Joe Leo
Founder & CEO, Def Method
I founded Def Method in 2014 with a simple premise: the most valuable engineering work happens when the stakes are high and getting it wrong isn't an option.
I've helped engineering teams at companies like Citi, Walmart, and Paxos modernize systems that couldn't afford to break. Revenue-critical platforms. Regulated environments. Operationally sensitive infrastructure.
I'm also the co-author of The Well-Grounded Rubyist (now in its 4th edition) and co-host of The Ruby AI Podcast, where we explore the intersection of Ruby and artificial intelligence.
That combination of high-stakes modernization work and practical AI exploration shaped GUIDE: our operating philosophy for helping teams preserve understanding, strengthen governance, set direction, and build evidence before implementation accelerates.
Speaking & Podcast
I regularly speak at conferences about Ruby, Rails, and engineering leadership. I'm also the co-host of The Ruby AI Podcast.
Latest Insights
View all →No More Free Rides
Amazon's retail site went down three times in a week. The cause: an AI agent acted on bad guidance from a stale internal wiki. This is what the Understanding principle is for.
Governance Like a Warm Hugging Face
An OpenAI model escaped its sandbox during an internal evaluation and breached Hugging Face. What made it a contained incident instead of a catastrophe was governance infrastructure Hugging Face had already built before any of this happened.
Somebody Has to GUIDE Us
AI unbundles implementation from understanding. The person writing the code no longer has to know the system. That's the gain — and that's the gap.
The Well-Grounded Rubyist
4th Edition — Co-authored with David A. Black
The Well-Grounded Rubyist is a comprehensive guide to Ruby that has helped thousands of developers master the language. Now in its 4th edition, it covers Ruby 3.x and modern Ruby idioms while maintaining the deep, thorough approach that made it a classic.
Whether you're new to Ruby or looking to deepen your understanding, this book provides the foundation you need to write confident, idiomatic Ruby code.
Want to work together?
If you're responsible for a high-stakes system that needs to modernize for AI-assisted development without losing engineering confidence, let's talk.