What are special tokens such as BOS, EOS, and PAD?
PICTURE THIS: A SENTENCE BECOMES TOKENS
The model does not read letters like humans. It reads these pieces, then predicts the next one.
Simple meaning
BOS marks the start of a sequence, EOS marks the end of generation, and PAD fills batches to equal length.
WHY — Tokens instead of words?
LLMs use tokens (not full words) because it helps them:
who only read docs from people who shipped.
and tied to GenAI / LLM work.
Each piece maps to a number the network can learn.
Fixed pieces are what transformers expect as input.
STEPS — What happens with tokens?
Before the model can read a sentence, it goes through these steps:
- 1BOS marks the start
of a sequence, EOS marks the end of generation, and PAD fills batches to equal length.
- 2Chat templates also add
role tokens.
- 3Misusing them can confuse
the model or leak template text into answers.
- 4Context mix
Attention looks at nearby tokens together.
- 5Next token
The model scores what should come next.
- 6Decode
IDs turn back into readable text.
EXAMPLE — See it in action
Let's see how a real sentence is tokenized (tokens may vary by model):
Note: Actual tokens and IDs depend on the tokenizer (e.g., GPT, Llama, etc.).
Key takeaway
BOS marks the start of a sequence, EOS marks the end of generation, and PAD fills batches to equal length. Chat templates also add role tokens.