Senior Software Engineer - Infrastructure

About Sixtyfour Sixtyfour helps teams understand the people, companies, and entities they interact with. With a primary focus on Compliance (AML/KYB), Trust & Safety, Cybersecurity, and Investigations. Our platform combines agents, LLMs, and large-scale data infrastructure to surface data other platforms can't.  The Role We're looking for a Senior Software Engineer to join our core Engineering team and help build the infrastructure Sixtyfour runs on. This is a hands-on, high-ownership role spanning backend systems, API design, billing, and infra scaling. This is the kind of work that shapes not just what we ship, but how we build for years to come. You won't have direct reports, but you'll operate as a technical leader from day one: setting the standards, shaping architecture decisions, and mentoring engineers as the team grows around you. If you're the kind of engineer who gets excited about API design, scaling infrastructure, and payments/billing systems, let’s talk. What You'll Do Own and evolve core backend infrastructure as we scale usage and traffic Design and build robust, well-architected APIs that power our core product and developer platform Build and maintain billing and payments infrastructure (Stripe) Build and productionize agent workflows and LLM-powered systems Drive technical decisions around scaling — performance, reliability, data pipelines, and search Operate like an SRE when needed. Work hands-on in the codebase daily while mentoring other engineers and helping raise the technical bar across the team Partner directly with leadership on technical strategy and roadmap as an early, influential member of the team What We're Looking For 3–7 years of professional software engineering experience Experience as a team lead or tech lead — comfortable providing technical direction and mentorship without needing direct reports to do so Strong API design and backend development experience Experience with observability  Experience with backend infrastructure and scaling systems in production Experience building or maintaining billing/payments systems (Stripe or similar) Proficiency in Python and JavaScript Experience with Kubernetes Experience with Apache Spark Experience with AWS: Glue, EC2,  Data Pepper & CloudWatch Experience with: Temporal  Experience with OpenRouter Experience with LLMs, scaling infrastructure, and search Comfortable in a fast-paced, early-stage environment with high ownership and ambiguity Willing to be based in or relocate to the San Francisco Bay Area. Nice to Have Experience building developer-facing products or platforms (APIs, SDKs, dev tools) Experience with Next.js and Vercel Prior startup experience, ideally at a similarly early stage Experience working in an AI company Experience developing agents   Benefits Unlimited PTO Lunch on us in the office Monthly gym membership reimbursement Monthly AI tools/productivity reimbursement 401k with company match Health / Dental / Vision  Flexible working hours