Loop Engine (internal)

Loop Engine (internal) is not a node you place. It's the machinery that makes Loop Start and Loop End work - the hidden recursion driver the pack spawns behind the scenes. If you've ever opened a workflow with a Pixaroma loop and noticed a node you didn't put there, this is almost certainly it. Its category is literally "Internal," and the pack's own docs are explicit: users never place it by hand.

So why does it deserve its own page? Because when it shows up in a graph and you don't know what it is, it looks like something you should wire up or delete - and doing either will break the loop. Understanding what it does saves you from "fixing" a working workflow.

What it actually does

When Loop End fires, it checks whether another round is due. If so, it builds this engine node, which clones the section between the brackets plus itself and re-runs the whole thing with the next index and the updated carried values - effectively tail recursion on top of ComfyUI's graph-expansion feature. Each round's index and total feed in on the index_in and total inputs, the loop wire keeps the topology straight, and the value1…value5 slots shuttle the carried values forward. When the last round is done, the engine stops recursing and emits the final values out of Loop End.

You can see it in the schema: required inputs loop (FLOW_CONTROL), index_in, and total, plus the same five carried-value slots Loop Start and Loop End expose. It's the same anatomy as the brackets, because it is the loop - just re-spawned each round.

How to treat it

Leave it alone. If it appears in a saved workflow, that's the pack working as designed - muting it, deleting it, or wiring it manually will break the loop in ways that are confusing to debug. If a loop "doesn't run," the cause is almost never this node: it's either the loop wire between Loop Start and Loop End not being connected, or an outdated ComfyUI (the engine needs graph-expansion support, and the loop raises a clear "update ComfyUI" error if it's missing).

Install and gotchas

It installs with the rest of the pack - ComfyUI Manager (search "Pixaroma") or:

cd ComfyUI/custom_nodes
git clone https://github.com/pixaroma/ComfyUI-Pixaroma

Restart. No dependencies or model downloads.

The one real gotcha is identical to the rest of the pack: after an update, nodes can look blank because the browser is serving a cached copy. Hard-refresh with Ctrl+Shift+R (Cmd+Shift+R on Mac) before you suspect a bug. And if you hand-edit workflows in a text editor and see PixaromaLoopEngine in the JSON - that's normal, and it's a sign the loop machinery is doing its job.