Lead Backend Engineer

About the role Calibrex is on a mission to democratize fitness coaching by giving everyone access to a world-class personal trainer for less than the cost of Netflix. We’re building GPS-like guidance for fitness: clear direction at every step, without excuses or guesswork. All you have to do is show up. As a Lead Backend Engineer, you’ll build the systems that make this experience intelligent, reliable, and scalable. You’ll design APIs, data models, authentication, integrations, and backend services that help athletes receive the right guidance at the right moment. You’ll work closely with frontend engineers, product designers, athletes, influencers, and leaders across fitness and professional sports. You’ll have meaningful input into product direction and technical strategy while helping establish the backend foundation for a rapidly growing product. Perks include working out in our office gym, collaborating with an elite team in a fast-growing space, high-protein catered meals, a premium Life Time or Equinox gym membership, and an office located three blocks from Grand Central. Calibrex offers an annual performance-based cash bonus and a generous equity package. The role includes great benefits and is an excellent wealth-building opportunity at a fast-growing pre-IPO company in a nascent and extremely exciting space. What you’ll bring 5+ years of professional backend or full-stack engineering experienceExperience designing and operating production systems used by 100,000+ usersStrong TypeScript and Node.js experienceDeep understanding of API design, data modeling, authentication, authorization, and asynchronous workflowsExperience with Firebase, Firestore, PostgreSQL, or comparable production databasesStrong understanding of security, privacy, validation, and protecting sensitive user dataExperience designing systems for reliability, observability, testing, and performanceAbility to lead technical architecture while remaining hands-on in implementationStrong product judgment and the ability to collaborate closely with frontend, design, and product teamsComfort using AI tools to accelerate development, debugging, documentation, and testingGenuine interest in fitness, athletic performance, or building products for athletes Preferred experience Early-Stage StartupBackend ArchitectureScaled consumer applicationsfitness or sports technologyreal-time or data-intensive systemsmobile application backendsthird-party API integrationsmedia or video systemstechnical leadershipproduct strategy Core skills TypeScript | Node.js | API design | Firebase | PostgreSQL | data modeling | authentication | cloud infrastructure | security | testing| performance optimization | AI-assisted development | claude/codex