In vLLM, --default-chat-template-kwargs sets chat-template options server-wide: it affects how incoming prompts are assembled and what behavior the model is asked to use. For Qwen3.8-27B, the documented defaults enable thinking and set reasoning effort to xhigh. These options do not change the model’s weights. You can also supply chat-template options for an individual request using chat_template_kwargs. Qwen’s model card and vLLM’s Qwen3.8-27B recipe describe the relevant behavior and configuration.
What the default setting does
The flag applies chat-template keyword arguments as defaults to requests handled by a vLLM server. A chat template formats conversation messages into the prompt sent to the model; its options can also specify requested behaviors such as whether to use thinking mode and the requested reasoning effort. Request-level chat_template_kwargs can specify options for an individual call.
For Qwen3.8-27B, Qwen says the model thinks before responding by default. Its thinking content can appear in a <think>...</think> block before the final answer. The documented default reasoning effort is xhigh. These are prompt/template settings, not a change to the model’s trained weights.
What each option controls
enable_thinking: whether to request thinking mode
Thinking is enabled by default. To request a direct response without thinking, vLLM’s recipe gives this JSON value:
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{"enable_thinking": false}reasoning_effort: requested reasoning depth
The documented values are xhigh, medium, and low. Qwen characterizes xhigh as suited to complex tasks, medium as a balance between accuracy and speed, and low as prioritizing speed and cost. To request lower effort, the recipe shows:
{"reasoning_effort": "low"}Effort and thinking mode are separate controls: disabling thinking is not the same as selecting a lower effort. A lower effort may make an individual turn faster, but Qwen warns that it can lead to failures and retries in multi-turn agent tasks, which can raise total latency and token use.
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preserve_thinking: whether earlier thinking blocks stay in context
This setting governs multi-turn history rather than whether thinking is enabled for a new answer. Qwen says historical thinking blocks are preserved by default; setting preserve_thinking to false retains only the thinking blocks from the latest user message. Its model-card API example explicitly passes enable_thinking: true, preserve_thinking: true, and reasoning_effort: "xhigh", matching the stated defaults.
How to configure Qwen3.8-27B in vLLM
The recipe documents --default-chat-template-kwargs for server-wide defaults and chat_template_kwargs for per-request options. The values shown above are JSON examples of the options, not complete command lines: the exact placement and syntax depend on the serving interface. Qwen’s model card demonstrates API-side parameters, while the vLLM recipe describes the vLLM flag. Check the interface and version you are using rather than treating those forms as interchangeable.
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Choosing settings for your workload
- For answers without thinking mode, set
enable_thinkingtofalse. - To keep thinking enabled but request a different depth, set
reasoning_efforttomediumorlowrather than the defaultxhigh. - For multi-turn conversations, decide whether earlier thinking blocks should remain in history by configuring
preserve_thinking. - For agent workflows, assess total task latency and token use across retries, not only the time or tokens for one response.
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