Dialogue agents powered by Large Language Models (LLMs) show superior performance in various tasks. Despite the better user understanding and human-like responses, their **lack of controllability** remains a key challenge, often leading to unfocused conversations or task failure. To address this, we introduce Standard Operating Procedure (SOP) to regulate dialogue flow. Specifically, we propose **ChatSOP**, a novel SOP-guided Monte Carlo Tree Search (MCTS) planning framework designed to enhance the controllability of LLM-driven dialogue agents. To enable this, we curate a dataset comprising SOP-annotated multi-scenario dialogues, generated using a semi-automated role-playing system with GPT-4o and validated through strict manual quality control. Additionally, we propose a novel method that integrates Chain of Thought reasoning with supervised fine-tuning for SOP prediction and utilizes SOP-guided Monte Carlo Tree Search for optimal action planning during dialogues. Experimental results demonstrate the effectiveness of our method, such as achieving a 27.95% improvement in action accuracy compared to baseline models based on GPT-3.5 and also showing notable gains for open-source models. Dataset and codes are publicly available.

Large language models (LLMs) have achieved remarkable progress in autonomous reasoning, evolving from basic text processing to sophisticated multimodal reasoning, a critical capability for general-purpose AI assistants. However, existing benchmarks usually fail to adequately capture the intricate multi-step reasoning demands inherent in real-world scenarios. To bridge this gap, we propose **C²RBench**: a **C**hinese **C**omplex **R**easoning **Bench**mark for evaluating multi-step, multimodal advanced reasoning capability of LLMs. C²RBench comprises 1,115 carefully curated Chinese tasks, which are organized into eight domain-specific subsets, each meticulously designed to mirror real-world challenges. This hierarchical benchmark features three difficulty tiers based on the number of reasoning steps required (average 8.44 steps per task), significantly exceeding existing benchmarks in cognitive complexity. Extensive evaluations of 20 LLMs (including DeepSeek-R1) and 24 multimodal large language models (MLLMs) on C²RBench reveal critical performance gaps: GPT-4.1 achieves only 52.11% accuracy, indicating substantial room for improvement. The dataset and evaluation code are publicly available.