Mechanistic probing found that RL-trained models leaned harder on what was already in the prompt, while concepts like board symmetry stayed weakly represented
AspireAI community · arXiv
Training language models with GRPO in the exhaustively analyzable domain of Tic-Tac-Toe, the authors tracked how internal representations changed and found that models reach high accuracy and prompt-robustness while sparse-autoencoder features show they rely more on information already explicit in the prompt, such as whose turn it is and board occupancy. High-level concepts like board symmetries and strategic forks remained weakly represented, and the paper reports that RL post-training reorganizes which features dominate rather than adding concept information, with the decision located downstream in the late-layer state. The domain is deliberately tiny and the authors frame the result as raising concern that reasoning benchmarks overestimate model abilities rather than as a general verdict.
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