What are the responsibilities and job description for the Fintech Software Engineer position at Phaxis?
Senior Backend Engineer
Engineering Lead Track
San Francisco, CA The RoleWe are hiring two Senior Backend Engineers to join us at a foundational moment. You will work directly with the technical founders to architect, build, and ship a highly performant, reliable, and secure financial services platform — handling real money, at scale, across international markets.
This is not a maintenance role or a feature-factory position. You will set engineering standards, make consequential technical decisions, and own core systems end to end. As the team grows, this role has a clear path to Backend or General Engineering Lead.
We are looking for engineers who have shipped production fintech software and understand the stakes: correctness, reliability, and security are not negotiable when real value is moving across borders.
What You'll Work On
- Core backend powering our iOS, Android, and Web frontends via REST API
- Integrations with banking service providers, payment rails, ledger systems, and auth infrastructure
- Cryptocurrency wallet integrations used as settlement channels
- Performance-critical services — open to Rust or Go where Python is the wrong tool
- Engineering standards and technical foundations for a system designed to scale internationally
- Python (FastAPI) monolith — our core backend
- PostgreSQL and Redis on AWS
- Rust and/or Go for performance-critical service breakouts
- REST API serving iOS, Android, and Web clients
- 5 years building and shipping production backend software in Python, Go, or Rust
- Strong fundamentals across the backend: PostgreSQL, Redis, AWS
- Experience in at least one: fintech, payments, stablecoins, banking infrastructure, ledgering, or risk/fraud systems
- Track record of shipping at scale in a production environment
- Direct experience in B2C financial services — payments processing, ledgering, stablecoin infrastructure, or fraud/risk management
- Full-stack capability — ability to flex into frontend when needed
- Experience at a fintech company (e.g. Mercury, Brex, Stripe, Chime, Marqeta, Lithic, or similar)
- Comfort working at true startup scale — small team, high ownership, significant ambiguity
- Two openings — you won't be the only one figuring it out from scratch
- Direct collaboration with technical founders from day one
- Greenfield architecture — no legacy debt, no inherited mess
- Real path to engineering lead as the team scales
- Mission that matters: expanding access to financial stability for people who don't have it