Parallel thinking has emerged as a promising paradigm for reasoning, yet it imposes significant computational burdens. Existing efficiency methods primarily rely on local, per-trajectory signals and lack principled mechanisms to exploit global dynamics across parallel branches. We introduce 2D probing, an interface that exposes the width-depth dynamics of parallel thinking by periodically eliciting intermediate answers from all branches. Our analysis reveals three key insights: non-monotonic scaling across width-depth allocations, heterogeneous reasoning branch lengths, and early stabilization of global consensus. Guided by these insights, we introduce Parallel-Probe, a training-free controller designed to optimize online parallel thinking. Parallel-Probe employs consensus-based early stopping to regulate reasoning depth and deviation-based branch pruning to dynamically adjust width. Extensive experiments across three benchmarks and multiple models demonstrate that Parallel-Probe establishes a superior Pareto frontier for test-time scaling. Compared to standard majority voting, it reduces sequential tokens by up to 35.8% and total token cost by over 25.8% while maintaining competitive accuracy.
@article{arxiv.2602.03845,
title = {Parallel-Probe: Towards Efficient Parallel Thinking via 2D Probing},
author = {Tong Zheng and Chengsong Huang and Runpeng Dai and Yun He and Rui Liu and Xin Ni and Huiwen Bao and Kaishen Wang and Hongtu Zhu and Jiaxin Huang and Furong Huang and Heng Huang},
journal= {arXiv preprint arXiv:2602.03845},
year = {2026}
}