Led a codebase-wide refactor of legacy Rails views and service layers, cutting load time on the highest-traffic screens from roughly 4s to under 1s. Introduced testing where there was none — RSpec suites from scratch with TDD, taking billing coverage from 0% to ~70%; review code across a team of 5 and mentor two mid-level developers.

Built two portfolio systems end to end; deepened Python/FastAPI, event-driven architecture and LLM integration.

Automated monthly billing for ~2,000 client companies across 5 countries — prorations, retries and country-specific edge cases — cutting the billing close from 3 days of manual work to under 2 hours. Cut p95 API response time by ~60% through ActiveRecord query optimisation and Redis caching; led end-to-end delivery of a product initiative with a 2-person team.

Built and maintained the billing side of a fintech product — invoicing flows, payment state handling and reconciliation against third-party providers, processing thousands of documents per month. Built REST integrations, webhooks and Sidekiq pipelines on AWS S3, plus ReactJS interfaces against the Rails API; containerised the environment with Docker on Linux.

Administered critical Linux servers and production Oracle and MySQL databases; diagnosed 24/7 incidents from logs and metrics across network, server and database layers. Designed and configured Cisco/Fortinet infrastructure for a nationwide operator — VPNs, firewalls, routing and segmentation — and automated monitoring and reporting with Python and C# scripts.