HindSight: Evaluating LLM-Generated Research Ideas via Future Impact

Evaluating AI-generated research ideas typically relies on LLM judges or human panels -- both subjective and disconnected from actual research impact. We introduce HindSight, a time-split evaluation framework that measures idea quality by matching generated ideas against real future publications and scoring them by citation impact and venue acceptance. Using a temporal cutoff~T, we restrict an idea generation system to pre-T literature, then evaluate its outputs against papers published in the subsequent 30 months. Experiments across 10 AI/ML research topics reveal a striking disconnect: LLM-as-Judge finds no significant difference between retrieval-augmented and vanilla idea generation (p=0.584), while HindSight shows the retrieval-augmented system produces 2.5× higher-scoring ideas (p<0.001). Moreover, HindSight scores are negatively correlated with LLM-judged novelty (ρ=-0.29, p<0.01), suggesting that LLMs systematically overvalue novel-sounding ideas that never materialize in real research.