🐧 Sigma Replace

Sometimes you don't want to add or remove a step - you want to change the noise level of one. Maybe your custom schedule has a step that lands awkwardly, a transition that's too aggressive, or a specific early step you want to soften. That's the exact niche of this node: replace the sigma value at one index and leave the rest of the curve alone.

It's part of the 🐧 SuperNodes pack by SuperCC's scheduling family, sitting alongside Sigma Insert and Sigma Remove.

How it works

Three inputs: sigmas, index, and value. 0 targets the first sigma, -1 the last (that's the full negative-index story here - no deeper tricks). The node swaps in your value at that position, then validates the schedule's monotonicity: the new value must be ≤ the previous sigma and ≥ the following one. Since a sigma schedule is a descending noise curve, a replacement that breaks that ordering produces a nonsense schedule, and the node refuses with a clear error instead of shipping it.

The output is the SIGMAS tensor with one value changed, everything else bit-identical.

When you'd reach for it

• Softening or sharpening a single step in a hand-built schedule - e.g. raising the last non-zero sigma to leave more denoise work for the final step.
• Fixing a jump in a custom scheduler's output where two adjacent values are too far apart, which can show up as a visible quality step.
• Fine-tuning one stage of the curve after you've graphed it (the pack's Sigmas Graph node is the natural companion: replace, re-graph, judge).

Install

ComfyUI Manager, search "ComfyUI-SuperNodes", or:

cd ComfyUI/custom_nodes
git clone https://github.com/sonnybox/ComfyUI-SuperNodes

Restart ComfyUI. No model downloads; the pack's only extra dependency is matplotlib, and it's only pulled in by the Sigmas Graph node. This pack uses ComfyUI's newer extension API - keep ComfyUI updated or the nodes won't register.

Gotchas

• The monotonicity check runs both directions: too big against the previous sigma or too small against the next one both error. On a schedule that's already out of order (from a buggy scheduler), you may not be able to set the value you want until you fix the neighbors.
• -1 means last only. If you want second-to-last you can't ask for it here - that's a Sigma Remove index, not a Sigma Replace one. Work from the front or chain nodes.
• It's a replacement, not a rescale: one step changes, the overall curve shape is preserved. If you want the whole curve shifted to a new range, that's Sigmas Rescale's job, not this node's.

A precision tool for a narrow job, and the validation is the whole reason to use it over typing a tensor value into a custom scheduler by hand - it catches the exact mistake that would otherwise produce a silent, schedule-shaped bug.