Mark Summerfield

The identifiers using bra and ket symbols are tokens, i.e. they are actually recognised and converted to a single "special" token by the tokenizer. For example, <|im_end|> is used as the EOS_TOKEN. (And, as an aside, you need to know this if you want to try fine-tuning these Qwen models, because you need to explicitly set eos_token='<|im_end|>' in your fine-tuning configuration.)

The XML-style identifiers are placeholders that define sections of the generated output. They are tokenized in the same manner as for any other text (i.e. they are probably converted to multiple tokens, but that depends on the vocabulary), they have no predetermined special meaning, and their use is learned by the model during training.

That said, the specific placeholders <think> and </think> are pretty standard across reasoning models.