fix: double bos token

[0xrushi]

Purpose

When using tokenizer.apply_chat_template() to manually format chat messages and then passing the resulting string to llm.generate(), vLLM incorrectly inserts an extra BOS token during tokenization.

Resolves #27486

Test Plan

def min_repro():
    from vllm import LLM, SamplingParams
    from transformers import AutoTokenizer
    
    tokenizer = AutoTokenizer.from_pretrained("meta-llama/Llama-3.2-1B-Instruct")
    MODEL_NAME = "meta-llama/Llama-3.2-1B-Instruct"
    prompt = "hello, how are you?"
    prompt_with_template = tokenizer.apply_chat_template([{"role": "user", "content": prompt}], tokenize=False, add_generation_prompt=True, add_special_tokens=False)
    llm = LLM(MODEL_NAME, max_model_len=1024, gpu_memory_utilization=0.60)
    outputs = llm.generate([prompt_with_template], sampling_params=SamplingParams(temperature=0.0, top_p=1.0, logprobs=1))
    print(outputs[0].prompt_token_ids)
    prompt_with_template_tokens = tokenizer.apply_chat_template([{"role": "user", "content": prompt}], tokenize=True, add_generation_prompt=True)
    print(prompt_with_template_tokens)
    assert len(outputs[0].prompt_token_ids) == len(prompt_with_template_tokens)
    

if __name__ == "__main__":
    min_repro()

Test Result

assert = success

---

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[gemini-code-assist]

Code Review

This pull request introduces a fix to prevent duplicate beginning-of-sequence (BOS) tokens during input preprocessing. The change checks if a prompt already starts with the BOS token text. If so, it disables the tokenizer's automatic addition of special tokens, provided the user hasn't explicitly configured this behavior. The implementation is safe and only applies to tokenizers that expose a bos_token string attribute. My review finds the logic to be sound and without any high or critical severity issues.

[DarkLight1337]

Someone else has already opened #27515, can you work with them on that PR instead?

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @0xrushi.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[0xrushi]

Someone else has already opened #27515, can you work with them on that PR instead?

Oh I didn't notice, closing this