It’s actually really simple, it just adds text to the system prompt section of the template.

https://huggingface.co/Qwen/Qwen3.8-27B/blob/main/chat_template.jinja#L45-L87

    {%- set resolved_reasoning_effort = reasoning_effort|default('xhigh') %}
    {%- if resolved_reasoning_effort not in ('xhigh', 'medium', 'low') %}
        {{- raise_exception('Unexpected reasoning effort ' ~ reasoning_effort ~ '. Supported types are xhigh (default), medium, and low.') }}
    {%- endif %}
    {%- if resolved_reasoning_effort == 'xhigh' %}
        {%- set reasoning_instructions = 'Reasoning effort is set to xhigh. Please think carefully through the task, validate key assumptions, consider plausible alternatives, and prioritize correctness, consistency, and clarity in the final answer.' %}
    {%- elif resolved_reasoning_effort == 'low' %}
        {%- set reasoning_instructions = 'Reasoning effort is set to low. Keep your thinking brief and focused, moving directly to the conclusion without unnecessary elaboration.' %}
    {%- endif %}

This method probably works decently for other/older models too, but I’m sure Qwen 3.8 was trained specifically for these prompts. medium means your system prompt is unchanged. The default is xhigh so you’d need to use --chat-template-kwargs '{"reasoning_effort":"medium"}' to manually override.