D&D 5E Ruling Question on Shapechanger & Hold Person

[Hriston]

Except to preserve the slaad's cover (and not give info to the players that their characters wouldn't have) I'd still go through the motions of rolling a save and ignore what the die told me.

Situations like this are why I don't usually do DM rolls in the open.

Whereas I would consider the absence of a saving throw to correspond to some quality of the spell's resolution perceptible to the characters, such as the slaad not having to expend any effort to avoid paralysis as a humanoid would, despite being unaffected as if it had made its save.

 

[jmartkdr2]

The spell has no visible effect, e.g. a ray or glow running from the caster to the target, right? Which means the only way the caster would know the spell didn't resolve properly is if the actual casting got messed up or interrupted somehow.

Same as when a demon or whatever has magic resistance that applies before any save: the caster has no way of knowing which effect blocked the spell from working, only that it didn't work.

I guess we see the fiction differently: I see this as the spell tries to affect the target but the target shrugs it off, with or without even knowing something was trying to affect it.

Which, when neither you nor the target can see or hear the bear trap, makes no appreciable difference.

I guess that’s where we differ: I assume the caster can sense the spell as it is forming (or not). So they could sense if it failed to form or formed incorrectly.

Magic resistance is different; it basically works as a bonus to saves. Like you wouldn’t know if a hasted monk dodged your attack because of haste or general monk dodginess, those effects look the same. But if the attack bounced off their iron skin that would be distinct (even though it’s still AC)

 

[Hriston]

“The slaad can use its action to polymorph into a small or medium humanoid.”

I’m reading that as “it turns into a humanoid.” If it becomes a humanoid, its type would change. If it doesn’t become a humanoid, it doesn’t turn into a humanoid.

Polymorph has a fairly loose definition as a verb. I would take it to mean "change its form" or "appearance" rather than "change its nature" or "essence". That way it doesn't conflict with the statement that the slaad doesn't change its type.