packages from PnP zip archives. There is no node_modules/ directory — PnP stores packages as .zip files in .yarn/cache/.

In the current TS 5.x setup, Yarn's TypeScript SDK (/.yarn/sdks/typescript/) provides patched resolution that understands PnP. The Go native tsc has no such integration.

Possible workaround: Switch to nodeLinker: node-modules in .yarnrc.yml, but this would be a major deviation from the current PnP setup.

What Works

yarn install with TS 7 — clean
tsc binary executes (native Go, fast)
SWC transpilation (@swc/core, @swc/jest) — no dependency on TS API
The 349 tsc errors are all config-level, not type-level — meaning once configs are fixed and type resolution works, the actual codebase likely compiles cleanly

Proposed Upgrade Plan

Phase 1: Prepare Configs (do now, works with TS 5.x too)

Remove baseUrl from all tsconfigs; replace with paths: {"*": ["./*"]} where needed (~58 files)
Change moduleResolution: "node" to "bundler" in all Next.js/SSR tsconfigs (6 files)
Remove target: es5 from root tsconfig; use at least ES2015
Fix rootDir in root tsconfig

Phase 2: Address Tooling API Gap (blocked on TS 7.1 or workaround)

Wait for TS 7.1 which will ship with a new JS API. The TS team has confirmed this is on the roadmap.
OR use side-by-side installation as recommended by the TS team:
```
"typescript": "npm:@typescript/typescript6@^6.0.2",
"@typescript/native": "npm:typescript@^7.0.2"
```
This keeps TS 6 for API-dependent tools (ts-node) and uses TS 7's tsc for compilation.

Phase 3: Replace/Update API-Dependent Tools

Replace ts-node with a TS-7-compatible alternative:
- tsx (uses esbuild, may be compatible)
- tsimp (uses SWC)
- Both ~32 ts-node --swc invocations and root scripts would need updating
Update @swc-node/register once a TS-7-compatible version is released
Remove or replace typescript-plugin-css-modules if needed (used in 1 package)

Phase 4: Resolve PnP Compatibility

Test if nodeLinker: node-modules resolves the TS2688 type resolution errors
OR wait for Yarn Berry to provide a TS 7 SDK with PnP-aware resolution (similar to existing .yarn/sdks/typescript/)

Risk Assessment

Risk	Severity	Mitigation
ts-node broken	High	Must replace all ts-node usage before upgrading
TS API not available until 7.1	High	Use side-by-side TS 6+7 or wait
PnP type resolution	Medium-High	May require `nodeLinker: node-modules`
@swc-node/register peer dep conflict	Medium	Updates expected after TS 7.1
Config changes may cascade	Low-Medium	Changes are mechanical and testable

Recommendation

Don't upgrade yet. The lack of a JS API in TS 7.0 makes this a non-starter for a project that heavily uses ts-node. Wait for TS 7.1 (expected ~Oct 2026, 3-4 months after 7.0) which will ship the new API.

In the meantime, do Phase 1 (config updates) which are backward-compatible with TS 5.x and reduce the surface area of changes needed later.

Update to TypeScript 6 #643

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