At Upside, we’re not just shipping features - we’re partnering with ambitious companies across the globe to solve complex, meaningful problems through technology. We work in small, focused teams, combining engineering, product thinking, and design, to deliver outcomes, not just output.Open-source is a strategic part of our work. We build and maintain a number of open-source projects that support our clients and internal R&D. Our latest release is Enthusiast, an AI Agent framework tailored for e-commerce use cases.Role Overview:As a Senior Backend Engineer, you will play a pivotal role in designing, developing, and maintaining production-grade platforms for global clients. This position is at the heart of our development efforts, contributing significantly to the backbone of client-facing solutions. You'll be part of a dynamic development team, sized between 3 to 9 members, dedicated to craft scalable and resilient backend systems.Job requirements Extensive commercial experience in building web-based application (+5 years). Strong proficiency in at least one of Upside’s core technology stack: Ruby, Python, JavaScript or the willingness to learn based on your current skills. Solid understanding of software architecture principles: monoliths, microservices, asynchronous systems, messaging patterns, and scalability. Deep knowledge of relational databases (we mostly use PostgreSQL) and performance optimization. Working understanding of frontend frameworks and tools (e.g. Vue.js, React.js). Practical experience with Continuous Integration/Continuous Deployment (CI/CD) practices and modern cloud environments. Strong consulting mindset - balancing technical excellence with client needs and communicating effectively with both technical and non-technical stakeholders. Fluent English, with clear written and verbal communication. Key Responsibilities: Developing complex digital platforms - from architecture decisions to implementation and deployment. Work in autonomous teams (3-9 people) delivering mission-critical systems. Collaborate directly with technology leaders, founders, and domain experts to shape solutions that drive real business outcomes. What We Offer Transparent progression path. Flexible work: office in Kraków, hybrid or remote from anywhere in Poland, with company on-sites. Additional paid days off beyond national holidays. A deeply collaborative, engineering-led company culture. Support for ongoing learning. If you didn't find all the information you were looking for, have a look at our handbook - it covers a variety of different matters that could be important at this stage. If there's anything else we did not mention that you're interested in - share your thoughts with us [email protected]:Senior Engineer: 20 800 - 25 900 PLN net + VATLead Engineer: >26 500 PLN net + VATIncluding additional days offLocation:Office in Kraków, hybrid, fully-remote.