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On vLLM 0.30.0, NVFP4 weights, an FP8 KV cache and MTP finally run together without crashes for the Qwen 3.6 and Qwen 3.8 models built on the Qwen 3.5 architecture. On older versions, the same combination crashed, stopped responding under load or needed features turned off, even MTP with FP8 models. This article lists the symptoms that suggest an upgrade, the problems we had on vLLM 0.25.0 to 0.29.0 and working start commands; only Qwen 3.8 Flash Next, on the experimental Qwen 4.0 architecture, still has errors.

Qwen 3.8 Flash Next is the best instruct model in our benchmarks, but its thinking mode is broken in our NVFP4 runs on vLLM 0.30.0: answers stop after the first token or the thinking block never closes. Until this is fixed, we recommend not using this setup and using Qwen 3.6 27B or Qwen 3.8 27B in thinking mode: they need far less VRAM and are much better at reasoning tasks.