SetFirstSigma_motorway_edition

Most people never touch the sigmas directly, which is exactly why SetFirstSigma exists as a thin, useful tool rather than a mysterious one. SetFirstSigma_motorway_edition is the clone of ComfyUI's built-in SetFirstSigma: it takes a SIGMAS schedule, overrides just the first entry - the starting noise level - and hands back the modified schedule. That single number is the "how much noise do we begin with" value, and changing it changes how much of the sampling trajectory actually happens.

Here's the intuition. A full denoise run starts at sigma ~14 (in the standard SD 1.5/SDXL scale), meaning "start from pure noise." If you set the first sigma to a lower value, you're saying "start from a partially denoised state" - the sampler effectively begins mid-way through the schedule, which is a bit like a denoise floor. That's the classic img2img-adjacent trick: feed in a latent that's already mostly the image you want, lower the first sigma, and the sampler only refines rather than rebuilds. It's also how some workflows keep high-frequency detail from being destroyed while still letting the sampler improve structure. The reverse - raising it - isn't a thing you'll normally do, because the schedule's natural start is already "as noisy as it gets."

The widgets:

• sigma (default 136.0, 0–20000) - the replacement first-sigma value. 136 is the conventional full-denoise starting point in the scheduler scale ComfyUI's newer samplers use; lower it toward 0 to cap how much denoising can happen. Start small and see what a fraction of the default does to your img2img stability.

Plumbing: it reads the schedule from the bus via INPUT_sigmas_key (default sigmas) and writes the modified one back under OUTPUT_SIGMAS_key (default SIGMAS), ready for the next sampling node in the chain.

Installing it

Part of ComfyUI_agilly1989_motorway. ComfyUI Manager: search ComfyUI_agilly1989_motorway, install, restart. Or:

cd ComfyUI/custom_nodes
git clone https://github.com/agilly1989/ComfyUI_agilly1989_motorway

No extra dependencies, no model files.

The catch

The pack's standing caveats: release 1.1.7 ships with the clones disabled (uncomment the two ClonedNodeMapping lines in custom_nodes/ComfyUI_agilly1989_motorway/__init__.py and restart), and the README - first line "BIG BROKEN WITH ASYNC WILL FIX WHEN I GET THE TIME/MOTIVATION" - is an honest beta from a solo dev. The node itself is a faithful clone, so it behaves like stock SetFirstSigma; just remember the sigma scale in your scheduler needs to line up with the number you type, or your "floor" will be somewhere you didn't intend.