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I'm curious about the utility of counterspelling with low level spells, if one chooses to feat for it. Particularly the feasibility of keeping low-level spells prepared specifically for use as counters. The rule for counterspell in Pathfinder 2E says:

[Witch, Wizard] You expend a prepared spell to counter the triggering creature’s casting of that same spell. You lose your spell slot as if you had cast the triggering spell. You then attempt to counteract the triggering spell.

Note that the countering spell could theoretically be a lower-level version of the triggering spell.

If a Wizard further takes the Clever Counterspell feat, they can use a different spell (again not necessarily of the same level):

At the GM’s discretion, you can instead use a spell that has an opposing trait or that otherwise logically would counter the triggering spell (such as using a cold or water spell to counter fireball or using remove fear to counter a fear spell). Regardless of what spell you expend, you take a –2 penalty to your counteract check, though the GM can waive this penalty if the expended spell is especially appropriate.

The rule for counteract checks appears to be the following (non-spell portions edited out)

When attempting a counteract check, add the relevant skill modifier or other appropriate modifier to your check against the target’s DC. ... use the caster’s DC. ... the counteract check modifier is your spellcasting ability modifier plus your spellcasting proficiency bonus, plus any bonuses and penalties that specifically apply to counteract checks. If an effect is a spell, its level is the counteract level.

Critical Success - Counteract the target if its counteract level is no more than 3 levels higher than your effect’s counteract level.

Say we have a Wizard who of course started his career at level 1 with 18 Int, and raises it at every attribute raising level. Now say both the Wizard and a baddie meet up at level 5. Baddie tries to cast fireball. The GM rules (as per above) that an ice spell can be used to counter it. The Wizard happens to have the level 1 spell Snowball prepared. What are the Wizard's odds of getting that Critical Success he needs?

Now say they meet again at level 20. Baddie heightens his fireball to a 10th level spell. Our Wizard, knowing he could face 10th level spells, has Snowball prepared into a 7th level spot. Now what are the Wizard's odds of counteracting that fireball spell cast?

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    \$\begingroup\$ 2 things real quick: 1) I'm pretty sure you can't Counterspell with a cantrip. It reads: "You lose your spell slot", which definitionally excludes cantrips. You can probably just replace it with Snowball in your question, heightened to appropriate levels. 2) Is the GM waiving the -2 penalty because you are using an "especially appropriate" spell? \$\endgroup\$
    – ESCE
    Jun 22 at 17:02
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    \$\begingroup\$ @ESCE - 2) I'm gonna assume the "normal" case I'm interested in it won't be waived, but if both cases are talked about in an answer, that wouldn't hurt my feelings. \$\endgroup\$
    – T.E.D.
    Jun 22 at 17:36
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    \$\begingroup\$ Re: 1-b): the wording is a bit ambiguous there admittedly, that itself might make for an interesting (separate) question. I think that the RAI is pretty clear, though, that you shouldn't have to expend the exact same level of spell as the opposing caster. But I think we have enough to make an answer now - it'll take some maths and charting so I wouldn't expect one quickly :). \$\endgroup\$
    – ESCE
    Jun 22 at 18:38
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    \$\begingroup\$ Voting to close because it's unclear to me which question is being asked. It starts with "how useful are lower level spell for Counterspell" but then the example uses cantrips for some reason. Given that cantrips are not 'lower level spells' for counteracting purposes the example doesn't line up with the explicit question at all, instead it implies a different question: "can you Counterspell with cantrips". \$\endgroup\$
    – Ruse
    Jun 22 at 21:08
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    \$\begingroup\$ @Ruse - You've convinced me to take ESCE's advice and remove the stuff about cantrips in favor of using Snowball as the example spell. Is this better? \$\endgroup\$
    – T.E.D.
    Jun 23 at 15:38

1 Answer 1

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The Odds are Low

  • Using the level 5 necromancer DC of 21 (just below the High DC for a creature of that level)
  • Our wizard's +11 trained spellcasting proficiency plus 19 Intelligence counteract modifier
  • The -2 penalty as they aren't casting something more specific like snowball-that-puts-out-fireballs
  • And that they need a Critical Success to counterspell the 3rd-level fireball using their 1st-level snowball

We can calculate that the wizard would need to roll a nat 20 to counterspell the fireball, as that bumps the degree of success up one step to a critical success. This is the same across all spell DC categories here, even an opponent with a moderate DC of 19 would still require that the wizard rolls a nat 20 to counterspell.

Higher levels do help a bit:

  • Using the level 20 pit fiend DC of 42 (exactly the High DC for a creature of that level)
  • Our wizard's +35 legendary spellcasting proficiency plus 24 Intelligence (Apex item assumed) counteract modifier
  • The -2 penalty
  • Critical Success still required

This time our wizard is able to counterspell on a roll of 19 using a 7th-level snowball against a 10th-level fireball. But at this higher level the math is much tighter, and could be altered if the opposed creature had a lowered or higher spell DC.

The minimum assuming nothing beyond an Extreme DC would be counterspelling on nat 20's only, but the maximum chance assuming the lowest Moderate DC at 39 would be counterspelling on a rolled 16 or higher, or a 25% to 5% chance at 20th level depending on the opponent.

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