vLLM V0 to V1: Correctness Before Corrections in RL

vLLM has updated from version 0 to version 1, focusing on improving correctness before applying reinforcement learning corrections. This update aims to enhance the reliability of language model outputs. The change is important for developers relying on accurate AI responses in production environments.

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PublishedWednesday, May 6, 2026 at 9:06 PMMay 6, 09:06 PM

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A Blog post by ServiceNow-AI on Hugging Face

TL;DR. vLLM V1 matched our vLLM V0 reference after we fixed four things: processed rollout logprobs, V1-specific runtime defaults, the inflight weight-update path, and the fp32 lm_head used for the final projection. We fixed the backend behavior before changing the RL objective.

The reference run used vLLM 0.8.5; the V1 runs used vLLM 0.18.1. Figure 1 shows the final result. The red run is the initial V1 attempt, and the green run is the final V1 run after the fixes described below.

Opening the briefing

vLLM V0 to V1: Correctness Before Corrections in RL

Original article excerpt