Founding Full-Stack Engineer (React.js / Next.js)

Salary Transparency:This position is intended for candidates with over 5 years of professional experience.The gross annual salary range is between €90,000 and €120,000, depending on experience.Location: Berlin (on-site)Three hires will be made for this position.Muxon is a small, globally active recruiting agency managing this search on behalf of an early-stage, Berlin-based startup. We are recruiting three hands-on Founding Software Engineers to join a compact, founder-led team on-site in Berlin — a role that offers real ownership, significant equity, and the opportunity to shape product and architecture from day one.Requirements:Minimum 5 years professional full-stack development experience.Proven experience with TypeScript, React.js, and Next.js.Experience with relational databases (preferably PostgreSQL) and cloud platforms (GCP or AWS).Strong product ownership, high velocity in ambiguous environments, and an entrepreneurial mindset.Professional English (C1) and willingness to work fully on-site in Berlin.Responsibilities:Own end-to-end development of user-facing features across frontend and backend.Design and implement scalable architecture for growth to millions of users.Build and optimize systems integrating LLMs and agentic tool-calling where relevant.Drive technical decisions, run evaluation frameworks (automated benchmarks, A/B tests), and optimize inference performance and cost.Collaborate directly with founders on roadmap and product direction.Benefits:Equity stake (founding stock options).Above-market salary and competitive package.Company-paid lunch & dinner.Work on a consumer AI product used by thousands daily with high founder impact.If you’re driven to build a category-defining product and want real ownership from day one, apply — We are actively sourcing strong candidates for this founding role.