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Principal Software Developer (Ruby on Rails & React) - Freelancer at Monterail. We’re specifically looking for a Principal Software Developer (Ruby on Rails & React) - Freelancer. What We’re Looking For. 8+ years of experience in software development, with deep, senior-level expertise in Ruby on Rails and React. A proven senior profile,  someone who doesn’t just execute, but defines direction, shapes systems, and confidently makes architectural and product decisions. Strong sense of end-to-end ownership: you are comfortable setting the roadmap, identifying gaps, and continuously building on top of existing systems without needing close guidance. A senior-level commitment to quality, with extensive experience designing and maintaining robust test suites; testing is a core part of how you build, not an afterthought. Clear and demonstrated focus on admin panels and internal tooling, with the maturity to design for usability, maintainability, and long-term evolution (not user-facing products). Experience working closely with internal stakeholders, with the communication skills and seniority to translate needs into well-structured solutions across varying levels of complexity. Background in smaller-scale environments (e.g., internal tools, limited user bases), where pragmatic decision-making and thoughtful trade-offs are more important than hyper-scale concerns. A self-directed, senior mindset, you bring structure to ambiguity, operate independently, and take responsibility for both the technical direction and its outcomes. What do we mean by freelance?. Read more at . Monterail Tech Network. Company Location: Poland.