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For some rolls in Pathfinder, such as attack rolls and saving throws, a natural 20 is an automatic success and a natural 1 is an automatic failure. The weakest character always has a 5% chance of success and the strongest character always has a 5% chance of failure.

For some other rolls, such as skill checks and initiative, there is no automatic success or failure. For skills, if the DC for a thing is greater than 20 plus your modifiers, then you simply can't do the thing. A level one commoner with a 3 dex and no ranks in acrobatics can't jump over the moon or leap across the ocean no matter what he rolls.

What about caster level checks, concentration checks, or rolls to overcome spell resistance? My gut tells me that automatic success failure should apply to those rolls, but I couldn't find the rule. Is there a rule somewhere that I've missed?

If the DC is greater than caster level + any applicable modifiers + 20, then is the caster simply incapable of doing whatever they're trying to do, or would rolling a natural 20 allow them to succeed anyway?

Also, if you think of any other d20 rolls that I haven't mentioned, is natural 20/natural 1 automatic success/failure for those rolls?

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Automatic success and failure applies only to those rolls where the rules explicitly say it does. It is the exceptional state; the default is for natural 1s and natural 20s to just add 1 or 20, respectively, to the bonus on the roll. If you cannot find such a rule, it likely means that it doesn’t exist and that those rolls do not exhibit automatic success or failure.

Known rolls that exhibit automatic failure and success include attack rolls and saving throws, and also, since combat maneuver checks are attack rolls, they follow the rules of attack rolls, e.g. they also have automatic success and failure. In addition to these, the Constitution check to stabilize has automatic success on a natural 20 (but no automatic failure on a natural 1). These are the only entries in d20PFSRD.com’s Combat page with automatic success or failure.

  • Attack rolls

    Automatic Misses and Hits

    A natural 1 (the d20 comes up 1) on an attack roll is always a miss. A natural 20 (the d20 comes up 20) is always a hit. A natural 20 is also a threat—a possible critical hit (see the attack action).

    • Combat maneuver checks

      When you attempt to perform a combat maneuver, make an attack roll and add your CMB in place of your normal attack bonus.

      (emphasis mine)

  • Saving throws

    Automatic Failures and Successes

    A natural 1 (the d20 comes up 1) on a saving throw is always a failure (and may cause damage to exposed items; see Items Surviving after a Saving Throw). A natural 20 (the d20 comes up 20) is always a success.

  • Stabilizing

    [Each round, a dying] character must make a DC 10 Constitution check to become stable. [...] A natural 20 on this check is an automatic success.

Ability checks, skill checks, concentration checks, caster level checks (to dispel, to overcome spell resistance, or really for any purpose at all) certainly all lack any automatic success or failure. A DC that is 21 or more greater than your bonus means that challenge is impossible for you; a DC that is no more than 1 greater than your bonus means you are guaranteed success.

Citing a negative is difficult, but for reference, the following places don’t mention any automatic success or failure rules:

  • Ability scores—which includes all the rules for ability checks, and the word “automatic” does not appear anywhere on this page.

  • Skills—which includes all the rules for skill checks, and the word “automatic” is used only to describe gaining new class skills when multiclassing, the automatic success you may have when taking-10, and that taking-20 “automatically” involves failing before success and the problems that causes with things that have penalties for failure.

  • Magic—which includes all the rules for various forms of caster level check (e.g. overcoming SR, dispelling), as well as for concentration checks. “Automatic” appears 10 times: automatic counterspells, automatic spell identification with arcane sight et al., automatic spell failure if you interrupt actions spent concentrating on casting a long spell, unconscious creatures automatically considered willing, conscious-but-helpless creatures not automatically considered willing, automatic success and failure of saving throws, automatic success deciphering magic writings with read magic, automatic success deciphering magic writings with the author’s assistance, a note that spells with automatic effects should be limited and/or higher level when designing a new spell, and that divine spellcasters learn all spells of a new level automatically.

  • Glossary—three uses of “automatic,” once in reference to the fact that saving throws don’t require an action, and then repeats of the descriptions for taking-10 and taking-20 on skill checks (same usage as the Skills page).

Those pages certainly cover any place I can think of to check for such rules. I can also assure you that the Pathfinder community is both clear and comfortable with these rules; a Google search on any of these topics will find Paizo forum threads confirming the lack of automatic success or failure on these checks. I couldn’t find any official commentary on them, however.

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  • \$\begingroup\$ A common houserule (I have seen it from different GMs) is to make a natural 20 count as 25, and natural 1 count as -5. Still not an automatic success though. \$\endgroup\$ – Anne Aunyme Feb 14 '18 at 17:05

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