TL;DR: Classifier-free guidance above a scale of ~7.5 pushed our SDXL product renders into oversaturation and clipped highlights. Adding CFG rescale at 0.7 plus dynamic thresholding fixed it with no retraining.
Around 18% of our automated product renders at Photoroom came back with blown-out highlights and oversaturated color once we raised the classifier-free guidance scale from 5.0 to 9.0 on our fine-tuned SDXL pipeline. The higher scale gave us sharper adherence to the prompt, which the catalog team wanted, but white backgrounds shifted toward grey-blue and metallic surfaces lost their specular detail. To be precise, the problem was not the prompt and not the fine-tune. It was the guidance arithmetic itself interacting with the noise schedule, and it is well documented if you know where to look.
What classifier-free guidance actually does
Classifier-free guidance combines two model predictions at each denoising step: one conditioned on the prompt and one unconditioned. The sampler extrapolates along the vector between them, scaled by a guidance weight. A weight of 1.0 means no guidance, and weights of 5 to 9 are typical for SDXL. Higher weights increase prompt adherence at the cost of pushing latents outside the distribution the model was trained on.
The method comes from Ho and Salimans in , where high guidance weights produced saturated, unnatural images. Their answer was dynamic thresholding. A second, complementary fix came later from Lin and colleagues in
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