Learning Explainable Dense Reward Shapes via Bayesian Optimization
Authors: name: Ryan Koo; name: Ian Yang; name: Vipul Raheja; name: Mingyi Hong; name: Kwang-Sung Jun; name: Dongyeop Kang
Published: 2025-04-22 21:09:33

Abstract: Current reinforcement learning from human feedback (RLHF) pipelines for large
language model (LLM) alignment typically assign scalar rewards to sequences,
using the final token as a surrogate indicator for the quality of the entire
sequence. However, this leads to sparse feedback and suboptimal token-level
credit assignment. In this work, we frame reward shaping as an optimization
problem focused on token-level credit assignment. We propose a reward-shaping
function leveraging explainability methods such as SHAP and LIME to estimate
per-token rewards from the reward model. To learn parameters of this shaping
function, we employ a bilevel optimization framework that integrates Bayesian
Optimization and policy training to handle noise from the token reward
estimates. Our experiments show that achieving a better balance of token-level
reward attribution leads to performance improvements over baselines on
downstream tasks and finds an optimal policy faster during training.
Furthermore, we show theoretically that explainability methods that are feature
additive attribution functions maintain the optimal policy as the original
reward.