What does octoprint use to look for layer count info?

Just set up Prusa Slicer to talk to octoprint to compare how it prints versus cura, and the most obvious difference is in the dashboard, where the layer info remains "Layer 0/0" for the duration of the entire print. What is octoprint normally parsing out of the gcode to determine number of layers and which layer it is printing? If I know that, I could probably whip out a perl script to post process the gcode to add what octoprint wants. (It gets the current height and total height correct, BTW).

I'm using the plugin OctoPrint-DisplayLayerProgress which has the ability to modify the string used to match the layer comment.

If there is a "standard" it may be how Cura outputs it. You could download and install Ultimaker Cura from https://ultimaker.com/software/ultimaker-cura/#links, slice the same object in both Cura and PrusaSlicer and you should be able to see the difference.

I believe it is Cura: ;LAYER:1 and PrusaSlicer: ;LAYER:1 Z=0.65

I found the docs on Prusa Slicer macros and the list of variables you can use. Then I found the advanced setting for custom code on layer change. That combination allowed me to output
And now things work better. I also added to the start gcode: