Scale

Scale (class Scale Mecha Recipe) is the simplest merge operation in the pack and the one you'll reach for constantly once you think in deltas. It takes a delta and multiplies it by a factor. That's it. One input, one strength, one output - but it's the control that turns "how much of this change do I actually want?" into a knob you can turn anywhere in a merge graph.

Mecha's mental model separates weights (actual models) from deltas (the difference between a model and a base - the "recipe" for a change). Deltas are the unit of merge composition: Add Difference applies a delta to a base, TIES combines deltas, Model Stock averages them. Scale is how you size any of those deltas before they get combined - the difference between "apply this fine-tune fully" and "apply a third of it."

How it works

Per tensor, it's output = input * alpha, computed lazily as part of the recipe. Two short-circuits keep it fast: alpha = 0 returns a literal 0 (the delta contributes nothing), and alpha = 1 returns the delta unchanged. Everything in between is a straight multiplication, and because the whole thing is a recipe, the scaling gets folded into the merge and executed key-by-key - no intermediate files, no extra memory.

The inputs that matter:

• a (delta) (MECHA_RECIPE, required) - the delta to scale. Note the merge space: this node expects a delta, not a full checkpoint. If you wire in weight space, validation will complain.
• alpha (1.0) - the scale factor. Because it's a MECHA_RECIPE input, you're not limited to a single number: drive it with per-component or per-block weights from the pack's Components Params/Blocks nodes, and you get selective scaling - full strength on the UNet, half on the text encoder, none on the VAE.
• recipe (output, delta) - the scaled delta, ready to feed Add Difference, Add Difference Ties, or any delta consumer.

The classic pattern: Subtract Recipe List (base → model) to get a delta, Scale to size it, then Add Difference to lay it back onto a base. Want your merge 30% weaker? Change one number. Want different parts at different strengths? Swap alpha for a components param.

Install

Part of the Mecha Merge Node Pack (ljleb/comfy-mecha):

• ComfyUI Manager → Install Custom Nodes → search "mecha" → Mecha Merge Node Pack, or manually:

cd ComfyUI/custom_nodes
git clone https://github.com/ljleb/comfy-mecha.git
pip install -r comfy-mecha/requirements.txt

Restart ComfyUI; it's under mecha. The only dependency is sd-mecha==1.1.7; no model downloads.

Common issues

• Delta in, delta out. Feed Scale a checkpoint and the merge-space check refuses it. The fix is to subtract a base first - Scale operates on changes, not models.
• Alpha = 0 removes the contribution, not the key. The delta's keys still exist in the recipe; they're just multiplied by zero. Usually equivalent, occasionally not if a downstream node counts keys.
• Negative alphas are legal and mean "reverse the change." That's sometimes exactly what you want for removing a trait - just know it's on the table.

Scale is the node that makes "strength" a first-class part of your merge instead of a vibe. Once your workflow runs on deltas, this little multiply button becomes the thing you're adjusting ninety percent of the time.