Mpi Logger

ComfyUI has no print() statement, which is a problem when you're debugging a workflow and you just need to know what value actually reached this point. Mpi Logger is the stand-in: an end-of-chain node with no output that prints to the console when its input arrives. Wire it at the end of a branch, run, and read the terminal. It's the least glamorous node in the pack and one of the most useful when something's misbehaving.

Three required inputs, one optional. mode is the switch: value (default) prints the input value itself, prefixed; message prints only the message text - the tooltip's framing is "logs that this point was reached without dumping the input." That distinction matters more than it sounds. In value mode, logging a huge tensor or a long prompt list dumps the whole thing to your terminal; in message mode you get a clean "we got here" breadcrumb. prefix (default [LOG]) is the label, and message is the text you type. The optional any input is where you attach the value you care about. Since it has no outputs, it must sit at the end of a chain - which is exactly what an output node (OUTPUT_NODE = True) is for.

The payoff is that you can drop several of these through a workflow with distinct prefixes - [stage1], [stage2] - and watch where values change, truncate, or vanish as a run progresses. When a number comes out wrong, the fastest diagnosis is usually "log it at every transformation and see which hop broke it." That's this node's entire job, and it does it without touching the data: the input is passed through untouched (well, not passed - it's a sink), so adding it to a graph for debugging can't alter results.

One thing beginners expect that it doesn't do: show anything in the ComfyUI UI. This prints to the server's console - the terminal where you launched ComfyUI (or the logs pane of whatever host app is running it, like Cubric Vision). If you're using the ComfyUI web UI on a remote box, "console" means the server process's stdout, not your browser. If you want to see a value on the canvas instead, you want a display/preview node, not this.

The distinction from MpiExecLogger is the useful one to remember: that sibling is a pass-through that logs and forwards the value so you can wire it inline mid-chain. Mpi Logger is the terminal one - no output, end of the road. For a workflow that's about to fork, put the logger on each branch with a different prefix and read which branches actually fired.

Install is the pack's standard no-drama story: ComfyUI Manager → search "ComfyUi-MpiNodes" → install → restart, or

cd ComfyUI/custom_nodes
git clone https://github.com/MadPonyInteractive/ComfyUi-MpiNodes

No dependencies, no models. Part of the Mad Pony Interactive utility pack (engine behind Cubric Vision). Cheap to add, invisible when unused, and the first thing you'll reach for when a run produces the wrong value and you need to know where.