[Bugfix][Model Runner V2] Fix min_tokens off-by-one in the V2 GPU sampler

[Sunt-ing]

Purpose

min_tokens=N should let EOS through at output index N (the N+1-th token), as the V1 MinTokensLogitsProcessor does. The V2 GPU sampler releases it one step late, so min_tokens=N silently forces N+1 non-EOS tokens. This is the default path for mainstream archs (Llama, Qwen3, Mistral, ...).

The kernel in vllm/v1/worker/gpu/sample/logit_bias.py suppresses stop tokens while pos < min_len, but pos is the last token's position (current length minus one), so it stops one step late. Compare the current length instead:

if num_stop_token_ids > 0 and pos + 1 < min_len:

min_tokens=0 is untouched (already guarded by num_stop_token_ids > 0).

Test Plan

Force EOS via logit_bias so it is selected the instant it is unblocked, then compare the generated length against V1.

from vllm import LLM, SamplingParams

llm = LLM("Qwen/Qwen3-0.6B", enforce_eager=True)
eos = llm.get_tokenizer().eos_token_id
out = llm.generate(
    "Hello",
    SamplingParams(temperature=0, min_tokens=4, max_tokens=32, logit_bias={eos: 100.0}),
)
print(len(out[0].outputs[0].token_ids))  # main: 6 (min_tokens + 2); fixed: 5 (min_tokens + 1)

Test Result

RTX 4090, Qwen/Qwen3-0.6B, forced EOS, generated length per min_tokens:

min_tokens	V1 reference	V2 + fix	V2 (main)
0	1	1	1
1	2	2	3
2	3	3	4
4	5	5	6

V2 with the fix matches V1 exactly; main is one token long for every min_tokens >= 1.

AI assistance was used to investigate, reproduce, and draft this change; the author reviewed the diff and validation output.

[Sunt-ing]

Hi @yewentao256, PTAL. No UT added :-)

[njhill]

Thanks @Sunt-ing, good catch