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.

Responsibilities

• Design and operate production systems used by 100,000+ users
• Build APIs, data models, authentication, authorization, and asynchronous workflows
• Develop backend services that deliver intelligent, real-time guidance to athletes
• Collaborate with frontend engineers, designers, and product teams
• Lead technical architecture while remaining hands-on in implementation
• Ensure reliability, observability, testing, and performance of systems
• Protect sensitive user data with strong security and privacy practices
• Use AI tools to accelerate development, debugging, documentation, and testing

Requirements

• 5+ years of professional backend or full-stack engineering experience
• Strong TypeScript and Node.js experience
• Deep understanding of API design and data modeling
• Experience with Firebase, Firestore, PostgreSQL, or comparable production databases
• Strong understanding of security, privacy, validation, and protecting sensitive user data
• Ability to design systems for reliability, observability, testing, and performance
• Strong product judgment and collaboration skills with cross-functional teams
• Genuine interest in fitness, athletic performance, or building products for athletes

Preferred experience

• Early-stage startups
• Backend architecture for scaled consumer applications
• Fitness or sports technology
• Real-time or data-intensive systems
• Mobile application backends
• Third-party API integrations
• Media or video systems
• Technical leadership and product strategy

Skills

• TypeScript
• Node.js
• API design
• Firebase
• PostgreSQL
• Data modeling
• Authentication
• Cloud infrastructure
• Security
• Testing
• Performance optimization
• AI-assisted development (e.g., Claude, Codex)

Benefits

• Work out in the office gym
• Collaborate with an elite team in a fast-growing space
• High-protein catered meals
• Premium Life Time or Equinox gym membership
• Office located three blocks from Grand Central
• Annual performance-based cash bonus
• Generous equity package