Why Your TypeScript 7 Upgrade Broke ESLint, ts-jest, and ts-morph

You installed TypeScript 7, ran your build, and something broke. Maybe ESLint crashed with a cryptic TypeError: Cannot read properties of undefined (reading 'Cjs') . Maybe ts-jest stopped transforming your test files. Maybe your CI pipeline just went red for no reason you can point to. You're not doing anything wrong. TypeScript 7 shipped tsgo, a genuine Go port of the type-checker, not a rewrite from scratch. But the tools that plug into TypeScript don't talk to the type-checker directly, they talk to a programmatic API. That API isn't stable yet, it lands in 7.1. Until then, a chunk of the ecosystem throws errors the moment you point typescript at the new version. The 10-second version Don't replace typescript in your dependencies with the 7.x line if you use typescript-eslint, ts-jest, ts-morph, or any tool doing programmatic type-checking. Keep typescript pinned to 6.x for those tools, and install @typescript/native-preview alongside it purely for fast type-checking in CI or a manual tsgo --noEmit command. Two compilers, living side by side, each doing a different job. Why this is happening The TypeScript team calls this Project Corsa: a line-by-line port of the compiler from the old JavaScript codebase (Strada) into Go (Corsa), preserving identical type-checking behavior while getting roughly 10x faster builds from real OS threads instead of Node's single-threaded event loop. That preservation is impressive, but it's a port, not a reimplementation with a new API surface. Tools like typescript-eslint depend on the programmatic API to walk your AST and pull type information out of the compiler, and that API isn't ready until 7.1. What's actually broken right now typescript-eslint — npm refuses to install alongside typescript@7 at all (ERESOLVE error), because the published peer range only allows versions below 6.1.0. Force it through and ESLint crashes deep inside typescript-estree . Tracked as typescript-eslint issue #12518, closed as not planned since the real