TypeScript 6.0 changed nine compiler defaults simultaneously and removed ES5 target plus AMD and UMD module formats breaking any project that did not explicitly set these options

TypeScript 6.0 shipped March 23 2026 with the largest breaking change set since TypeScript 2.0. Nine defaults changed at once: strict now on by default, ES2025 the new target default, moduleResolution now bundler by default. ES5 target was removed entirely along with AMD, UMD, and SystemJS module output. Projects that relied on implicit defaults, shipped legacy bundles for IE or older Node, or used AMD module federation now get compiler errors with no graceful fallback. Any project that does a version bump without auditing tsconfig gets broken builds. No automated migration scanner exists that reads a tsconfig, identifies every option now in conflict with TS6 defaults, and outputs a prioritized fix list with per-option rationale.