100K-line React 16 → 19 + TypeScript migration, solo, with AI coding assistants.

What it was

A 100K+ line production React 16 codebase migrated to React 19 with TypeScript. Solo project, used AI coding assistants as the primary engineering surface.

What I learned

• AI changed the calculus of what a single engineer can ship — but only after the harness (tests, types, observability) was right
• Architecture docs are now agent-readable artifacts, not just team-readable
• The hardest moments were the ones the model couldn't see: dependency conflicts, ambient runtime assumptions, and team-specific conventions

Stack

React 19, TypeScript, Vite, Claude Code, custom prompts and review checklists.