AI Under the Hood (Beta)

Appearance

Understanding Tokenizers in AI: A Deep Dive into ChatGPT, Grok, and Gemini ​

Checkout on Linkedin ​

Introduction ​

Tokenization is one of the most underrated yet most foundational components in Natural Language Processing (NLP) and modern Large Language Models (LLMs). Before a model like ChatGPT, Grok, or Gemini can interpret text, it must convert raw text into tokens — numerical units that form the input sequence for transformer architectures.

While humans read words, LLMs read token IDs.

And the tokenizer determines:

  • how text is segmented,
  • how long your input becomes,
  • how much you pay (tokens = cost),
  • how efficiently the model learns,
  • and even how well the model handles languages like Hindi, Tamil, or Japanese.

Poor tokenization leads to inflated token counts, truncated inputs, weaker multilingual performance, and degraded reasoning.

This article explores:

  • how tokenizers work,
  • different tokenization methods,
  • and the specific tokenizers used in ChatGPT (OpenAI), Grok (xAI), and Gemini (Google) — based on official documentation and open-source releases.

How Tokenization Works ​

Tokenization generally involves two steps:

  1. Splitting text into tokens (words, subwords, characters).
  2. Mapping each token to an integer ID from the vocabulary.

Example using a typical LLM tokenizer:

"Hello, world!"
→ ["Hello", ",", " world", "!"]
→ [15496, 11, 995, 0]

Why not use full words?

Because:

  • words vary a lot (“run”, “running”, “runs”)
  • new words constantly appear
  • multilingual corpora explode vocabulary sizes
  • rare words are inefficient to represent
  • model size scales with vocabulary size

This is why modern tokenization uses subword approaches — allowing flexible combinations while keeping vocabularies manageable.

The Mathematical Bridge: From Tokens to Embeddings ​

Tokenization serves as the crucial link that transforms human-readable text into a mathematical format that LLMs can process. Once text is split into tokens and mapped to integer IDs, these IDs are fed into an embedding layer—a trainable matrix that converts each discrete ID into a high-dimensional continuous vector (e.g., a 512- or 4096-dimensional float array).

This vector representation enables the application of linear algebra operations, which form the backbone of transformer architectures. For instance:

  • Matrix Multiplications: Embeddings are multiplied by weight matrices in feed-forward layers to capture patterns.
  • Attention Mechanisms: Involve dot products between query, key, and value vectors to weigh contextual importance, followed by softmax for probability normalization.
  • Other Algorithms: Gradients during training (via backpropagation), normalization layers, and optimizations like Adam rely on these vector spaces.

Without tokenization's conversion to numbers, none of these mathematical tricks—rooted in calculus, probability, and linear algebra—would be possible, turning raw text into computable data for learning and inference.

How Math Makes LLMs "Intelligent" (A Quick Note) ​

LLMs "sound intelligent" by predicting the next token through probabilistic computations over vast vector spaces, leveraging massive matrix operations on GPUs to model semantic relationships and generate coherent responses. This math simulates reasoning but is fundamentally pattern matching at scale. (Stay tuned for a dedicated article diving deeper into the math powering LLM intelligence.)

Types of Tokenizers in NLP ​

1. Word-Level Tokenization ​

Splits based on spaces/punctuation. Fast but problematic for:

  • inflection-heavy languages
  • languages without spaces (Chinese, Japanese)
  • spelling variations

Not used in modern LLMs.

2. Character-Level Tokenization ​

Every character is a token.
Pros: tiny vocabulary
Cons: extremely long sequences → inefficient

Occasionally used in niche research.

3. Subword Tokenization (Modern LLM Standard) ​

This is the foundation of nearly all major LLMs. Subword methods include:

Byte Pair Encoding (BPE) ​

WordPiece ​

SentencePiece (BPE or Unigram) ​

  • Used in Gemini (Google), Grok (xAI), LLaMA (Meta)
  • Language-agnostic
  • Trains on raw text (no whitespace assumptions)
  • Reference: (Kudo & Richardson, 2018)
    https://arxiv.org/abs/1808.06226

Unigram LM Tokenizer ​

  • Probabilistic subword selection
  • Used by Google’s T5, Gemini
  • Excellent for multilingual corpora

4. N-Gram Tokenization ​

Rarely used in modern LLMs — included for completeness.

Why Subword Tokenization Became the Standard ​

Because it balances:

  • Vocabulary size
  • Handling of rare words
  • Ability to represent new words
  • Computational efficiency

Words like “electroencephalography” are broken into smaller reusable units, reducing memory and training cost.

Tokenizers in ChatGPT (OpenAI) ​

OpenAI’s GPT models use Byte Pair Encoding (BPE) implemented through the tiktoken library:

➡️ https://github.com/openai/tiktoken

Key Tokenizers ​

ModelTokenizerNotes
GPT-3 / 3.5cl100k_base~50k vocabulary
GPT-4Updated BPEBetter multilingual
GPT-4o / GPT-4.1o-series tokenizer~200k vocabulary, multimodal
GPT-5 prototypesO-series evolutionUnder NDA

Why OpenAI still uses BPE ​

  • Efficient
  • Fast inference
  • Stable for code
  • Supports all Unicode via byte fallback

Tokenizers in Grok (xAI) ​

xAI’s Grok-1 release confirmed:

➡️ https://github.com/xai-org/grok-1

Grok Tokenizer ​

  • SentencePiece
  • 131,072 vocabulary
  • Unigram/BPE hybrid
  • Great for multilingual + code-mixed text (Hinglish, Spanglish)

Reference:
https://x.ai/blog/grok-1

Tokenizers in Gemini (Google) ​

Gemini uses SentencePiece Unigram, optimized for multilingual and multimodal tasks.

Sources ​

Google’s Gemini Technical Report (2024):
https://arxiv.org/abs/2408.04227
SentencePiece Library (Google Research):
https://github.com/google/sentencepiece

Why Unigram for Gemini? ​

  • Excellent multilingual compression
  • Great for 100+ languages
  • Supports images/audio → token representations
  • Shorter token sequences on average

🔍 Comparison: ChatGPT vs Grok vs Gemini Tokenizers ​

FeatureChatGPT (OpenAI)Grok (xAI)Gemini (Google)
TokenizerBPESentencePieceSentencePiece Unigram
Vocabulary Size~50k–200k131kUndisclosed
StrengthsCode, EnglishMultilingual, noisy textMultimodal, multilingual
WeaknessesIndian-language fragmentationSlightly longer sequencesComplex multimodal token counts

Common Tokenizer Challenges ​

1. Poor support for Indian languages ​

Sequence inflation for words like:

स्वतंत्रता → स् + वत + ंत + र + ता

2. Numbers ​

Long numbers often split sub-optimally.

3. Emojis ​

Some tokenizers break emojis into bytes.

4. Multimodal complexity ​

Images/audio → add token overhead.

The Future: Unified Tokenization ​

Next-gen models will use learned multimodal tokenizers representing:

  • Text
  • Image patches
  • Audio segments
  • Video frames
  • Code structures
  • Proteins

OpenAI GPT-4o and Gemini 2.0 already hint at this direction.

Conclusion ​

Tokenizers shape how LLMs see and understand the world.

If you work with LLMs, dataset design, or prompt engineering — understanding tokenization is essential. It's the invisible layer that controls cost, performance, and reasoning quality.

References ​

Last updated: