Senior Backend Engineer Kotlin​/JVM Sofia, Bulgaria

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.

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.
• Enrollment 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 service (up to 35), along with public…