FWIW, their readme states /nothink - and that's what works for me.
>/think and /nothink instructions: Use those words in the system or user message to signify whether Qwen3 should think. In multi-turn conversations, the latest instruction is followed.
>/think and /nothink instructions: Use those words in the system or user message to signify whether Qwen3 should think. In multi-turn conversations, the latest instruction is followed.
https://github.com/QwenLM/Qwen3/blob/main/README.md