SimpleTIR: End-to-End Reinforcement Learning for Multi-Turn
  Tool-Integrated Reasoning
Authors: name: Zhenghai Xue; name: Longtao Zheng; name: Qian Liu; name: Yingru Li; name: Xiaosen Zheng; name: Zejun Ma; name: Bo An
Published: 2025-09-02 16:30:19

Abstract: Large Language Models (LLMs) can significantly improve their reasoning
capabilities by interacting with external tools, a paradigm known as
Tool-Integrated Reasoning (TIR). However, extending TIR to multi-turn scenarios
using Reinforcement Learning (RL) is often hindered by training instability and
performance collapse. We identify that such instability is primarily caused by
a distributional drift from external tool feedback, leading to the generation
of low-probability tokens. This issue compounds over successive turns, causing
catastrophic gradient norm explosions that derail the training process. To
address this challenge, we introduce SimpleTIR , a plug-and-play algorithm that
stabilizes multi-turn TIR training. Its core strategy is to identify and filter
out trajectories containing void turns, i.e., turns that yield neither a code
block nor a final answer. By removing these problematic trajectories from the
policy update, SimpleTIR effectively blocks the harmful, high-magnitude
gradients, thus stabilizing the learning dynamics. Extensive experiments show
that SimpleTIR achieves state-of-the-art performance on challenging math
reasoning benchmarks, notably elevating the AIME24 score from a text-only
baseline of 22.1 to 50.5 when starting from the Qwen2.5-7B base model.
Furthermore, by avoiding the constraints of supervised fine-tuning, SimpleTIR
encourages the model to discover diverse and sophisticated reasoning patterns,
such as self-correction and cross-validation.