Kostyantyn C.

Most clients contact me when a system already exists, the requested change looks deceptively small, and nobody is quite sure what else it might break. That is the kind of work I do best. I help teams understand complex backend behavior, untangle integrations and business rules, and deliver production-safe changes. My recent work has involved payment and payout flows, recurring transactions, multi-currency rules, KYC/PCI processes, external APIs, data reconciliation, and legacy modernization. I can help when you need to: • diagnose a difficult production or data problem; • design or repair an API or payment integration; • modernize a mature backend without rewriting everything; • turn ambiguous requirements into an implementable plan; • take a cross-system change from investigation through production. I bring more than 15 years of software engineering experience, a PhD in mathematical modeling, and over 7,300 hours of Upwork work with 100% Job Success. I work AI-first. I use AI during discovery, design, implementation and verification to challenge assumptions, trace edge cases and shorten feedback loops. It helps me move faster; I remain responsible for the decisions and the result. My deepest implementation experience is in C#/.NET, SQL Server and PostgreSQL. I am not limited to one stack, but I am most valuable where the real problem sits between code, data, integrations and business rules. A good first engagement is often a focused paid review. Show me the system and the problem, and I will help determine what is actually happening, what is risky, and what should be done next.