Senior Software Engineer - Security/Infrastructure

ABOUT AFTERQUERY AfterQuery https://www.afterquery.com/ is an applied research lab curating data solutions for foundation model development. We serve every frontier AI lab with the mission of delivering the best data to power the best models. In doing so, we can make expertise that once took a lifetime to build available to anyone who needs it. Our customers are the ones building the foundation models themselves and our work sits directly in the loop of how those systems improve. This is a rare opportunity to join a company at a defining moment in AI. Since raising our $30M Series A at a $300M valuation, AfterQuery has grown well over a $100M revenue run rate. We're based in San Francisco and backed by leading investors including Altos Ventures, BoxGroup, and Y Combinator and angels from Google DeepMind, OpenAI, Anthropic, Meta Superintelligence Labs, and Microsoft AI. WHY APPLY Massive Opportunity: We are one of the fastest-growing YC companies in our batch, and we believe we can become one of the fastest-growing YC companies of all time. Founding Impact: You will own and architect core infrastructure systems that power our platform from the ground up. Equity & Growth: Competitive salary and meaningful equity. As we scale, you’ll have the opportunity to shape the engineering organization and lead major technical initiatives. Strong Team: Our founding team has experience from Citadel Securities, Meta, Google, Silver Lake, and Morgan Stanley — work alongside world-class engineers and researchers. OVERVIEW You’ll be responsible for protecting a system that handles highly sensitive workflows across AI training, expert networks, and enterprise integrations with leading model labs. This role is deeply technical. You’ll design and implement the infrastructure that keeps our systems resilient, rather than managing off-the-shelf security tools. We move fast and rely heavily on AI internally. You should be comfortable using LLMs to accelerate debugging, threat analysis, and system design—and just as comfortable replacing repetitive work with code. Security here isn’t isolated. When you’re not working on security problems, you’ll be building core systems—especially human-in-the-loop platforms and data infrastructure that power how frontier AI models are developed. We work in-person in San Francisco.   RESPONSIBILITIES Build systems that identify and respond to real security events across our stack Replace manual security workflows with programmatic, scalable solutions Strengthen our cloud and production environments (GCP/AWS, containers, networking) Design and enforce access control systems across internal teams, experts, and customers Integrate security checks directly into development workflows without slowing shipping speed Own incident handling systems and processes from detection through resolution Secure surfaces unique to AI systems, including data flows and model interaction layers Develop human-in-the-loop systems used in AI training and evaluation Contribute to core infrastructure and internal tooling used across the company Ship features across the stack that directly impact customers and model performance   REQUIRED QUALIFICATIONS You’ve built internal tools or systems to solve security problems—not just configured existing products Strong programming ability in Python, Go, or TypeScript Experience working with cloud environments (IAM, networking, containers) Ability to identify vulnerabilities directly in code and system design Experience creating or maintaining signal-based monitoring and alerting systems Comfortable owning production issues when they arise Able to move fluidly between security work and general engineering 2-7 years building software or security systems PREFERRED QUALIFICATIONS Background in fast-scaling startups or high-output engineering teams Exposure to AI systems, data pipelines, or model evaluation workflows Familiarity with offensive security techniques Experience building tools that other engineers depend on Evidence of building and shipping systems from zero to production