TR2-D2: Tree Search Guided Trajectory-Aware Fine-Tuning for Discrete
  Diffusion
Authors: name: Sophia Tang; name: Yuchen Zhu; name: Molei Tao; name: Pranam Chatterjee
Published: 2025-09-29 17:58:45

Abstract: Reinforcement learning with stochastic optimal control offers a promising
framework for diffusion fine-tuning, where a pre-trained diffusion model is
optimized to generate paths that lead to a reward-tilted distribution. While
these approaches enable optimization without access to explicit samples from
the optimal distribution, they require training on rollouts under the current
fine-tuned model, making them susceptible to reinforcing sub-optimal
trajectories that yield poor rewards. To overcome this challenge, we introduce
TRee Search Guided TRajectory-Aware Fine-Tuning for Discrete Diffusion
(TR2-D2), a novel framework that optimizes reward-guided discrete diffusion
trajectories with tree search to construct replay buffers for trajectory-aware
fine-tuning. These buffers are generated using Monte Carlo Tree Search (MCTS)
and subsequently used to fine-tune a pre-trained discrete diffusion model under
a stochastic optimal control objective. We validate our framework on single-
and multi-objective fine-tuning of biological sequence diffusion models,
highlighting the overall effectiveness of TR2-D2 for reliable reward-guided
fine-tuning in discrete sequence generation.