Principal Engineer, AI Systems

Block builds simple, powerful tools that make progress towards an economy that's truly open to all. Each of our brands unlocks different aspects of the economy for more people. Square makes commerce and financial services accessible to sellers. Cash App is the easy way to spend, send, and store money. Afterpay is transforming the way customers manage their spending over time. TIDAL is a music platform that empowers artists to thrive as entrepreneurs. Bitkey is a simple self-custody wallet built for bitcoin. Proto is a suite of bitcoin mining products and services. Together, we're helping build a financial system that is open to everyone. Join us. The Role Block is building toward one of the most ambitious technical visions in our history: an AGI-enabled entity that fundamentally transforms how we deliver economic empowerment. We're assembling a world-class team of AI experts to design and ship autonomous agents and agentic workflows that operate across Square, Cash App, and the broader Block ecosystem. This work is about creating solutions—you'll be building AI that acts, decides, and completes real tasks for real users from across all of Block's business functions. As a Principal Engineer, you'll join as a senior individual contributor with significant technical leadership responsibilities. You Will Ship, architect, build, and own end-to-end delivery of autonomous agents and agentic workflows that deliver real business value for Block, ensuring reliability, safety, and performance at scale Design agent orchestration systems including planning, tool use, memory, evaluation, and multi-agent coordination at production scale Integrate and optimize frontier LLMs into agent architectures, making decisions on model selection, fine-tuning, prompt engineering, and context retrieval strategies Drive deeper model optimization work (fine-tuning, distillation, RLHF) where it unlocks agent capability or efficiency Lead detailed technical planning by breaking down ambitious objectives into concrete, sequenced tasks with clear ownership and execution paths Provide technical mentorship and guidance to engineers across experience levels, elevating team capabilities through code review, pairing, and knowledge sharing Partner closely with technical and non-technical stakeholders to translate business objectives into agent-powered product experiences Keep Block at the frontier by continuously evaluating emerging AI capabilities and making pragmatic tradeoffs across model performance, latency, cost, and user experience Foster a culture of technical excellence, high-quality delivery, rapid experimentation, and learning within your team and beyond You Have 15+ years of experience in software engineering or machine learning, with recent professional experience building autonomous agents or agentic workflows in production Deep experience building autonomous agents or agentic workflows in production environments—not just prototypes or demos Fluency in the core primitives of agentic systems: context management, planning, tool use, memory, evaluation, and multi-step reasoning Experience bringing frontier LLM capabilities into production products, with hands-on experience in prompt engineering, retrieval-augmented generation, and model optimization A track record of taking AI-powered products from zero to scale in fast-paced, product-driven environments, with the judgment that comes from operating in production Strong software engineering fundamentals with the ability to write production-quality code and make sound architectural decisions Experience providing technical leadership within teams—you've shaped technical direction, driven execution, and elevated others Product-minded engineering approach—you think in terms of user outcomes, not just model metrics Excellent collaboration and communication skills, with ability to build alignment across engineering, product, and design Comfort navigating extreme ambiguity in a domain that's evolving weekly Alignm