Tokenized form

<|im_start|>system
You are helpful.<|im_end|>
<|im_start|>user
Hi<|im_end|>
<|im_start|>assistant
Hello!<|im_end|>
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Special tokens

<|im_start|> and <|im_end|>. Added to vocab. Reserved for chat delimiting. Model learns these mark role transitions.

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Why matters

Fine-tuning tools apply ChatML (or equivalent per model). Data as messages JSON, tokenizer converts to ChatML string.