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After reading this question: Bless/Bane and their effects on Passive checks, I want to ask about the general bonus rolls to passive ability checks.

The description for passive check is:

A passive check is a special kind of ability check that doesn’t involve any die rolls. Such a check can represent the average result for a task done repeatedly, such as searching for secret doors over and over again, or can be used when the DM wants to secretly determine whether the characters succeed at something without rolling dice, such as noticing a hidden monster. Here’s how to determine a character’s total for a passive check:

10 + all modifiers that normally apply to the check

There are modifiers that come as dice rolls, like guidance and Bardic Inspiration. Do these apply to passive ability checks? How do they calculated, especially when stacked with each other?

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Only if they last long enough and are done automatically

When Passive checks are used, they are used for two completely different reasons, as explained in the PHB (emphases mine):

A passive check is a special kind of ability check that doesn't involve any die rolls. Such a check can represent the average result for a task done repeatedly, such as searching for secret doors over and over again, or can be used when the DM wants to secretly determine whether the characters succeed at something without rolling dice, such as noticing a hidden monster.

Besides the admonition that such checks don't involve die rolls and the two spells you mention create die rolls, in neither of these use-cases would the benefit of guidance or bardic inspiration apply, because they are both single-use effects whose values are known to the players.

Suppose the DM has decided to use a Passive Perception Check because a character is searching repeatedly for secret doors. Searching repeatedly is clearly an action that is repeated over multiple turns. Yet both guidance and bardic inspiration can affect only a single action on a single turn:

Guidance, duration 1 minute, emphases mine:

Once before the spell ends, the target can roll a d4 and add the number rolled to one ability check of its choice.

Bardic inspiration, emphases mine:

Once within the next 10 minutes, the creature can roll the die and add the number rolled to one ability check, attack roll, or saving throw it makes.

Some quick actions might be repeated over the course of 10 minutes in the case of bardic inspiration, but in either case the bonus would be applied to only a single roll, rather than each one of a series of repeated rolls, which is what a Passive Check represents in this case. Being slightly better during a specific six seconds of a prolonged search would not make you significantly more likely to have a better outcome during the entire search.

Another time a DM might use a Passive Check is when they don't want the players to see the result of rolls, such as when searching for Hidden monsters. In this case, the DM is using the Passive Check specifically to combat the player meta-gaming that results from them knowing the 'result' of a roll. Suppose the party is confronted with a chamber and three passages leading out. They listen down each of the passages and the DM calls for active Perception checks. The players see the results of the rolls: 10, 14, 22 and the DM tells them they don't hear anything down any of the passages. They determine that they still don't know anything about two of the passages, but they can be pretty sure that there is nothing down the third because they 'rolled a 22' and 'nothing has that good a Stealth at our level'. In order to combat this meta-gaming (the characters shouldn't know how well they listened), when the players announce that they listen down each passage, the DM says that they are using Passive Perception, and heard nothing down all three. If the players then respond with three uses of guidance, and see the rolls of 1, 1, and 4, and would like to apply them to their Passive score, they again know that they 'listened harder' down one of the passages, which has defeated the purpose of the Passive Check.

Thus in neither of the cases for which Passive Checks are intended should guidance or bardic inspiration apply, because they are antithetical to the intent of using Passive Checks.

That is not to say that Passive Checks cannot be modified, just that such a modification would need to be of a long-term nature, and automatically applied to every roll. This is the case, for example, with the foresight spell (duration 8 hours), where:

For the duration, the target can't be surprised and has advantage on attack rolls, ability checks, and saving throws.

In this case, eight hours is plenty of time to take repeated actions, and the automatic advantage on every single ability check regardless of whether the character chooses to apply it in any single check means that such a spell should indeed affect a Passive Check. And the rules for Passive Checks state that:

If the character has advantage on the check, add 5.

Thus the effect of being under a foresight spell is a +5 to your Passive score.

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  • \$\begingroup\$ "Only can be used once instead of repeatedly" is actually another excellent reason why it should not apply to passive check. Can 'guidance' be excluded since it can be cast repeatedly? \$\endgroup\$
    – Vylix
    Sep 21, 2023 at 6:04
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    \$\begingroup\$ @Vylix If you are willing to have a dedicated caster follow the checker around and give them guidance every six seconds, and have the checker use that guidance every six seconds, for the entire duration of their repeated activity, then I think the argument can be made that guidance would in that case apply to the passive score, perhaps for an 'average' bonus of 2 or 3. But I would also say that if you are going to tie down two characters for the duration, why not just use a repeated Help action, for advantage that would translate as a +5? \$\endgroup\$
    – Kirt
    Sep 21, 2023 at 6:10
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    \$\begingroup\$ My players would certainly argue that they cast guidance as a cantrip over and over on every single one of those repeated rolls in a longer search process. \$\endgroup\$ Sep 21, 2023 at 6:55
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    \$\begingroup\$ @NobodytheHobgoblin Have you watched Critical Role, aka 8 friends yelling “guidance” at each other for four hours on Thursdays? \$\endgroup\$ Sep 21, 2023 at 7:16
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    \$\begingroup\$ @NobodytheHobgoblin My players call it 'massaging' since it has a range of touch. As in 'I follow the scout around and massage his shoulders while he looks for secret doors'. \$\endgroup\$
    – Kirt
    Sep 21, 2023 at 14:56
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No, because these are not modifiers in the sense of the rules

The PHB clearly outlines what it means when it uses the term modifier on page 171, under Ability Scores and Modifiers:

The three main rolls of the game—the ability check, the saving throw, and the attack roll—rely on the six ability scores. The book’s introduction describes the basic rule behind these rolls: roll a d20, add an ability modifier derived from one of the six ability scores, and compare the total to a target number.

And:

Each ability also has a modifier, derived from the score and ranging from 5 (for an ability score of 1) to +10 (for a score of 30).

Only the ability score bonuses and maluses and a few other static modifiers are called modifiers in the rules. Not everything that modifies the outcome of a roll is a modifier in the game's sense.

The proficiency bonus is also a modifier, and some features have static modifier they call out explicitly, like the scrying spell, that has a table of modifiers ranging from +5 to -10 depending on circumstances when casting the spell. Cover and Advantage and Disasvantage also can create modifiers (p. 194). In the context of an attack, things that influcence the roll are discussed as modifiers, but attacks are never passive. All mentions of the term in the rules otherwise refer to ability modifiers.

In contrast, Bardic Inspiration (p. 59 PHB), never calls what it does a modifier. It only calls it a "bardic inspiration die, a d6". Guidance likewise does not call what it does a modifier, it says:

Once before the spell ends, the target can roll a d4 and add the number rolled to one ability check of its choice.

Because these dice do not count as modifiers, there is no need to worry about how you would calculate them in case of a passive roll: they do not interact with passive rolls.

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  • \$\begingroup\$ Would that change if the party was in combat and a PC readied an action to use a spell like Guidance on themselves or another party member? \$\endgroup\$
    – Senmurv
    Sep 21, 2023 at 6:02
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    \$\begingroup\$ @Senmurv Not sure how that would change anything. Guidance still does not create a modifier in the narrow sense of the rules. It's also unlikely that a check would be passive in combat (although not impossible, maybe a passive perception of some kind). \$\endgroup\$ Sep 21, 2023 at 6:53

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