Mid-to-Senior Full-Stack Software Engineer

← Careers Full-time Zagreb or Remote Mid to Senior Full-Stack Software Engineer We're looking for a Software Engineer to work across high-impact US and EU engagements. You'll build production-grade full-stack systems — from React frontends and REST APIs to cloud infrastructure and AI-integrated backends — with full ownership of the code you ship. This role is for engineers who move fast and take pride in clean, scalable work. About Seyn Seyn is an AI-native engineering firm headquartered in Zagreb, Croatia. We're building the operating layer where AI runs businesses — learning how they work, then automating them. Right now we do that through client work: custom AI solutions deployed in days, for companies that are done waiting. Every engagement moves us closer to the infrastructure we're actually here to build. What you'll do —Build and maintain full-stack applications across the entire product lifecycle — from database schema to pixel-perfect UI.—Own your code from design through deployment — accountable for quality, performance, and reliability in production.—Design and build RESTful APIs and backend services that are clean, well-documented, and built to scale.—Integrate AI capabilities into product features — LLM calls, structured outputs, AI-driven automation — working alongside AI Engineers to ship coherent systems.—Participate in agile ceremonies directly with US and EU clients — sprint planning, standups, retrospectives.—Use AI-augmented development tools daily as a force multiplier: code assistants, automated testing, documentation generation.—Instrument applications with logging and monitoring tooling; respond to production incidents with urgency and ownership. What makes you a strong fit —4+ years of professional software engineering experience, with meaningful full-stack production work.—Strong proficiency in TypeScript or JavaScript (Node.js) on the backend and React or similar on the frontend.—Solid understanding of relational databases (PostgreSQL or similar), REST API design, and distributed system fundamentals.—Experience with testing (unit, integration, E2E) and a genuine belief that code review makes the team better.—Comfortable working with AI tools (Cursor, Copilot, Claude) as part of your daily workflow — curious and fast.—Strong written and spoken English — you will communicate daily with US-based teams and clients.—Bonus: experience calling LLM APIs from application code, working with structured outputs, or building AI-powered product features. Nice to have —Experience with Python for backend services or scripting.—CI/CD pipelines (GitHub Actions, Jenkins) and containerisation (Docker, Kubernetes).—Cloud platform experience: AWS, GCP, or Azure.—Redis, RabbitMQ, or similar queuing and caching layers. Why Seyn —Work on production systems used by emergency services, global real estate companies, public safety infrastructure, and high-growth US tech companies.—AI-native culture — AI-augmented workflows are not optional here; they are how we work. You'll be trained, supported, and pushed to use AI as a core part of your daily process.—Seyn's founders previously exited Flow and Form, a software engineering firm acquired by Robin. You're joining a team that has done this before.—Competitive salary, flexible hours, remote-friendly. Compensation EUR 3,500 – 5,200 gross / month Employment contract EUR 10,000 – 30,000 / year Equity grant or profit pool (estimated) Apply now → Or reach out directly at doris@seyn.so