Decompose the Distillation: Interpretable Single-Pass Guidance for Diffusion Models

Classifier-Free Guidance (CFG) is a widely used inference-time guidance rule for conditional image generation with diffusion models, which exposes a guidance scale that adjusts conditioning strength after training. This flexibility requires conditional and unconditional model evaluations at every sampling step, so an $N$-step trajectory takes $2N$ forward passes. Single-pass alternatives reduce such cost, yet most collapse CFG's two-component structure, forfeiting the separable base flow and guidance direction that support post-hoc scale control and per-step diagnostics. This work introduces CtrlFlow, a single-pass guidance method that preserves CFG's separable base-flow and guidance-direction interface. To preserve both components at one-pass cost, CtrlFlow attaches two lightweight LoRA adapters to a frozen backbone, each adding $2.5\%$ parameters, and predicts the base flow and guidance direction from shared features. Because the guidance scale multiplies the guidance direction only after the forward pass, a single trained checkpoint remains adjustable at inference without retraining. Preserving two outputs introduces a training mismatch: inference amplifies guidance-direction errors by the chosen scale, while naive component-wise losses weight both outputs equally. To align training with this composed inference-time error, CtrlFlow uses a unified multi-scale distillation loss. Residual guidance errors can still persist at extreme scales, where even small guidance-direction errors are amplified by the guidance scale. To reduce this inference-time error, CtrlFlow applies scale-adaptive modulation to damp velocity-field errors and time-grid warping to concentrate solver evaluations where the trajectory changes most rapidly. On ImageNet $256\times256$ with the DiT-XL/2 backbone, CtrlFlow matches CFG within a pre-specified tolerance in 24 of 25 $(N, s)$ configurations and achieves a measured $1.93\times$ wall-clock speedup at $N{=}25$.