CAST: Causal Advantage-Structured Training with Spatially Grounded Compositional Rewards for Diffusion Models

Online reinforcement learning has been successfully extended to flow matching for diffusion model (DM) image generation. However, this paradigm faces three limitations: (1) **Window selection.** Existing methods typically manually set the stochastic differential equation (SDE) sampling window, i.e., the denoising steps where exploration noise is injected. We instead determine it from each model's denoising trajectory. (2) **Reward saturation.** Current methods rely heavily on scoring models trained on human annotations; we find that such scores are extremely high and nearly indistinguishable on the latest SOTA open-source DMs, making advantage estimation largely ineffective. (3) **Sample inefficiency.** A single scalar reward collapses different failure modes into almost identical scores, leaving minimal gradient guidance for targeted improvement. To address these issues, we propose **CAST** (**C**ausal **A**dvantage-**S**tructured **T**raining), an RL fine-tuning method for pretrained DMs, which (1) identifies the denoising step at which each model fixes the objects and their spatial arrangement in the image and uses that timing to set the SDE window, (2) decomposes each prompt via Causal Scene Graphs (CSG) into *verifiable-atoms*, i.e., minimal semantic units such as an object, count, attribute, or spatial relation that can each be checked independently, and rewards each atom separately, and (3) projects the signed atom-level advantages into pixel space through teacher-forced attention and uses them to spatially weight the SDE policy objective. We fine-tune two of the strongest open-source DMs, FLUX.2-dev and Qwen-Image-2512, with CAST, and evaluate them on GenEval 2, a compositional benchmark on which these models still fail frequently, and on Qwen-Image-Bench for overall quality. Within almost the same training budget, CAST's improvement over the base model on the most challenging GenEval 2 prompts is up to $3.07\times$ that of Flow-GRPO, while overall generation quality also improves.