About this role.
Flex is hiring a Senior Backend Software Engineer to build and maintain financial-platform services involving card processing, issuing, and core transaction ledgering. The role emphasizes end-to-end ownership of scalable, secure, highly available backend systems, from API contracts and data models through observability and production operations. Candidates need strong Node.js and TypeScript experience, cloud familiarity, architectural judgment, and at least four years of relevant industry experience. This is an early-stage, high-accountability environment with mentoring responsibilities, on-call participation, and a strong execution-oriented culture.
Role DNA
A quick view of the complexity, pace, ownership and collaboration implied by the job description.
Job Complexity
5/5Pace & Pressure
5/5Autonomy Level
5/5Communication Load
4/5Salary analysis
Estimated compensation compared with the broader US market for similar roles.
Core skills
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Sample interview questions
I would use an immutable double-entry ledger model where each transaction creates balanced debit and credit entries. I would enforce idempotency with durable request keys, maintain clear transaction states, retain an audit trail, and use reconciliation processes to identify discrepancies with external payment systems.
I require clients to provide an idempotency key, persist the key and request fingerprint before initiating side effects, and return the original result for repeated valid requests. I also define behavior for concurrent requests, expiration, partial failures, retries, and downstream provider timeouts.
I would begin with domain boundaries, team ownership, change frequency, reliability requirements, and operational maturity. A modular monolith is often preferable early because it reduces deployment and consistency complexity, while separate services become justified when a domain needs independent scaling, isolation, release velocity, or clearly distinct ownership.
I would implement structured logs with correlation IDs, distributed traces across internal and provider calls, and metrics for throughput, latency, error rates, retries, queue depth, failed reconciliation, and transaction-state transitions. I would pair those signals with actionable alerts, dashboards, runbooks, and business-level monitoring such as payment success and settlement completion rates.
I present the decision in terms of the problem, viable options, assumptions, risks, cost, delivery impact, and expected customer outcomes. I make my recommendation explicit, document why alternatives were rejected, invite challenge from relevant stakeholders, and define measurable criteria for revisiting the decision after implementation.
Flex is building the AI-native private bank for business owners.
We’re re-architecting the entire financial system for entrepreneurs—from the first dollar a business earns to how that value compounds, moves, and is ultimately spent in real life. Banking, credit, payments, personal finance, and financial operations—rebuilt from the ground up as a single, intelligent system. Flex is the full financial home for ambitious owners.
Since launching publicly in September 2023, Flex has scaled from zero to nine-figure annualized revenue, with a clear path to profitability by late 2026. We move fast, ship relentlessly, and operate with extreme ownership.
Our customers are affluent business owners ($3–$200M in revenue)—the backbone of the economy and one of the most underserved segments in finance. They’re stuck with outdated banks and fragmented tools. We’re replacing all of it. The opportunity is massive: a ~$1T+ revenue market hiding in plain sight. Our ambition is to build a $100B+ company by delivering a product that is fundamentally better—not incrementally improved.
Flex has raised $100M+ in equity and $300M+ in debt.
– Mission-critical problems: We build software that directly controls how money moves at scale.
– High bar, low ego: Small teams, exceptional people, real ownership.
– Speed over comfort: We prioritize execution, quality, clarity, and results.
– Enduring impact: What we’re building will define how a generation of owners runs their businesses.
Team & Locations
We hire exceptional people who want to build hard things and see their work matter immediately. Roles are available in: San Francisco, Miami, New York, and fully remote.
Flex Fuels Ambition.
The Role
We are looking for engineers who are excited to be part of our early story and help us build a diverse and vibrant company. As a senior software engineer you will have the opportunity to work across the entire stack with a focus on the back-end. You will have the opportunity to learn and drive the creation of financial products and experiences to help our customers reach their full potential. You should have a strong sense of ownership and enjoy taking projects from inception to release. As an early employee, you’ll be working with a nimble team of committed and talented engineers and having a large, long-term impact on technical design and engineering culture.
We are open to candidates from Canada, the United States, and Colombia.
What You’ll Do
- Ownership over large domains and features related to card processing/issuing, and core ledgering of financial transactions
- Play a critical role in the development, design, and maintenance of Flex’s platform and services.
- Architect well thought-out services and data models that ensure Flex’s continued growth
- Own problems end-to-end, thinking through everything from api contracts, data, scalability, security, observability, and metrics.
- Lead by example and mentor engineers in all aspects of planning, development and testing.
- Be a catalyst for driving best-in-class technology frameworks and tooling while staying engaged with the latest technology trends.
- Be on call, with the expectation to ability to solve problems and find solutions for the servicing team
What You Need
- Proficiency in NodeJS+Typescript and server design principles.
- An understanding and interest in architectural patterns for highly available, scalable, low-latency, asynchronous systems
- Ability to hold yourself and the team to high standards.
- Strong communication (written and verbal) and interpersonal skills.
- Ability to provide in-depth evaluation of multiple technical directions and determine tradeoffs of each.
- Strong propensity to make data-driven decisions on technical architecture and project prioritization.
- Exposure to modular monoliths and micro-services along with the benefits and challenges they bring.
- Experience with any cloud based provider like Google Cloud, AWS, or Azure.
- Four or more years of relevant industry experience with a track record of shipping high-quality products and features at scale.
- A strong work ethic and are ready to grind
- Have built and shipped a working project or product, ideally something complex
Compensation
Additional Information
Interested? We’d love to hear from you
At Flex, we value passion, determination, and honesty. Even if you don’t fully match the job specifics, we encourage you to apply. Unusual career paths and unique skills can help you stand out. We believe diversity drives our success. Join us at jobs@flex.one
Why Join Us
Build something generational — Capture the full lifecycle of money for ambitious business owners.
Work on real money, real risk — Payments, credit, and banking at serious scale.
Solve hard problems — AI, underwriting, compliance, and global finance from first principles.
True ownership — Small teams, high trust, real accountability.
Founder-level exposure — Direct access to leadership, customers, and investors.
High bar, high taste — Move fast without cutting corners.
Elite peers — People here are builders, not tourists.
Real upside — Meaningful equity if you help build something big.
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