YCU

Problem

Build and ship a consumer social product — group chat, video, challenges — across iOS, Android and web, as the only engineer. Everything that a team would normally divide up (client, backend, billing, moderation, release management) lands on one person, so the architecture has to be chosen for how little it can demand rather than how much it can do.

Constraints

• One engineer, three platforms. Anything that requires maintaining the same feature three times is disqualified.
• User-generated content is a launch blocker, not a follow-up. A social app with chat and video comments cannot ship moderation later — Apple will not approve it, and more to the point it would be the wrong thing to do.
• Two payment systems, one truth. Apple requires IAP for in-app purchases; the web needs Stripe. The product must not care which one a given user came through.

Approach

One monorepo, shared everywhere. Expo/React Native for iOS and Android, Next.js 16 for web, on Turborepo with pnpm, over Supabase. Types, validation and business rules are written once and consumed by every client.

Entitlements as the single source of truth. Rather than letting Stripe state and Apple state leak into feature code, both providers reconcile through webhooks into one resolved entitlement record. Every feature gate asks that record one question — is this user entitled? — and never needs to know about trials, partner codes, receipt validation, or which store the subscription came from. This is the piece I would defend hardest: it is the difference between adding a second payment provider and rewriting the app around one.

Safety classification before publish, not after. Every piece of user-generated content — group chat messages, video and challenge comments — passes an LLM safety classifier on the write path. Clean content publishes; anything flagged routes to an admin moderation queue rather than to another user. Reviewing after the fact means somebody already saw it.

Outcome

Live on the App Store, with 50+ releases through Apple App Review, running on iOS, Android and web from one codebase.

The dual-billing normalisation and the pre-publish classifier are the two decisions that made the rest of the product tractable for one person. Both are cases of paying an architectural cost early to avoid paying a per-feature cost forever.