I'm Xabier Ariznabarreta. I was born and raised in Madrid, did high school at Trinity College in San Sebastián de los Reyes, and now study AI Engineering at Universidad Francisco de Vitoria in Madrid. I started building AI projects at the beginning of my second year of the degree and shipped my first production agent a few months later. Since then I've sent several to production: commercial agents integrated with Salesforce, MCP servers with security-review loops, Android field assistants, multilingual institutional tooling.
The work was useful. It was also a teacher. The bugs were instructive, the customers were patient, and the experience showed me where the bottleneck is — not in the engineering, but in the science underneath.
So I'm pivoting. I'm working on my first empirical paper (an ablation study of agent-harness components), going through ARENA in parallel, and using this site as the public notebook for both. Builder in the concrete, learner in the deep.
What you'll find here
Four kinds of posts:
- Technical deep-dives — rigorous explanations of LLM concepts, transformer internals, mech interp, eval methodology. Depth before speed.
- Research notes — work-in-progress on the harness-ablation paper. Hypotheses, design choices, null findings included.
- Production lessons — honest case studies from systems I've shipped, including the expensive mistakes.
- Meta journey — reflections on the builder-to-researcher pivot at 19, ARENA progress, decisions I'm reversing, honest self-critique.