Thomas Karatzas

Hi, I’m Thomas.

I love building APIs and infrastructure, and I’m passionate about helping developers build the best products for their users.

I’m currently building AI infrastructure at Terse, a venture-backed company. Before that, I was a Staff Engineer at Apple, working on platforms and Apple Intelligence.

I also build things to learn how they work, and occasionally write about what I’m learning.

What I’m working on

Durable Actors at Terse

I’m building Terse with Olivier Simard-Morissette. My current focus is Durable Actors: a TypeScript actor framework for apps where people and AI agents work with shared state. Actors persist their state and serialize access to it, with typed clients and WebSocket support. I want to make it easier for developers to build collaborative apps and multi-agent systems without having to manage all that concurrency themselves.

A few side projects

  • GuitarGPT — I used Gemini to give feedback on videos of guitar practice.
  • ReSkin AI — a Chrome extension I built to restyle websites with AI.
  • lit-cli — a CLI for keeping track of work while building.

Writing

Substack

Before this

I spent 2018–2025 at Apple, growing from junior engineer to Staff. A lot of my work was building platforms and helping engineers across teams adopt them. Later, I worked on AI initiatives and cross-functional architecture.

Before Apple, I studied software engineering at McGill, where I got involved in the AI community and entrepreneurship. I’ve also returned as a mentor through McGill Engine.

I wrote more about that journey in my post about going from junior to Staff.

Say hello

If you’re working on developer tools or AI infrastructure, or just want to compare notes, find me on X.