Lead Backend Engineer

country: DE all locations: [Berlin] commitment: Full time department: location: Berlin team: Product The Role: You'll own the architecture of the Tonsser platform as we move into this next phase. That means making the foundational technical decisions for a product used by a generation of athletes and setting the bar for how a small, ambitious team builds in 2026. This is a hands-on lead role. You'll write code, shape the system, and set the technical direction. You'll also help us level up as a team, particularly around how we adopt AI as a core part of how we build and what we ship. What you'll do: Lead the architectural direction of our backend platform as we evolve from where we are today toward what's next Drive the technical roadmap for our new flagship project Set the standard for how the team works with AI, both in our development workflow and as a product capability Ship production code, review PRs, and mentor the engineers around you Partner closely with product, mobile, and design to turn ambitious ideas into things that actually work at scale Make pragmatic calls about what to keep, what to evolve, and what to rebuild Our current stack: Ruby monolith with a GraphQL API SQL (PostgreSQL) Hosted on Heroku Native iOS and Android apps, with the ambition to move to Kotlin Multiplatform We're not dogmatic about the stack, we're looking for someone who can help us decide what stays, what changes, and what comes next.   What we're looking for: Significant experience as a senior or lead backend engineer, ideally with time spent owning architecture decisions in a production environment Deep comfort with at least one of Ruby, Python, Elixir, Go, or a similar modern backend language, and the judgement to know when language choice matters and when it doesn't Strong opinions on API design, data modelling, and system evolution Hands-on experience integrating AI/LLM capabilities into real products, or a strong, demonstrable interest in doing so Experience leading or mentoring other engineers Comfortable working remotely across European timezones Bonus: experience with consumer-scale mobile backends, GraphQL, or Kotlin Multiplatform The shape of the role: Location: Remote, anywhere in Europe Engagement: Open to starting as a freelance contract, with the clear intention of moving to a permanent role Compensation: €80,000–€90,000, with flexibility based on experience How to apply: Send us a short note about what you've built, what you're looking for next, and why this caught your eye. We read every application.