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Some spells for a wizard state that they need an ability check vs. spell save DC (SSDC). I'm not understanding how this is supposed to work.

If the spell says a creature can see through the effect if it passes an Int vs. the SSDC, how does that work? A d20 is rolled by the DM and the Int mod (if any) of the creature is added and it has to beat (be greater than [or equal to?]) my wizard's SSDC (8 + Wis mod + proficiency bonus)?

Note that this is not about how to calculate this SSDC; I already know how to do that.

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Your analysis is correct (mostly).

Minor illusion is a good example of this. The text (PHB 260) says:

... the creature can determine that it is an illusion with a successful Intelligence (Investigation) check against your spell save DC.

If a creature under the control of the DM uses its action to investigate the illusion, the DM rolls the d20, adds the creature's Investigation bonus (or just its Intelligence bonus if it doesn't have a modifier specified for the Investigation skill), and checks if it is greater than or equal to the caster's spell save DC.

If the check is greater than or equal to the caster's spell save DC, the creature determines the illusion is false.

However, Wizards use Intelligence, not Wisdom, for their spells.

The Wizard's spell save DC, therefore, is 8 + [Intelligence modifier] + [Proficiency bonus].

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  • \$\begingroup\$ Thanks, I knew it was Int...just a kind of typo. \$\endgroup\$ – Tanstaafl Mar 28 '19 at 17:58
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    \$\begingroup\$ @Tanstaafl No worries, we kind of figured it was, but this answer covered it just in case. FYI if you find an answer that solves your problem you can indicate that by accepting it by clicking the checkmark next to it. Sometimes it is good to wait 24 hours to do so but you are welcome to do it whenever (or not at all) \$\endgroup\$ – Rubiksmoose Mar 28 '19 at 18:07
  • \$\begingroup\$ What gives you proficiency on spellsaves? \$\endgroup\$ – Miles Bedinger Mar 28 '19 at 20:18
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    \$\begingroup\$ @MilesBedinger Your class gives you two saving throw proficiencies, if that's what you mean, but a spellcasting class calculates their spell save DC with 8 + [their spellcasting modifier] + [their proficiency bonus]. A spellcaster is always proficient in the spells they cast \$\endgroup\$ – Blake Steel Mar 28 '19 at 20:25
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There's no functional difference between rolling an ability check against a spell DC and rolling any other ability check against any other DC; it's just a different person rolling the dice, and the difficulty is coming off your character sheet rather than the DM's notes.

Meeting the DC of a check exactly is in fact a success, as described in step 3 of "The d20" in the introduction to the Player's Handbook (page 7). My group always forgets whether meeting a DC is a success or not, so I remind them, "The guy with the die wins a tie." (And in the case where more than one person are rolling in opposition -- a Contest, per PHB p.174 -- a tie is really a tie, and the situation is not resolved in either character's favor.)

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