[Feature]: separating reasoning and answer for reasoning models

[jamesbraza]

🚀 The feature, motivation and pitch

When hitting the DeepSeek R1 API at deepseek-reasoner (link), the API responds with reasoning and the answer in separate aspects of the payload. This enables clients like:

• LiteLLM to house reasoning and the answer in separate fields (link).
• OpenRouter's include_reasoning opt-in parameter (link).

It would be really useful if vllm serve had some capability for this.

Alternatives

Manually post-processing a response is error prone and arduous. It's better to do it within the model serving code.

Additional context

DeepSeek R1 model on Hugging Face

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[jamesbraza]

Actually this was done (tho currently unreleased) in #12473

It's docs: https://docs.vllm.ai/en/latest/features/reasoning_outputs.html

vllm serve ... --enable-reasoning --reasoning-parser xyz