Hey Andy, no I’m not using ATC. fluidNC will just change its output signal based on a tool number. This allows for the laser and the spindle to be on different outputs or even signal methods.
I dug into the code and it’s possible the fluidnc is being detected as a grblhal instance which looks like it lacks the code for passthrough of the M6. If I modify this and drop the replace for (M6) from M6 it works.
In the Grbl code I see the code and it looks clean but no matter the position of the pass through switch the line is replaced. I’m going to be at the machine in a few hours where I will run some debugging.