---
title: Elias Castro Hernandez
description: Elias Castro Hernandez is an applied AI engineer and educator. Projects, teaching and writing on building AI systems that hold up in the real world.
url: https://ehcastroh.me/
---

# Elias Castro Hernandez

**Applied AI engineer & educator.** I make models into fast, reliable features.

Research tells you what's possible. Shipping tells you what holds up at scale, with real users and real limits. I build the systems that get a model from one to the other, and I teach the engineering behind them.

Now building: LLM wiki.

## One hard idea, made simple: Tokenization (text becomes numbers)

Before a model reads your prompt, it cuts the text into tokens. Common words stay whole, rarer ones get split into pieces it has seen before, and each piece is swapped for a number. Those numbers are all the model ever sees.

Example: "Tokenization isn’t magic." becomes 6 tokens:

| Token | ID |
|---|---|
| `Token` | 3404 |
| `ization` | 2065 |
| `·isn` | 2125 |
| `’t` | 956 |
| `·magic` | 11204 |
| `.` | 13 |

The dot marks a space, which usually rides along with the word after it. The split and the numbers here are illustrative. Every model ships its own tokenizer and cuts text a little differently.

It matters because you pay, wait and run out of context in tokens, not words.

## About

Microsoft, UC Berkeley.

I care about helping people do their best work. Some days that means making a model answer faster. More often it means explaining how these systems work to the people who have to build with them, buy them or manage the teams that do.

I teach graduate and undergraduate students, run workshops for senior leaders and VPs, teach online on edX and run summer boot camps.

I also run Village Games, a nonprofit that uses video game design and programming to get underrepresented minorities and women interested in careers in STEM.

## Projects

Systems I built, and what each one taught me.

Coming soon. Project write-ups are in progress. Each one will split the build into phases and link back to its commits.

## Teaching

Graduate and undergraduate courses, leadership workshops, edX, summer boot camps and Village Games.

- **Behavioral analytics**. Talk · Stanford University.
- **Data visualization for business process discovery**. Talk · UC Berkeley.
- **Village Games**. Nonprofit. Game design and programming as a way into STEM.

## Writing

What I learned from a project or a class, written down while it was fresh.

Coming soon. The first reflections are being written. Each one will link to the project or talk it came from.

## Contact

- **Consulting.** Bring me in on a hard AI problem. Scaling up, users who don’t behave the way the plan assumed, and the edge cases that only show up once a model is live.
- **Speaking & teaching.** Invite me to lecture or teach your team. Guest lectures, courses, and focused sessions for executive boards and leadership teams.

- GitHub: https://github.com/ehcastroh