What does the alpha coefficient control?

Alpha is the blend ratio. In a weighted-sum merge, alpha is the fraction of model B mixed in — alpha 0 is pure model A and alpha 1 is pure model B. Sigmoid methods reshape that curve to favour one model more strongly in the middle range.

When should I use add-difference?

Add-difference transfers what makes model B special relative to a base model C. The formula is A + (B - C) * alpha. Use it to graft a fine-tune's style onto another checkpoint while keeping that checkpoint's structure.

Do merged models need re-training?

No. Merging is a weighted average of weights and happens in seconds with no GPU training. That is why it is so popular for quickly combining the strengths of two community checkpoints.

Why is my merge producing washed-out or broken images?

Extreme alphas or merging incompatible architectures (for example an SD 1.5 model with an SDXL model) breaks the weights. Keep alpha in a sensible range, only merge same-architecture checkpoints and test at a few alpha values.

SD Model Merge Recipe Builder

Model merge recipe builder

Merging Stable Diffusion checkpoints is a weighted average of their weights — no training required. The result inherits traits from each parent in proportion to the alpha coefficient and the merge method you choose. This builder assembles a clean, reproducible recipe and the exact Automatic1111 Checkpoint Merger settings.

How it works

Three methods take two models: weighted sum (A*(1-a) + B*a), sigmoid and inverse-sigmoid, which apply a non-linear blend curve so one model dominates more in the middle of the range. The add-difference method takes three models and computes A + (B - C) * a, transferring the difference between B and a base model C onto A. The tool validates that you supplied model C when add-difference is selected and clamps coefficients to the valid 0-1 range.

Tips for clean merges

Only merge same-architecture models. SD 1.5 with SD 1.5, SDXL with SDXL — never mix the two.
Sweep alpha. Generate the same prompt and seed at alpha 0.3, 0.5 and 0.7 to find the sweet spot rather than guessing once.
Use add-difference for style transfer. It is the cleanest way to move a fine-tune’s aesthetic onto a stronger base without dragging in its quirks.
Bake the VAE separately. Merging does not change the VAE; pick a known-good VAE at inference time for consistent colour.