You can use any message you want, but the model was tested to react reasonably well to the specific token sequence of "\nConsidering the limited time by the user, I have to give the solution based on the thinking directly now.\n</think>.\n\n" (from a Alibaba paper, struggling to find it now)
Since i have tools to prune context and run subagents, i just tell it to do either since both require summarization which is usually what it needs to avoid the long if...then chains
Edit: arXiv:2505.09388 Qwen3 Technical Report