Senior Backend Engineer Kotlin​/JVM

Sum Up builds simple, affordable financial tools that help over 4 million small businesses across 37 markets get paid, manage their money, and grow. Beyond card payments, we offer a full suite of financial products - business banking, lending, invoicing, and money transfers - giving merchants everything they need in one place. We're a global fintech with over 3,000 people across 20+ offices, and Sofia has been one of our most important engineering hubs since the very beginning.

Transfers Excellence squad sits within Sum Up's Global Bank
, and owns the core services that power how merchants and consumers move money.

E.g. IBAN-based transfers, money in and out, and peer-to-peer payments via the Sum Up consumer app. With the high volume transfers per hour across our European markets. That means reliability isn't a nice-to-have - it's the baseline for us. Right now, the team is replacing a set of market-specific implementations with a single, unified global transfer platform that will serve Europe, the UK, Brazil, Chile, and eventually the US and Canada.

Join our team and contribute directly to platform-level architecture that will shape how Sum Up moves money at scale - work that directly impacts millions of small businesses every day.

Our tech stack includes Kotlin, Java, Go, a bit of Elixir, AWS, Kafka, PostgreSQL, and Kubernetes, supported by a strong observability toolchain with Prometheus, Grafana, and Honeycomb. We also actively use AI‑assisted development tools such as Cursor, Git Hub Copilot, and others.

Get a feel for life in our Sofia office.

• Build and maintain backend services that sit at the heart of Sum Up's transfer infrastructure, taking end-to-end ownership from development through to observability, testing and production support
• Contribute to the migration from market-specific services toward a unified global transfer platform, working across a mix of active services and legacy systems being decomposed
• Raise well-scoped pull requests, conduct thorough code reviews, and help maintain high standards of code quality and test coverage across the team
• Participate in incident reviews and the on-call rotation, taking accountability for the reliability of the services you build
• Contribute to the technical roadmap by participating in architectural discussions and proposing solutions for complex, distributed systems challenges
• Apply AI tooling to accelerate your own engineering workflows and contribute to raising the team's overall AI literacy

• Solid experience as a Backend Software Engineer in modern languages (e.g. Kotlin, Go, Java, etc.). Kotlin is the team's primary language and we also work with Java, Golang, and Elixir.
• Solid understanding of distributed systems and event-driven architecture. Experience with async processing and messaging tools like Kafka (or similar).
• Experience working with microservice architectures, including Spring Boot and event-driven patterns, alongside solid knowledge of relational databases and API design
• Comfortable working in high-throughput, regulated environments where reliability matters. Defensive coding and attention to edge cases are part of the job
• Clear communicator in written and spoken English, with a collaborative approach to working across global teams
• Curious about the product impact, not just the code. Comfortable thinking about how backend decisions affect the end user experience.

• Opportunity to work with Sum Uppers globally on large-scale fintech products used by millions of businesses worldwide, from our modern Sofia office. This involves an office-first setup
• Commitment to Diversity and Inclusion: be part of a workplace that values and promotes diversity, fostering an inclusive environment where everyone's perspectives are respected and embraced
• Enrolment onto our Virtual Stock Option programme: you will own a stake in Sum Up's future success
• A dedicated annual L&D budget of €2000 and 10 paid days of educational leave which can be used to attend conferences and/or advance your career through further education
• Generous time off: 25 days of paid leave, plus one extra day per year of…