Simobi+.
Bank Sinarmas' mobile banking app, rewritten from a 10-year-old codebase. 1M+ downloads, rated 4.9 on Google Play.
About the project.
SimobiPlus covers transfers, bill payments, bonds, and mutual funds. On the 10-person team that rewrote the app in seven months, I led those four features, built the shared component library published to npm, and cut app size from 100MB+ to 42MB and cold start from 5.79s to 1.12s.
Challenges & decisions.
The old app
Ten years of accumulated patches had left the app at 100MB+ install size, a 5.79s cold start, and 92.62% crash-free users. Under the hood: outdated Redux with Thunk, most screens still class components, and every data fetch forced through the store.
Why a rewrite
Incremental fixes couldn't undo a decade of structural decisions, so a 10-person team rebuilt the app from scratch and shipped it in seven months.
Old stack → New stack
Aspect | Old Stack | New Stack | Why |
Components | Mostly class components | Function components + hooks (React 19) | Less boilerplate, easier to test and reuse logic |
State management | Outdated Redux + Thunk | Zustand | No action/reducer/thunk ceremony; one file per store |
Data fetching | Every fetch forced through the Redux store | TanStack Query | Server state cached and synced automatically; screens stop hand-managing loading states |
Testing | None | Jest + Testing Library, 70% coverage | Regressions caught before release |
What I did
Feature lead for Transfer, Bill Payment, Bonds, and Mutual Funds.
Organised the codebase into domain-driven feature modules so squads could ship in parallel without collisions.
Built the @bsim-red shared component library: 20+ packages published to the bank's private npm, reused across its other mobile apps.
Drove unit-test coverage from 0 to 70% (SonarQube Security A, Reliability A).
Set up the team's Claude Code workflow: wrote the conventions, module structure, and component rules agents had to follow, and reviewed every change before merge.
Where it landed
App size 100MB+ → 42MB.
Cold start 5.79s → 1.12s.
Crash-free users 92.62% → 99.03% (Sentry).
1M+ downloads, 4.9 on Google Play.
Built with.
A look inside.
