Senior Software Engineer — API, Billing & Scalable Infra

Sixtyfour; San Francisco (CA); On-site; USD 180,000 - 230,000; Full time

Benefits offered by this job

Unlimited PTO

Lunch on us in the office

Monthly gym membership reimbursement

Monthly AI tools/productivity reimburs

401k with company match

Health / Dental / Vision

Flexible working hours

Job summary

Sixtyfour is seeking a Senior Software Engineer to own and evolve core backend infrastructure, API design, and billing systems as we scale. You’ll lead architectural decisions, mentor engineers, and work on AI-powered workflows and data pipelines in a fast-growing startup.

You’ll collaborate with leadership on the technical strategy while delivering robust, scalable solutions in Python/JavaScript, Kubernetes, and cloud services. Relocation to the SF Bay Area is expected for this early-stage team.

Qualifications

• 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.

Responsibilities

• 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 productionise billing and payments infrastructure (Stripe)
• Build and productionise 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

Skills

API design

Backend development

Observability

Backend infrastructure

Billing/payments systems

Python

JavaScript

Kubernetes

Apache Spark

AWS

Temporal

OpenRouter

LLMs

Search

Mentorship

Leadership

Tools

Stripe

Job description

Sixtyfour is seeking a Senior Software Engineer to own and evolve core backend infrastructure, API design, and billing systems as we scale. You’ll lead architectural decisions, mentor engineers, and work on AI-powered workflows and data pipelines in a fast-growing startup.

You’ll collaborate with leadership on the technical strategy while delivering robust, scalable solutions in Python/JavaScript, Kubernetes, and cloud services. Relocation to the SF Bay Area is expected for this early-stage team.