Mettaton, dungeon - does pressing Z with the yellow heart screw up the pacifist run?

[pac man's nuts]

yup

[CyperAleksi]

Nope.

[He Who Is Vile]

Your puny pellets can't hurt him.

He was only acting.

[Strider VM]

A general rule is whatever happened to a monsters using the ACT command is not an aggression.

[Seraphita]

You kinda have to attack him with the yellow hearts for this battle... I got true pacifist ending on my first run so yeah. As long as Fight is not used (except on a few exceptions) you're fine.

[Nostril]

The only way to interrupt a pacifist run is by killing another monster with the FIGHT command. If you don't kill a monster, it will remain pacifist.
You can even attack other monsters as long as you spare them in the end.

[brickey.8]

Is he dead by the end of the scene? No? Then you're allowed to do it in a pacifist run.

[Brassqund]

Originally posted by StriderVM:

A general rule is whatever happened to a monsters using the ACT command is not an aggression.

You can be use attack all the time in pacifist. (if a monster is half health, you can spare it)
It is just not a good idea because you can easily messup.
KILLING is what cancel your pacifist run.