vLLM hits prefix-cache snag with Qwen 3.5

This is not a launch story so much as a sharp reminder that hybrid-model support still has rough edges when you push open-source inference stacks into real agent workloads.

• –vLLM’s docs explicitly list Qwen 3.5 among “hybrid-only” models, which means text serving behavior is not always identical to plain decoder-only LLMs
• –The documented `--language-model-only` flag is the main workaround for text-only deployments, since it disables multimodal modules and frees more GPU memory for KV cache
• –Even so, the Reddit complaint highlights the difference between “model runs” and “model serves efficiently for long chats,” which is the real bar for agentic use
• –For AI infra teams, this is the kind of issue that decides whether a model is viable in production, regardless of its raw benchmark quality