Let’s Build the GPT Tokenizer: A Complete Guide to Tokenization in LLMs

📰 Fast.ai Blog

Build a complete guide to tokenization in LLMs based on Andrej Karpathy's video

intermediate Published 15 Oct 2025
Action Steps
  1. Watch Andrej Karpathy's 2h13m tokenizer video
  2. Read the translated book chapter on tokenization
  3. Implement key pieces of code from the chapter
  4. Experiment with different tokenization techniques for LLMs
Who Needs to Know This

NLP engineers and researchers on a team can benefit from understanding tokenization in LLMs to improve model performance and efficiency. This knowledge can also be useful for AI engineers and ML researchers working with language models.

Key Insight

💡 Tokenization is a key component of how LLMs work

Share This
📚 Build your own GPT tokenizer with this complete guide!

Key Takeaways

Build a complete guide to tokenization in LLMs based on Andrej Karpathy's video

Full Article

18 months ago, Andrej Karpathy set a challenge : “Can you take my 2h13m tokenizer video and translate the video into the format of a book chapter”. We’ve done it, and the chapter is below, including key pieces of code inlined, and images from the video at key points (hyperlinked to the video timestamp). It’s a great video for learning this key piece of how LLMs work, and this new text version is great too.
Read full article → ← Back to Reads

Related Videos

5 Levels of AI Agents - From Simple LLM Calls to Multi-Agent Systems
5 Levels of AI Agents - From Simple LLM Calls to Multi-Agent Systems
Dave Ebbelaar (LLM Eng)
Gemini AI + Nano Banana: Deep Research to Full eBook FAST
Gemini AI + Nano Banana: Deep Research to Full eBook FAST
LoverFighterWriter
How to Use Google Gemini AI For Beginners (Full Tutorial)
How to Use Google Gemini AI For Beginners (Full Tutorial)
LoverFighterWriter
Claude vs ChatGPT: Which AI Writer Crushes Competitors?
Claude vs ChatGPT: Which AI Writer Crushes Competitors?
LoverFighterWriter
Off-Page Topical Map: Why Third-Party Corroboration Improves LLM Visibility (Karl ft James)
Off-Page Topical Map: Why Third-Party Corroboration Improves LLM Visibility (Karl ft James)
James Dooley
AI Reputation Tree - Getting The LLMs To Be Your 24/7 Sales Engine (Karl Hudson ft James Dooley)
AI Reputation Tree - Getting The LLMs To Be Your 24/7 Sales Engine (Karl Hudson ft James Dooley)
James Dooley