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Elias Castro Hernandez

Applied AI engineer & educator

I make models intofast, reliablefeatures.

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.

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One hard idea, made simpleNo. 01 · Tokenization

Tokenizationtext 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.

25Characters
6Tokens

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

01 Text

Tokenization isn’t magic.

Cut wherever the pieces match the tokenizer’s vocabulary

02 Tokens, then 03 Numbers

  1. Tokentoken id 3404
  2. izationtoken id 2065
  3. isntoken id 2125
  4. ’ttoken id 956
  5. magictoken id 11204
  6. .token id 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.

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.

Do good work

Projects

No. 06

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

No. 05

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

  • Behavioral analyticsTalk · Stanford University
  • Data visualization for business process discoveryTalk · UC Berkeley
  • Village GamesNonprofitGame design and programming as a way into STEM.

Writing

No. 04

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.

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.

Start a consulting conversation →

Speaking & teaching

Invite me to lecture or teach your team.

Guest lectures, courses, and focused sessions for executive boards and leadership teams.

Ask about a session →
Everything else