Title: EAT: Entropy After $\textlangle \tt /Think \textrangle$ for reasoning model early exiting
Primary area: foundation or frontier models, including LLMs
Keywords: Reasoning LLM; Early exiting; Uncertainty

Abstract: Large reasoning models show improved performance with longer chains of thought.
However, recent work has highlighted (qualitatively) their tendency to overthink, continuing to revise answers even after reaching the correct solution.
We quantitatively confirm this inefficiency by tracking Pass@1 for answers averaged over a large number of rollouts and find that the model often begins to always produce the correct answer early in the reasoning, making extra reasoning a waste of tokens.
To detect and prevent overthinking, we propose a simple and inexpensive novel signal---Entropy After `</Think>`
 (EAT)---for monitoring and deciding whether to exit reasoning early. 
By appending a stop thinking token (`</think>`) and monitoring the entropy of the following token as the model reasons, we obtain a trajectory that decreases and stabilizes when Pass@1 plateaus; thresholding its variance under an exponential moving average yields a practical stopping rule.
Importantly, our approach enables adaptively allocating compute based on the EAT trajectory, allowing us to spend compute in a more efficient way compared with fixing the token budget for all questions.
Empirically, on MATH-500 and AIME-2025, EAT reduces token usage by 13–21\% without harming accuracy, and it remains effective in black-box settings where logits from the reasoning model are not accessible, and EAT is computed with proxy models.