When Valid Signals Fail: Regime Boundaries Between LLM Features and RL Trading Policies

Can large language models (LLMs) generate continuous numerical features that improve reinforcement learning (RL) trading agents? We build a modular pipeline where a frozen LLM serves as a stateless feature extractor, transforming unstructured daily news and filings into a fixeddimensional vector consumed by a downstream PPO agent. We introduce an automated promptoptimization loop that treats the extraction prompt as a discrete hyperparameter and tunes it directly against the Information Coefficient—the Spearman rank correlation between predicted and realized returns—rather than NLP losses. The optimized prompt discovers genuinely predictive features (IC above 0.15 on held-out data). However, these valid intermediate representations do not automatically translate into downstream task performance: during a distribution shift caused by a macroeconomic shock, LLM-derived features add noise, and the augmented agent under-performs a price-only baseline. In a calmer test regime the agent recovers, yet macroeconomic state variables remain the most robust driver of policy improvement. Our findings highlight a gap between feature-level validity and policy-level robustness that parallels known challenges in transfer learning under distribution shift.