Senior Fullstack Engineer

At Upside, we’re partnering with ambitious companies across the globe to solve complex, meaningful problems through technology. We work in 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, a customizable agentic AI toolkit for e-commerce. What it means for you You’ll have a 'Founder' mindset. You won’t just build a UI or a database - you’ll own the entire product experience. You’ll be a technical partner to stakeholders, helping them understand what’s possible with modern tech and AI. You’ll have the autonomy to experiment across the stack and the responsibility to deliver tools that actually solve business problems. You’ll work at the intersection of product thinking and engineering, deploying AI-augmented features across the entire stack. Role Overview As a Senior Fullstack Engineer, you’ll design and scale resilient systems from the ground up. You’ll bridge the gap between backend logic and frontend experience, ensuring a seamless, production-grade product. ResponsibilitiesEnd-to-End Ownership: Lead the design and implementation of both backend systems and frontend interfaces. Proactive Solutions: Don't just flag blockers—come with proposals and experiments to bypass them. Consulting: Translate complex business needs into elegant, scalable fullstack architecture. Engineering Standards: Advocate for code quality, security, and performance across the entire repo. Continuous Improvement: Constantly evaluate new tech (especially AI/LLM) to improve delivery. RequirementsExperience: 5+ years of building complex web applications. Backend: Strong proficiency in Python or Ruby. Frontend: Mastery of Vue.js or React.js. Infrastructure: Solid understanding of SQL (PostgreSQL), CI/CD, and cloud environments. AI-Native: You use tools like Claude or Cursor to build better and faster. English: Fluent (C1) for direct client collaboration. Tech Stack: Python, Ruby, Vue.js, React.js, PostgreSQL, AWS, Docker. Value PropositionBuilt for the long run We hire for the company, not just a project - looking for passion driven engineers who want to grow with Upside over timeSmall teams, real accountability. 3-9 people per engagement. We're small by design. That means your decisions carry weight and you are visible. Mission-critical work: We handle complex projects that move the needle for international brands. Our clients are scaling companies, established international brands, and large institutions. The systems you build will be under real load, with real stakes. Real Ownership: You take part in the decision making process on technology and implementation.. You’re empowered to take initiative, experiment, and drive solutions that create real impact. No Micromanagement: We trust you to own your work and deliver outcomes - no unnecessary oversight. Collaborative R&D: Contribute to targeted Open-source projects and internal AI research. Growth through challenges: Complex problems and direct client exposure means you’ll grow by action. Applied AI as a first-class discipline. We don't bolt AI onto projects as an afterthought. You'll identify where it genuinely moves the needle and lead the implementation - on client work and on our own open-source tooling. The tools you want. If something makes you faster or sharper, we want you using it. Transparent progression path. We have defined engineering levels with clear criteria - you'll know exactly what each role looks like, and what it takes to get from one to the other. No ambiguity, no waiting for someone above you to move onIf 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 careers@upsidelab.ioCompensation & LocationSalary: Senior Engineer: 20 800 - 25 900 PLN net + VATLead Engineer: >25 900 PLN net + VATLocation: Office in Kraków, hybrid, fully-remote.