COMPLLLM: Fine-tuning LLMs to Discover Complementary Signals for Decision-making

Multi-agent decision pipelines can outperform single agent workflows when complementarity holds, i.e., different agents bring unique information to inform a final decision. We propose ComplLLM, a post-training framework based on decision theory that fine-tunes a decision-assistant LLM to output signals that complement existing agent decisions, using complementary information as reward. We validate ComplLLM on synthetic and real-world tasks involving domain experts, demonstrating how the approach recovers known complementary information, produces plausible explanations of complementary signals to support downstream decision-makers, and improves human and LLM agent decision performance in controlled studies.