ZyfinBot — Global personal finance SaaS

Problem

Household expense tracking fails when capture requires opening an application, navigating to a form and completing multiple fields. WhatsApp provides a lower-friction entry point in a channel people already use, while international households also need multi-currency reporting and a secure way to collaborate with financial advisers.

Approach

I designed ZyfinBot as a modular Ruby on Rails application whose WhatsApp webhook runs in-process, reducing early operational complexity while the product evolves into a multi-tenant financial platform with Hotwire and Flutter clients.

Key architectural decisions:

• Integrated only the official Meta WhatsApp Cloud API, avoiding unofficial clients and account-ban risk.
• Implemented a deterministic parser before introducing an LLM, handling shorthand entries such as 02/06 Mercado Alimentação 130,28 and using interactive controls as a testable fallback.
• Stored money in integer minor units with exchange-rate snapshots, preventing historical reports from changing when currency rates move.
• Designed tenant-per-family isolation, starting with embedded SQLite for the MVP and retaining a migration path to self-hosted PostgreSQL with Row-Level Security.
• Designed authentication for potential phone loss with email, password and mandatory WebAuthn or TOTP 2FA, without relying on SMS or WhatsApp OTPs.
• Defined an end-to-end infrastructure path from a Raspberry Pi k3s development cluster behind Cloudflare Tunnel to future Terraform-provisioned cloud infrastructure, Kamal 2 deployments and GitHub Actions publishing images to GHCR.

Impact

• Delivered the local v0.1.0 foundation with HMAC webhook verification, idempotent message handling, expense parsing and persistence covered by RSpec golden tests.
• Split delivery into independently shippable vertical slices covering ingestion, budgets, reports, PostgreSQL/RLS, web UI and Open Finance.
• Modelled Open Finance independently of providers, allowing Pluggy in Brazil and GoCardless or TrueLayer in Europe without schema migrations.