Senior Full Stack Software Engineer Jobs in Philadelphia, PA

Circle is building the world’s leading AI-powered, all-in-one platform for digital businesses. We make it possible for creators, coaches, educators, and businesses to bring together their audience with engaging discussions, live streams, events, chat, courses, and payments — all in one place, all under their own brand.

We’re proud to be a fully remote company of around 270 (and growing!) team members from 30+ countries around the world. We seek exceptional individuals around the world, set them up to do the best work of their lives, and in turn, create a meaningful impact in their own lives. We don't track hours, but we do manage for high expectations very closely. We collaborate across time zones, are highly async, and like to document a lot.

Twice a year, we bring the whole company together in beautiful places around the world for our company offsites. So far, we’ve hosted offsites in Turkey, Portugal, Mexico, Thailand, Colombia, Italy, Ireland, and more, with still more to come!

Check out our Careers page for more about working at Circle.

The AI Core team at Circle is at the forefront of leveraging AI to drive innovation for our customers through transformative applications of large language models (LLMs).

We're looking for a strong full-stack engineer who is proficient in web frameworks, backend development, and infrastructure to spearhead the development of full-stack AI solutions across Circle. You will work on exciting projects such as AI agents, Retrieval-Augmented Generation, AI evaluations and observability, and building infrastructure for LLM inference.

While deep AI specialization or expertise isn't required, we care about some hands-on experience building AI features. Beyond that, we’re looking for a strong experimentation mindset. If you're excited about rapidly testing ideas, running A/B experiments, and iterating on real production use cases of LLMs, this role is for you.

The Applied AI team is composed of different backgrounds. Some team members joined with deep Rails experience and solid AI fundamentals, while other team members joined with deep AI experience and ramped up on Rails. Either path works as long as you bring hands-on experience with both Rails and AI and have genuine curiosity to grow.

• Ship full-stack AI projects end-to-end on a product built in Ruby on Rails backend and React frontend.

• Design and run experiments (including A/B tests, AI evaluations, or others) to validate AI features, measure impact, and guide iteration.

• Build and integrate components for AI infrastructure and optimize them for performance and reliability to support production-level inferencing.

• Improve processes, tools, and systems to scale AI features into production-ready solutions.

• Help us stay up-to-date with the cutting-edge AI research, techniques, and tools.

• Work closely with Circle's leaders and designers throughout the feature design process.

• Foster a bias for action by prioritizing speed of iteration, experimentation, and learning, while maintaining high standards of code quality and user experience.

• 6+ years experience working as a full-stack engineer on high-traffic production applications. You've dealt with the challenges that come with scale — query optimization, careful migrations, performance bottlenecks.

• Strong proficiency in Ruby on Rails, MySQL / Postgresql, ReactJS. Familiarity with both frontend and backend is necessary, but a high degree of proficiency in both is not a requirement.

• Hands-on experience building AI features in production — whether RAG systems, AI agents, or LLM-powered tools. You've shipped at least one real AI use case, not just prototypes.

• Strong experimentation mindset — you’re comfortable designing and running A/B tests, measuring results, and iterating quickly to discover what works and what doesn’t.

• A desire to work in an environment which values speed of iteration and individual autonomy, while also embracing personal accountability and the ability to collaborate effectively as part of a dynamic team.

• Comfortable in a fast-paced environment with a certain level of ambiguity, especially when learning and picking up new technologies when projects require it.

• Strong alignment with our values, find our values on our career page if you haven’t read up on them yet.

• You are proficient in English (spoken, written, and reading) at a CEFR Level C2 / ILR Level 5.

Circle offers U.S.-benchmarked compensation globally, equity in the company with ongoing refresh grants, and 35 days of paid time off each year.

We’re a remote-only team that comes together twice a year for company retreats in incredible destinations around the world. Alongside incredible flexibility and autonomy, we offer a benefits package that supports health, wellbeing, and professional growth. Learn more in our Candidate Hub.

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• Visit our Candidate Hub to learn more about working at Circle, our benefits, and our hiring process.

Compensation Range: $130K - $140K